generating string from exited characters

generating string from exited characters - c#

I have a character array as shown below :
char[] pwdCharArray = "abcdefghijklmnopqrstuvwxyzABCDEFG" +
"HIJKLMNOPQRSTUVWXYZ0123456789`~!##$%^&*()-_=+[]{}\\|;:'\",<" +
".>/?".ToCharArray();
and from this char array, i want to generate a string of minimum length 7 and all the characters in the string should be from the above char array.
How to do this operation?

What's the maximum length? (You may want to parameterize the methods below to specify a minimum and maximum length.) Here's a simple way of doing it for exactly 7 characters:
Here's the C# version:
public string GeneratePassword(Random rng)
{
char[] chars = new char[7];
for (int i = 0; i < chars.Length; i++)
{
chars[i] = pwdCharArray[rng.Next(pwdCharArray.Length)];
}
return new string(chars);
}
Note that the Random instance should be passed in to avoid the common problem of creating many instances of Random. I have an article describing this problem and ways around it. In essence, you should use one instance of Random per thread - don't create a new instance every time you want to use one, and don't reuse the same instance across multiple threads.
In fact, for a genuine password which is guarding sensitive information, you probably shouldn't be using Random at all, but rather something like RNGCryptoServiceProvider (or less directly, the results of RandomNumberGenerator.Create()). This can be somewhat harder to use, but will give you much more secure random numbers.
In Java it would be pretty similar, but then I'd use SecureRandom (which is fortunately rather easier to use than its .NET counterpart). In this case, you can create a new instance each time:
public String generatePassword() {
char[] chars = new char[7];
SecureRandom rng = new SecureRandom();
for (int i = 0; i < chars.length; i++) {
chars[i] = pwdCharArray[nextInt(pwdCharArray.length)];
}
return new String(chars);
}

Generate 7 random numbers between 0 and yourArray.length -1.
Pick the corresponding char in the array and put it in your final String.
Here is the code with a StringBuilder:
StringBuilder sb = new StringBuilder();
Random random = new Randdom();
for(int i=0; i<7; i++) {
sb.append(pwdCharArray[random.nextInt(0, pwdCharArray.length -1)]);
}
return sb.toString();

(C# example)
Random rand = new Random();
char[] arr = new char[rand.Next(7,15)];
for(int i = 0 ; i < arr.Length ; i++) {
arr[i] = pwdCharArray[rand.Next(pwdCharArray.Length)];
}
string pwd = new string(arr);

Here is a solution in Java, with variable length of the word, and test of working:
import java.util.*;
public class RandomWord {
// Valid characters
private static String VALID_CHARACTERS = "abcdefghijklmnopqrstuvwxyzABCDEFG" +
"HIJKLMNOPQRSTUVWXYZ0123456789`~!##$%^&*()-_=+[]{}\\|;:'\",<.>/?";
// Minimal length thw word can have
private static int MIN_LENGTH = 7;
// Maximal length will be MIN_LENGTH + THRESHOLD - 1
private static int THRESHOLD = 10;
// Generate a random number generator.
// The time in millis is used as seed prevents the numbers to be always the same in same order
Random randomGenerator = new Random(System.currentTimeMillis());
public String generateWord () {
// Actual length of the word (from MIN_LENGTH to MIN_LENGTH + THRESHOLD - 1)
int length = MIN_LENGTH + randomGenerator.nextInt(THRESHOLD);
// Loop for every character
StringBuilder word = new StringBuilder();
for (int i = 0; i < length; i++) {
// Appends one more random char
word.append(VALID_CHARACTERS.charAt(randomGenerator.nextInt(VALID_CHARACTERS.length())));
}
// Returns the random word
return word.toString();
}
// Test the class
public static void main (String[] args) {
// Instantiates and tests the class
RandomWord randomWord = new RandomWord();
for (int i = 0; i < 30; i++) {
String word = randomWord.generateWord();
System.out.println(word + " (" + word.length() + ")");
}
}
}

Related

Randomize string in C#

I am trying to randomize string elements, but my code is repeating strings. Can someone explain what is wrong with my code?
string[] words = Console.ReadLine().Split();
//input = "Welcome and have fun learning programming"
Random number = new Random();
for (int i = 0; i < words.Length; i++)
{
int currRandomNumber = number.Next(0, words.Length);
words[i] = words[currRandomNumber];
}
Console.WriteLine(string.Join(' ', words));
//output = "have learning learning learning learning programming"
I am facing problems with the words repeating, and it is not randomized? If you don't understand what I mean, see the comments which I added in the code. Any help will be appreciated!

The words are repeating because you are doing this:
words[i] = words[currRandomNumber];
The line means "copy the word at index currRandomNumber to the index i". As long as i and currRandomNumber are different, you are guaranteed to have a duplicate word.
What you meant to do was probably to swap the words at currRandomNumber and i:
var temp = words[i];
words[i] = words[currRandomNumber];
words[currRandomNumber] = temp;
// in C# 7, you could swap two values very easily by:
// (words[currRandomNumber], words[i]) = (words[i], words[currRandomNumber]);
Alternatively, you could use a Fisher Yates shuffling algorithm, which makes sure that each permutation have equal chances of occurring:
static void Shuffle<T>(T[] array)
{
int n = array.Length;
for (int i = 0; i < n; i++)
{
int r = i + _random.Next(n - i);
T t = array[r];
array[r] = array[i];
array[i] = t;
}
}
// Shuffle(words);

Alternatively, you can put each of the words into a new list in a random order:
List<string> words = Console.ReadLine().Split().ToList();
//input = "Welcome and have fun learning programming"
Random number = new Random();
var newwords = new List<string> ();
while (words.Count > 0)
{
int currRandomNumber = number.Next(0, words.Count);
newwords.Add( words[currRandomNumber]);
words.RemoveAt(currRandomNumber);
}
Console.WriteLine(string.Join(' ', newwords));

Random String generator with number of alpha and numeric chars C# [duplicate]

How can I generate a random 8 character alphanumeric string in C#?

I heard LINQ is the new black, so here's my attempt using LINQ:
private static Random random = new Random();
public static string RandomString(int length)
{
const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
return new string(Enumerable.Repeat(chars, length)
.Select(s => s[random.Next(s.Length)]).ToArray());
}
(Note: The use of the Random class makes this unsuitable for anything security related, such as creating passwords or tokens. Use the RNGCryptoServiceProvider class if you need a strong random number generator.)

var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
var stringChars = new char[8];
var random = new Random();
for (int i = 0; i < stringChars.Length; i++)
{
stringChars[i] = chars[random.Next(chars.Length)];
}
var finalString = new String(stringChars);
Not as elegant as the Linq solution.
(Note: The use of the Random class makes this unsuitable for anything security related, such as creating passwords or tokens. Use the RNGCryptoServiceProvider class if you need a strong random number generator.)

UPDATED for .NET 6. RNGCryptoServiceProvider is marked as obsolete. Instead, call RandomNumberGenerator.Create(). The code in the answer has been updated accordingly.
UPDATED based on comments. The original implementation generated a-h ~1.95% of the time and the remaining characters ~1.56% of the time. The update generates all characters ~1.61% of the time.
FRAMEWORK SUPPORT - .NET Core 3 (and future platforms that support .NET Standard 2.1 or above) provides a cryptographically sound method RandomNumberGenerator.GetInt32() to generate a random integer within a desired range.
Unlike some of the alternatives presented, this one is cryptographically sound.
using System;
using System.Security.Cryptography;
using System.Text;
namespace UniqueKey
{
public class KeyGenerator
{
internal static readonly char[] chars =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
public static string GetUniqueKey(int size)
{
byte[] data = new byte[4*size];
using (var crypto = RandomNumberGenerator.Create())
{
crypto.GetBytes(data);
}
StringBuilder result = new StringBuilder(size);
for (int i = 0; i < size; i++)
{
var rnd = BitConverter.ToUInt32(data, i * 4);
var idx = rnd % chars.Length;
result.Append(chars[idx]);
}
return result.ToString();
}
public static string GetUniqueKeyOriginal_BIASED(int size)
{
char[] chars =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
byte[] data = new byte[size];
using (RNGCryptoServiceProvider crypto = new RNGCryptoServiceProvider())
{
crypto.GetBytes(data);
}
StringBuilder result = new StringBuilder(size);
foreach (byte b in data)
{
result.Append(chars[b % (chars.Length)]);
}
return result.ToString();
}
}
}
Based on a discussion of alternatives here and updated/modified based on the comments below.
Here's a small test harness that demonstrates the distribution of characters in the old and updated output. For a deep discussion of the analysis of randomness, check out random.org.
using System;
using System.Collections.Generic;
using System.Linq;
using UniqueKey;
namespace CryptoRNGDemo
{
class Program
{
const int REPETITIONS = 1000000;
const int KEY_SIZE = 32;
static void Main(string[] args)
{
Console.WriteLine("Original BIASED implementation");
PerformTest(REPETITIONS, KEY_SIZE, KeyGenerator.GetUniqueKeyOriginal_BIASED);
Console.WriteLine("Updated implementation");
PerformTest(REPETITIONS, KEY_SIZE, KeyGenerator.GetUniqueKey);
Console.ReadKey();
}
static void PerformTest(int repetitions, int keySize, Func<int, string> generator)
{
Dictionary<char, int> counts = new Dictionary<char, int>();
foreach (var ch in UniqueKey.KeyGenerator.chars) counts.Add(ch, 0);
for (int i = 0; i < REPETITIONS; i++)
{
var key = generator(KEY_SIZE);
foreach (var ch in key) counts[ch]++;
}
int totalChars = counts.Values.Sum();
foreach (var ch in UniqueKey.KeyGenerator.chars)
{
Console.WriteLine($"{ch}: {(100.0 * counts[ch] / totalChars).ToString("#.000")}%");
}
}
}
}
Update 7/25/2022
Based on a question in the comments, I was curious whether the distribution is truly random.
I'm not a statistician, but I could probably play one on TV. If an actual statistician wants to chime in, that would be most welcome.
There are 62 possible output values (A-Za-Z0-9) and int.MaxValue numbers used to pick an array index. int.MaxValue % 62 is 1, so one character will be selected 1-in-4 billionish times more often than the others. We could further reduce that selection bias by randomly rotating the array of output values prior to indexing.
A T-test or other statistical measure would be the best approach to determine whether there is bias in the output results, but that's not something I can accomplish on my lunch break, so instead I leave you with a modification of the above code that measures deviation from expectation. Note that it tends toward zero.
using System.Security.Cryptography;
using System.Text;
const int REPETITIONS = 1_000_000;
const int KEY_SIZE = 32;
int TASK_COUNT = Environment.ProcessorCount - 1;
var expectedPercentage = 100.0 / KeyGenerator.chars.Length;
var done = false;
var iterationNr = 1;
var totalRandomSymbols = 0L;
var grandTotalCounts = new Dictionary<char, long>();
foreach (var ch in KeyGenerator.chars) grandTotalCounts.Add(ch, 0);
while (!done)
{
var experiments = Enumerable.Range(0, TASK_COUNT).Select(i => Task.Run(Experiment)).ToArray();
Task.WaitAll(experiments);
var totalCountsThisRun = experiments.SelectMany(e => e.Result)
.GroupBy(e => e.Key)
.Select(e => new { e.Key, Count = e.Select(_ => _.Value).Sum() })
.ToDictionary(e => e.Key, e => e.Count);
foreach (var ch in KeyGenerator.chars)
grandTotalCounts[ch] += totalCountsThisRun[ch];
var totalChars = grandTotalCounts.Values.Sum();
totalRandomSymbols += totalChars;
var distributionScores = KeyGenerator.chars.Select(ch =>
new
{
Symbol = ch,
OverUnder = (100.0 * grandTotalCounts[ch] / totalChars) - expectedPercentage
});
Console.WriteLine($"Iteration {iterationNr++}. Total random symbols: {totalRandomSymbols:N0}");
foreach (var chWithValue in distributionScores.OrderByDescending(c => c.OverUnder))
{
Console.WriteLine($"{chWithValue.Symbol}: {chWithValue.OverUnder:#.00000}%");
}
done = Console.KeyAvailable;
}
Dictionary<char, long> Experiment()
{
var counts = new Dictionary<char, long>();
foreach (var ch in KeyGenerator.chars) counts.Add(ch, 0);
for (int i = 0; i < REPETITIONS; i++)
{
var key = KeyGenerator.GetUniqueKey(KEY_SIZE);
foreach (var ch in key) counts[ch]++;
}
return counts;
}
public class KeyGenerator
{
internal static readonly char[] chars =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
public static string GetUniqueKey(int size)
{
byte[] data = new byte[4 * size];
using (var crypto = RandomNumberGenerator.Create())
{
crypto.GetBytes(data);
}
StringBuilder result = new StringBuilder(size);
for (int i = 0; i < size; i++)
{
var rnd = BitConverter.ToUInt32(data, i * 4);
var idx = rnd % chars.Length;
result.Append(chars[idx]);
}
return result.ToString();
}
}

Solution 1 - largest 'range' with most flexible length
string get_unique_string(int string_length) {
using(var rng = new RNGCryptoServiceProvider()) {
var bit_count = (string_length * 6);
var byte_count = ((bit_count + 7) / 8); // rounded up
var bytes = new byte[byte_count];
rng.GetBytes(bytes);
return Convert.ToBase64String(bytes);
}
}
This solution has more range than using a GUID because a GUID has a couple of fixed bits that are always the same and therefore not random, for example the 13 character in hex is always "4" - at least in a version 6 GUID.
This solution also lets you generate a string of any length.
Solution 2 - One line of code - good for up to 22 characters
Convert.ToBase64String(Guid.NewGuid().ToByteArray()).Substring(0, 8);
You can't generate strings as long as Solution 1 and the string doesn't have the same range due to fixed bits in GUID's, but in a lot of cases this will do the job.
Solution 3 - Slightly less code
Guid.NewGuid().ToString("n").Substring(0, 8);
Mostly keeping this here for historical purpose. It uses slightly less code, that though comes as the expense of having less range - because it uses hex instead of base64 it takes more characters to represent the same range compared the other solutions.
Which means more chance of collision - testing it with 100,000 iterations of 8 character strings generated one duplicate.

Here's an example that I stole from Sam Allen example at Dot Net Perls
If you only need 8 characters, then use Path.GetRandomFileName() in the System.IO namespace. Sam says using the "Path.GetRandomFileName method here is sometimes superior, because it uses RNGCryptoServiceProvider for better randomness. However, it is limited to 11 random characters."
GetRandomFileName always returns a 12 character string with a period at the 9th character. So you'll need to strip the period (since that's not random) and then take 8 characters from the string. Actually, you could just take the first 8 characters and not worry about the period.
public string Get8CharacterRandomString()
{
string path = Path.GetRandomFileName();
path = path.Replace(".", ""); // Remove period.
return path.Substring(0, 8); // Return 8 character string
}
PS: thanks Sam

The main goals of my code are:
The distribution of strings is almost uniform (don't care about minor deviations, as long as they're small)
It outputs more than a few billion strings for each argument set. Generating an 8 character string (~47 bits of entropy) is meaningless if your PRNG only generates 2 billion (31 bits of entropy) different values.
It's secure, since I expect people to use this for passwords or other security tokens.
The first property is achieved by taking a 64 bit value modulo the alphabet size. For small alphabets (such as the 62 characters from the question) this leads to negligible bias. The second and third property are achieved by using RNGCryptoServiceProvider instead of System.Random.
using System;
using System.Security.Cryptography;
public static string GetRandomAlphanumericString(int length)
{
const string alphanumericCharacters =
"ABCDEFGHIJKLMNOPQRSTUVWXYZ" +
"abcdefghijklmnopqrstuvwxyz" +
"0123456789";
return GetRandomString(length, alphanumericCharacters);
}
public static string GetRandomString(int length, IEnumerable<char> characterSet)
{
if (length < 0)
throw new ArgumentException("length must not be negative", "length");
if (length > int.MaxValue / 8) // 250 million chars ought to be enough for anybody
throw new ArgumentException("length is too big", "length");
if (characterSet == null)
throw new ArgumentNullException("characterSet");
var characterArray = characterSet.Distinct().ToArray();
if (characterArray.Length == 0)
throw new ArgumentException("characterSet must not be empty", "characterSet");
var bytes = new byte[length * 8];
var result = new char[length];
using (var cryptoProvider = new RNGCryptoServiceProvider())
{
cryptoProvider.GetBytes(bytes);
}
for (int i = 0; i < length; i++)
{
ulong value = BitConverter.ToUInt64(bytes, i * 8);
result[i] = characterArray[value % (uint)characterArray.Length];
}
return new string(result);
}

The simplest:
public static string GetRandomAlphaNumeric()
{
return Path.GetRandomFileName().Replace(".", "").Substring(0, 8);
}
You can get better performance if you hard code the char array and rely on System.Random:
public static string GetRandomAlphaNumeric()
{
var chars = "abcdefghijklmnopqrstuvwxyz0123456789";
return new string(chars.Select(c => chars[random.Next(chars.Length)]).Take(8).ToArray());
}
If ever you worry the English alphabets can change sometime around and you might lose business, then you can avoid hard coding, but should perform slightly worse (comparable to Path.GetRandomFileName approach)
public static string GetRandomAlphaNumeric()
{
var chars = 'a'.To('z').Concat('0'.To('9')).ToList();
return new string(chars.Select(c => chars[random.Next(chars.Length)]).Take(8).ToArray());
}
public static IEnumerable<char> To(this char start, char end)
{
if (end < start)
throw new ArgumentOutOfRangeException("the end char should not be less than start char", innerException: null);
return Enumerable.Range(start, end - start + 1).Select(i => (char)i);
}
The last two approaches looks better if you can make them an extension method on System.Random instance.

Just some performance comparisons of the various answers in this thread:
Methods & Setup
// what's available
public static string possibleChars = "abcdefghijklmnopqrstuvwxyz";
// optimized (?) what's available
public static char[] possibleCharsArray = possibleChars.ToCharArray();
// optimized (precalculated) count
public static int possibleCharsAvailable = possibleChars.Length;
// shared randomization thingy
public static Random random = new Random();
// http://stackoverflow.com/a/1344242/1037948
public string LinqIsTheNewBlack(int num) {
return new string(
Enumerable.Repeat(possibleCharsArray, num)
.Select(s => s[random.Next(s.Length)])
.ToArray());
}
// http://stackoverflow.com/a/1344258/1037948
public string ForLoop(int num) {
var result = new char[num];
while(num-- > 0) {
result[num] = possibleCharsArray[random.Next(possibleCharsAvailable)];
}
return new string(result);
}
public string ForLoopNonOptimized(int num) {
var result = new char[num];
while(num-- > 0) {
result[num] = possibleChars[random.Next(possibleChars.Length)];
}
return new string(result);
}
public string Repeat(int num) {
return new string(new char[num].Select(o => possibleCharsArray[random.Next(possibleCharsAvailable)]).ToArray());
}
// http://stackoverflow.com/a/1518495/1037948
public string GenerateRandomString(int num) {
var rBytes = new byte[num];
random.NextBytes(rBytes);
var rName = new char[num];
while(num-- > 0)
rName[num] = possibleCharsArray[rBytes[num] % possibleCharsAvailable];
return new string(rName);
}
//SecureFastRandom - or SolidSwiftRandom
static string GenerateRandomString(int Length) //Configurable output string length
{
byte[] rBytes = new byte[Length];
char[] rName = new char[Length];
SolidSwiftRandom.GetNextBytesWithMax(rBytes, biasZone);
for (var i = 0; i < Length; i++)
{
rName[i] = charSet[rBytes[i] % charSet.Length];
}
return new string(rName);
}
Results
Tested in LinqPad. For string size of 10, generates:
from Linq = chdgmevhcy [10]
from Loop = gtnoaryhxr [10]
from Select = rsndbztyby [10]
from GenerateRandomString = owyefjjakj [10]
from SecureFastRandom = VzougLYHYP [10]
from SecureFastRandom-NoCache = oVQXNGmO1S [10]
And the performance numbers tend to vary slightly, very occasionally NonOptimized is actually faster, and sometimes ForLoop and GenerateRandomString switch who's in the lead.
LinqIsTheNewBlack (10000x) = 96762 ticks elapsed (9.6762 ms)
ForLoop (10000x) = 28970 ticks elapsed (2.897 ms)
ForLoopNonOptimized (10000x) = 33336 ticks elapsed (3.3336 ms)
Repeat (10000x) = 78547 ticks elapsed (7.8547 ms)
GenerateRandomString (10000x) = 27416 ticks elapsed (2.7416 ms)
SecureFastRandom (10000x) = 13176 ticks elapsed (5ms) lowest [Different machine]
SecureFastRandom-NoCache (10000x) = 39541 ticks elapsed (17ms) lowest [Different machine]

One line of code Membership.GeneratePassword() does the trick :)
Here is a demo for the same.

The code written by Eric J. is quite sloppy (it is quite clear that it is from 6 years ago... he probably wouldn't write that code today), and there are even some problems.
Unlike some of the alternatives presented, this one is cryptographically sound.
Untrue... There is a bias in the password (as written in a comment), bcdefgh are a little more probable than the others (the a isn't because by the GetNonZeroBytes it isn't generating bytes with a value of zero, so the bias for the a is balanced by it), so it isn't really cryptographically sound.
This should correct all the problems.
public static string GetUniqueKey(int size = 6, string chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890")
{
using (var crypto = new RNGCryptoServiceProvider())
{
var data = new byte[size];
// If chars.Length isn't a power of 2 then there is a bias if
// we simply use the modulus operator. The first characters of
// chars will be more probable than the last ones.
// buffer used if we encounter an unusable random byte. We will
// regenerate it in this buffer
byte[] smallBuffer = null;
// Maximum random number that can be used without introducing a
// bias
int maxRandom = byte.MaxValue - ((byte.MaxValue + 1) % chars.Length);
crypto.GetBytes(data);
var result = new char[size];
for (int i = 0; i < size; i++)
{
byte v = data[i];
while (v > maxRandom)
{
if (smallBuffer == null)
{
smallBuffer = new byte[1];
}
crypto.GetBytes(smallBuffer);
v = smallBuffer[0];
}
result[i] = chars[v % chars.Length];
}
return new string(result);
}
}

My simple one line code works for me :)
string random = string.Join("", Guid.NewGuid().ToString("n").Take(8).Select(o => o));
Response.Write(random.ToUpper());
Response.Write(random.ToLower());
To expand on this for any length string
public static string RandomString(int length)
{
//length = length < 0 ? length * -1 : length;
var str = "";
do
{
str += Guid.NewGuid().ToString().Replace("-", "");
}
while (length > str.Length);
return str.Substring(0, length);
}

We also use custom string random but we implemented is as a string's helper so it provides some flexibility...
public static string Random(this string chars, int length = 8)
{
var randomString = new StringBuilder();
var random = new Random();
for (int i = 0; i < length; i++)
randomString.Append(chars[random.Next(chars.Length)]);
return randomString.ToString();
}
Usage
var random = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".Random();
or
var random = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789".Random(16);

A slightly cleaner version of DTB's solution.
var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
var random = new Random();
var list = Enumerable.Repeat(0, 8).Select(x=>chars[random.Next(chars.Length)]);
return string.Join("", list);
Your style preferences may vary.

public static string RandomString(int length)
{
const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
var random = new Random();
return new string(Enumerable.Repeat(chars, length).Select(s => s[random.Next(s.Length)]).ToArray());
}

Another option could be to use Linq and aggregate random chars into a stringbuilder.
var chars = "abcdefghijklmnopqrstuvwxyz123456789".ToArray();
string pw = Enumerable.Range(0, passwordLength)
.Aggregate(
new StringBuilder(),
(sb, n) => sb.Append((chars[random.Next(chars.Length)])),
  sb => sb.ToString());

Question: Why should I waste my time using Enumerable.Range instead of typing in "ABCDEFGHJKLMNOPQRSTUVWXYZ0123456789"?
using System;
using System.Collections.Generic;
using System.Linq;
public class Test
{
public static void Main()
{
var randomCharacters = GetRandomCharacters(8, true);
Console.WriteLine(new string(randomCharacters.ToArray()));
}
private static List<char> getAvailableRandomCharacters(bool includeLowerCase)
{
var integers = Enumerable.Empty<int>();
integers = integers.Concat(Enumerable.Range('A', 26));
integers = integers.Concat(Enumerable.Range('0', 10));
if ( includeLowerCase )
integers = integers.Concat(Enumerable.Range('a', 26));
return integers.Select(i => (char)i).ToList();
}
public static IEnumerable<char> GetRandomCharacters(int count, bool includeLowerCase)
{
var characters = getAvailableRandomCharacters(includeLowerCase);
var random = new Random();
var result = Enumerable.Range(0, count)
.Select(_ => characters[random.Next(characters.Count)]);
return result;
}
}
Answer: Magic strings are BAD. Did ANYONE notice there was no "I" in my string at the top? My mother taught me not to use magic strings for this very reason...
n.b. 1: As many others like #dtb said, don't use System.Random if you need cryptographic security...
n.b. 2: This answer isn't the most efficient or shortest, but I wanted the space to separate the answer from the question. The purpose of my answer is more to warn against magic strings than to provide a fancy innovative answer.

After reviewing the other answers and considering CodeInChaos' comments, along with CodeInChaos still biased (although less) answer, I thought a final ultimate cut and paste solution was needed. So while updating my answer I decided to go all out.
For an up to date version of this code, please visit the new Hg repository on Bitbucket: https://bitbucket.org/merarischroeder/secureswiftrandom. I recommend you copy and paste the code from: https://bitbucket.org/merarischroeder/secureswiftrandom/src/6c14b874f34a3f6576b0213379ecdf0ffc7496ea/Code/Alivate.SolidSwiftRandom/SolidSwiftRandom.cs?at=default&fileviewer=file-view-default (make sure you click the Raw button to make it easier to copy and make sure you have the latest version, I think this link goes to a specific version of the code, not the latest).
Updated notes:
Relating to some other answers - If you know the length of the output, you don't need a StringBuilder, and when using ToCharArray, this creates and fills the array (you don't need to create an empty array first)
Relating to some other answers - You should use NextBytes, rather than getting one at a time for performance
Technically you could pin the byte array for faster access.. it's usually worth it when your iterating more than 6-8 times over a byte array. (Not done here)
Use of RNGCryptoServiceProvider for best randomness
Use of caching of a 1MB buffer of random data - benchmarking shows cached single bytes access speed is ~1000x faster - taking 9ms over 1MB vs 989ms for uncached.
Optimised rejection of bias zone within my new class.
End solution to question:
static char[] charSet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".ToCharArray();
static int byteSize = 256; //Labelling convenience
static int biasZone = byteSize - (byteSize % charSet.Length);
public string GenerateRandomString(int Length) //Configurable output string length
{
byte[] rBytes = new byte[Length]; //Do as much before and after lock as possible
char[] rName = new char[Length];
SecureFastRandom.GetNextBytesMax(rBytes, biasZone);
for (var i = 0; i < Length; i++)
{
rName[i] = charSet[rBytes[i] % charSet.Length];
}
return new string(rName);
}
But you need my new (untested) class:
/// <summary>
/// My benchmarking showed that for RNGCryptoServiceProvider:
/// 1. There is negligable benefit of sharing RNGCryptoServiceProvider object reference
/// 2. Initial GetBytes takes 2ms, and an initial read of 1MB takes 3ms (starting to rise, but still negligable)
/// 2. Cached is ~1000x faster for single byte at a time - taking 9ms over 1MB vs 989ms for uncached
/// </summary>
class SecureFastRandom
{
static byte[] byteCache = new byte[1000000]; //My benchmark showed that an initial read takes 2ms, and an initial read of this size takes 3ms (starting to raise)
static int lastPosition = 0;
static int remaining = 0;
/// <summary>
/// Static direct uncached access to the RNGCryptoServiceProvider GetBytes function
/// </summary>
/// <param name="buffer"></param>
public static void DirectGetBytes(byte[] buffer)
{
using (var r = new RNGCryptoServiceProvider())
{
r.GetBytes(buffer);
}
}
/// <summary>
/// Main expected method to be called by user. Underlying random data is cached from RNGCryptoServiceProvider for best performance
/// </summary>
/// <param name="buffer"></param>
public static void GetBytes(byte[] buffer)
{
if (buffer.Length > byteCache.Length)
{
DirectGetBytes(buffer);
return;
}
lock (byteCache)
{
if (buffer.Length > remaining)
{
DirectGetBytes(byteCache);
lastPosition = 0;
remaining = byteCache.Length;
}
Buffer.BlockCopy(byteCache, lastPosition, buffer, 0, buffer.Length);
lastPosition += buffer.Length;
remaining -= buffer.Length;
}
}
/// <summary>
/// Return a single byte from the cache of random data.
/// </summary>
/// <returns></returns>
public static byte GetByte()
{
lock (byteCache)
{
return UnsafeGetByte();
}
}
/// <summary>
/// Shared with public GetByte and GetBytesWithMax, and not locked to reduce lock/unlocking in loops. Must be called within lock of byteCache.
/// </summary>
/// <returns></returns>
static byte UnsafeGetByte()
{
if (1 > remaining)
{
DirectGetBytes(byteCache);
lastPosition = 0;
remaining = byteCache.Length;
}
lastPosition++;
remaining--;
return byteCache[lastPosition - 1];
}
/// <summary>
/// Rejects bytes which are equal to or greater than max. This is useful for ensuring there is no bias when you are modulating with a non power of 2 number.
/// </summary>
/// <param name="buffer"></param>
/// <param name="max"></param>
public static void GetBytesWithMax(byte[] buffer, byte max)
{
if (buffer.Length > byteCache.Length / 2) //No point caching for larger sizes
{
DirectGetBytes(buffer);
lock (byteCache)
{
UnsafeCheckBytesMax(buffer, max);
}
}
else
{
lock (byteCache)
{
if (buffer.Length > remaining) //Recache if not enough remaining, discarding remaining - too much work to join two blocks
DirectGetBytes(byteCache);
Buffer.BlockCopy(byteCache, lastPosition, buffer, 0, buffer.Length);
lastPosition += buffer.Length;
remaining -= buffer.Length;
UnsafeCheckBytesMax(buffer, max);
}
}
}
/// <summary>
/// Checks buffer for bytes equal and above max. Must be called within lock of byteCache.
/// </summary>
/// <param name="buffer"></param>
/// <param name="max"></param>
static void UnsafeCheckBytesMax(byte[] buffer, byte max)
{
for (int i = 0; i < buffer.Length; i++)
{
while (buffer[i] >= max)
buffer[i] = UnsafeGetByte(); //Replace all bytes which are equal or above max
}
}
}
For history - my older solution for this answer, used Random object:
private static char[] charSet =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".ToCharArray();
static rGen = new Random(); //Must share, because the clock seed only has Ticks (~10ms) resolution, yet lock has only 20-50ns delay.
static int byteSize = 256; //Labelling convenience
static int biasZone = byteSize - (byteSize % charSet.Length);
static bool SlightlyMoreSecurityNeeded = true; //Configuration - needs to be true, if more security is desired and if charSet.Length is not divisible by 2^X.
public string GenerateRandomString(int Length) //Configurable output string length
{
byte[] rBytes = new byte[Length]; //Do as much before and after lock as possible
char[] rName = new char[Length];
lock (rGen) //~20-50ns
{
rGen.NextBytes(rBytes);
for (int i = 0; i < Length; i++)
{
while (SlightlyMoreSecurityNeeded && rBytes[i] >= biasZone) //Secure against 1/5 increased bias of index[0-7] values against others. Note: Must exclude where it == biasZone (that is >=), otherwise there's still a bias on index 0.
rBytes[i] = rGen.NextByte();
rName[i] = charSet[rBytes[i] % charSet.Length];
}
}
return new string(rName);
}
Performance:
SecureFastRandom - First single run = ~9-33ms. Imperceptible. Ongoing: 5ms (sometimes it goes up to 13ms) over 10,000 iterations, With a single average iteration= 1.5 microseconds.. Note: Requires generally 2, but occasionally up to 8 cache refreshes - depends on how many single bytes exceed the bias zone
Random - First single run = ~0-1ms. Imperceptible. Ongoing: 5ms over 10,000 iterations. With a single average iteration= .5 microseconds.. About the same speed.
Also check out:
https://bitbucket.org/merarischroeder/number-range-with-no-bias/src
https://stackoverflow.com/a/45118325/887092
These links are another approach. Buffering could be added to this new code base, but most important was exploring different approaches to removing bias, and benchmarking the speeds and pros/cons.

Simplest and most flexible solution with cryptographic security (.NET Core 3.0+):
If you're using .NET Core 3.0 or above, you can use the new static GetInt32 method (which is cryptographically secure) on the RandomNumberGenerator class to generate a random index for a given character set and populate the result that way quite easily.
This approach is far simpler than, for instance, the one proposed in this answer; and it also provides complete flexibility since you can pass in any character set you want.
public static string GenerateRandomString(int length, IEnumerable<char> charSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")
{
var charArray = charSet.Distinct().ToArray();
char[] result = new char[length];
for (int i = 0; i < length; i++)
result[i] = charArray[RandomNumberGenerator.GetInt32(charArray.Length)];
return new string(result);
}
Usage:
string randomAlphanumericString = GenerateRandomString(length: 10);

A simple and highly secure way might be generating the cryptography Aes key.
public static string GenerateRandomString()
{
using Aes crypto = Aes.Create();
crypto.GenerateKey();
return Convert.ToBase64String(crypto.Key);
}

I was looking for a more specific answer, where I want to control the format of the random string and came across this post. For example: license plates (of cars) have a specific format (per country) and I wanted to created random license plates.
I decided to write my own extension method of Random for this. (this is in order to reuse the same Random object, as you could have doubles in multi-threading scenarios).
I created a gist (https://gist.github.com/SamVanhoutte/808845ca78b9c041e928), but will also copy the extension class here:
void Main()
{
Random rnd = new Random();
rnd.GetString("1-###-000").Dump();
}
public static class RandomExtensions
{
public static string GetString(this Random random, string format)
{
// Based on http://stackoverflow.com/questions/1344221/how-can-i-generate-random-alphanumeric-strings-in-c
// Added logic to specify the format of the random string (# will be random string, 0 will be random numeric, other characters remain)
StringBuilder result = new StringBuilder();
for(int formatIndex = 0; formatIndex < format.Length ; formatIndex++)
{
switch(format.ToUpper()[formatIndex])
{
case '0': result.Append(getRandomNumeric(random)); break;
case '#': result.Append(getRandomCharacter(random)); break;
default : result.Append(format[formatIndex]); break;
}
}
return result.ToString();
}
private static char getRandomCharacter(Random random)
{
string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
return chars[random.Next(chars.Length)];
}
private static char getRandomNumeric(Random random)
{
string nums = "0123456789";
return nums[random.Next(nums.Length)];
}
}

Now in one-liner flavour.
private string RandomName()
{
return new string(
Enumerable.Repeat("ABCDEFGHIJKLMNOPQRSTUVWXYZ", 13)
.Select(s =>
{
var cryptoResult = new byte[4];
using (var cryptoProvider = new RNGCryptoServiceProvider())
cryptoProvider.GetBytes(cryptoResult);
return s[new Random(BitConverter.ToInt32(cryptoResult, 0)).Next(s.Length)];
})
.ToArray());
}

Try to combine two parts: unique (sequence, counter or date ) and random
public class RandomStringGenerator
{
public static string Gen()
{
return ConvertToBase(DateTime.UtcNow.ToFileTimeUtc()) + GenRandomStrings(5); //keep length fixed at least of one part
}
private static string GenRandomStrings(int strLen)
{
var result = string.Empty;
using (var gen = new RNGCryptoServiceProvider())
{
var data = new byte[1];
while (result.Length < strLen)
{
gen.GetNonZeroBytes(data);
int code = data[0];
if (code > 48 && code < 57 || // 0-9
code > 65 && code < 90 || // A-Z
code > 97 && code < 122 // a-z
)
{
result += Convert.ToChar(code);
}
}
return result;
}
}
private static string ConvertToBase(long num, int nbase = 36)
{
const string chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; //if you wish to make the algorithm more secure - change order of letter here
// check if we can convert to another base
if (nbase < 2 || nbase > chars.Length)
return null;
int r;
var newNumber = string.Empty;
// in r we have the offset of the char that was converted to the new base
while (num >= nbase)
{
r = (int)(num % nbase);
newNumber = chars[r] + newNumber;
num = num / nbase;
}
// the last number to convert
newNumber = chars[(int)num] + newNumber;
return newNumber;
}
}
Tests:
[Test]
public void Generator_Should_BeUnigue1()
{
//Given
var loop = Enumerable.Range(0, 1000);
//When
var str = loop.Select(x=> RandomStringGenerator.Gen());
//Then
var distinct = str.Distinct();
Assert.AreEqual(loop.Count(),distinct.Count()); // Or Assert.IsTrue(distinct.Count() < 0.95 * loop.Count())
}

For both crypto & noncrypto, efficiently:
public static string GenerateRandomString(int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890") =>
new Random().GenerateRandomString(length, charset);
public static string GenerateRandomString(this Random random, int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890") =>
RandomString(random.NextBytes, length, charset.ToCharArray());
public static string GenerateRandomCryptoString(int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890")
{
using (var crypto = new System.Security.Cryptography.RNGCryptoServiceProvider())
return crypto.GenerateRandomCryptoString(length, charset);
}
public static string GenerateRandomCryptoString(this RNGCryptoServiceProvider random, int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890") =>
RandomString(random.GetBytes, length, charset.ToCharArray());
private static string RandomString(Action<byte[]> fillRandomBuffer, int length, char[] charset)
{
if (length < 0)
throw new ArgumentOutOfRangeException(nameof(length), $"{nameof(length)} must be greater or equal to 0");
if (charset is null)
throw new ArgumentNullException(nameof(charset));
if (charset.Length == 0)
throw new ArgumentException($"{nameof(charset)} must contain at least 1 character", nameof(charset));
var maxIdx = charset.Length;
var chars = new char[length];
var randomBuffer = new byte[length * 4];
fillRandomBuffer(randomBuffer);
for (var i = 0; i < length; i++)
chars[i] = charset[BitConverter.ToUInt32(randomBuffer, i * 4) % maxIdx];
return new string(chars);
}
Using generators & LINQ. Not the fastest option (especially because it doesn't generate all the bytes in one go) but pretty neat & extensible:
private static readonly Random _random = new Random();
public static string GenerateRandomString(int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890") =>
new string(_random.GetGenerator().RandomChars(charset.ToCharArray()).Take(length).ToArray());
public static string GenerateRandomCryptoString(int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890")
{
using (var crypto = new System.Security.Cryptography.RNGCryptoServiceProvider())
return new string(crypto.GetGenerator().RandomChars(charset.ToCharArray()).Take(length).ToArray());
}
public static IEnumerable<char> RandomChars(this Func<uint, IEnumerable<uint>> randomGenerator, char[] charset)
{
if (charset is null)
throw new ArgumentNullException(nameof(charset));
if (charset.Length == 0)
throw new ArgumentException($"{nameof(charset)} must contain at least 1 character", nameof(charset));
return randomGenerator((uint)charset.Length).Select(r => charset[r]);
}
public static Func<uint, IEnumerable<uint>> GetGenerator(this Random random)
{
if (random is null)
throw new ArgumentNullException(nameof(random));
return GeneratorFunc_Inner;
IEnumerable<uint> GeneratorFunc_Inner(uint maxValue)
{
if (maxValue > int.MaxValue)
throw new ArgumentOutOfRangeException(nameof(maxValue));
return Generator_Inner();
IEnumerable<uint> Generator_Inner()
{
var randomBytes = new byte[4];
while (true)
{
random.NextBytes(randomBytes);
yield return BitConverter.ToUInt32(randomBytes, 0) % maxValue;
}
}
}
}
public static Func<uint, IEnumerable<uint>> GetGenerator(this System.Security.Cryptography.RNGCryptoServiceProvider random)
{
if (random is null)
throw new ArgumentNullException(nameof(random));
return Generator_Inner;
IEnumerable<uint> Generator_Inner(uint maxValue)
{
var randomBytes = new byte[4];
while (true)
{
random.GetBytes(randomBytes);
yield return BitConverter.ToUInt32(randomBytes, 0) % maxValue;
}
}
}
a simpler version using LINQ for only non-crypto strings:
private static readonly Random _random = new Random();
public static string RandomString(int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890") =>
new string(_random.GenerateChars(charset).Take(length).ToArray());
public static IEnumerable<char> GenerateChars(this Random random, string charset)
{
if (charset is null) throw new ArgumentNullException(nameof(charset));
if (charset.Length == 0) throw new ArgumentException($"{nameof(charset)} must contain at least 1 character", nameof(charset));
return random.Generator(charset.Length).Select(r => charset[r]);
}
public static IEnumerable<int> Generator(this Random random, int maxValue)
{
if (random is null) throw new ArgumentNullException(nameof(random));
return Generator_Inner();
IEnumerable<int> Generator_Inner() { while (true) yield return random.Next(maxValue); }
}

Horrible, I know, but I just couldn't help myself:
namespace ConsoleApplication2
{
using System;
using System.Text.RegularExpressions;
class Program
{
static void Main(string[] args)
{
Random adomRng = new Random();
string rndString = string.Empty;
char c;
for (int i = 0; i < 8; i++)
{
while (!Regex.IsMatch((c=Convert.ToChar(adomRng.Next(48,128))).ToString(), "[A-Za-z0-9]"));
rndString += c;
}
Console.WriteLine(rndString + Environment.NewLine);
}
}
}

A solution that includes all alphabet characters and numbers which you can change as you wish:
public static string RandomString(int length)
{
Random rand = new Random();
string charbase = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
return new string(Enumerable.Range(0,length)
.Select(_ => charbase[rand.Next(charbase.Length)])
.ToArray());
}
If you like one-liner methods ;)
public static Random rand = new Random();
public const string charbase = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
public static string RandomString(int length) =>
new string(Enumerable.Range(0,length).Select(_ => charbase[rand.Next(charbase.Length)]).ToArray());

A solution without using Random :
var chars = Enumerable.Repeat("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789", 8);
var randomStr = new string(chars.SelectMany(str => str)
.OrderBy(c => Guid.NewGuid())
.Take(8).ToArray());

Here is a variant of Eric J's solution, i.e. cryptographically sound, for WinRT (Windows Store App):
public static string GenerateRandomString(int length)
{
var chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
var result = new StringBuilder(length);
for (int i = 0; i < length; ++i)
{
result.Append(CryptographicBuffer.GenerateRandomNumber() % chars.Length);
}
return result.ToString();
}
If performance matters (especially when length is high):
public static string GenerateRandomString(int length)
{
var chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
var result = new System.Text.StringBuilder(length);
var bytes = CryptographicBuffer.GenerateRandom((uint)length * 4).ToArray();
for (int i = 0; i < bytes.Length; i += 4)
{
result.Append(BitConverter.ToUInt32(bytes, i) % chars.Length);
}
return result.ToString();
}

I know this one is not the best way. But you can try this.
string str = Path.GetRandomFileName(); //This method returns a random file name of 11 characters
str = str.Replace(".","");
Console.WriteLine("Random string: " + str);

I don't know how cryptographically sound this is, but it's more readable and concise than the more intricate solutions by far (imo), and it should be more "random" than System.Random-based solutions.
return alphabet
.OrderBy(c => Guid.NewGuid())
.Take(strLength)
.Aggregate(
new StringBuilder(),
(builder, c) => builder.Append(c))
.ToString();
I can't decide if I think this version or the next one is "prettier", but they give the exact same results:
return new string(alphabet
.OrderBy(o => Guid.NewGuid())
.Take(strLength)
.ToArray());
Granted, it isn't optimized for speed, so if it's mission critical to generate millions of random strings every second, try another one!
NOTE: This solution doesn't allow for repetitions of symbols in the alphabet, and the alphabet MUST be of equal or greater size than the output string, making this approach less desirable in some circumstances, it all depends on your use-case.

If your values are not completely random, but in fact may depend on something - you may compute an md5 or sha1 hash of that 'somwthing' and then truncate it to whatever length you want.
Also you may generate and truncate a guid.

Unique random numbers in C#

I'm trying to create a binary search tree with unique random numbers. I'm using SortedSet to represent my tree and then I make it into an array and then I use Contains to see if a certain number is in the tree. My problem is that I can't figure out how to get all the random numbers different in a simple way. I used the methods Unik and Nålen_Unik but in this code it only generates 1 number to the array
Random random = new Random();
Program Tal = new Program();
string nål = Tal.Nålen_Unik();
string TalIArray = Tal.Unik();
bool found = false;
SortedSet<string> Tree = new SortedSet<string>();
for (int x = 0; x < 50000; x++)
{
Tree.Add(TalIArray);
}
int y = 0;
string[] TreeArray = Tree.ToArray<string>();
while (y < TreeArray.Length)
{
Console.WriteLine(TreeArray[y]);
y = y + 1;
}
private string Unik()
{
int maxSize = 4;
char[] chars = new char[10000];
string a;
a = "0123456789";
chars = a.ToCharArray();
int size = maxSize;
byte[] data = new byte[1];
RNGCryptoServiceProvider crypto = new RNGCryptoServiceProvider();
crypto.GetNonZeroBytes(data);
size = maxSize;
data = new byte[size];
crypto.GetNonZeroBytes(data);
StringBuilder result = new StringBuilder(size);
foreach (byte b in data)
{
result.Append(chars[b % (chars.Length - 1)]);
}
return result.ToString();
}
private string Nålen_Unik()
{
int maxSize = 1;
char[] chars = new char[62];
string a;
a = "0123456789";
chars = a.ToCharArray();
int size = maxSize;
byte[] data = new byte[1];
RNGCryptoServiceProvider crypto = new RNGCryptoServiceProvider();
crypto.GetNonZeroBytes(data);
size = maxSize;
data = new byte[size];
crypto.GetNonZeroBytes(data);
StringBuilder result = new StringBuilder(size);
foreach (byte b in data)
{
result.Append(chars[b % (chars.Length - 1)]);
}
return result.ToString();

There are mainly three approaches that are used to get random numbers without collisions:
Keep all numbers that you have picked, so that you can check a new number against all previous.
Create a range of unique numbers, shuffle them, and pick one at a time from the result.
Create such a huge random number that the risk of collisions is so small that it's negligible.
The second approach is the same principle as shuffling a deck of cards. The third approach is used when creating a GUID, which is basically a random 128 bit value.

You could use the Random class and a HashSet which will guarantee no duplicate items.
The HashSet class provides high-performance set operations. A set is a collection that contains no duplicate elements, and whose elements are in no particular order.
E.g:
HashSet<int> t = new HashSet<int>();
Random r = new Random();
while(t.Count < 50)//or the desired length of 't'
{
t.Add(r.Next(0,1000));//adjust min/max as needed
}
foreach (int i in t)
{
Console.WriteLine(i);
}
Console.Read();
Will give you a collection of 50 guaranteed unique random integers.
Since the number of elements in the set is not a requirement for this question it seems irrelevant to even mention, though if this is a requirement you can simply modify the line t.Count < ? to get a set of a desired length.

You could use Guid.NewGuid() or new Random().Next()

Assuming that you want unique numbers in a limited range, one (simple but possibly inefficient) way to do it is to create a list of all possible values (say, 0-99). Then you use System.Random to pick a random between 0 and (number of elements in list - 1). Get that index from the list, output it and remove the element. If you repeat the process, that element can no longer be generated and the numbers will be unique until you've used all possible values.

Create one instance of Random class. Make sure it is one!
then use this code
private Random random = new Random();
random.Next();

How can I generate random alphanumeric strings?

How can I generate a random 8 character alphanumeric string in C#?

I heard LINQ is the new black, so here's my attempt using LINQ:
private static Random random = new Random();
public static string RandomString(int length)
{
const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
return new string(Enumerable.Repeat(chars, length)
.Select(s => s[random.Next(s.Length)]).ToArray());
}
(Note: The use of the Random class makes this unsuitable for anything security related, such as creating passwords or tokens. Use the RNGCryptoServiceProvider class if you need a strong random number generator.)

var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
var stringChars = new char[8];
var random = new Random();
for (int i = 0; i < stringChars.Length; i++)
{
stringChars[i] = chars[random.Next(chars.Length)];
}
var finalString = new String(stringChars);
Not as elegant as the Linq solution.
(Note: The use of the Random class makes this unsuitable for anything security related, such as creating passwords or tokens. Use the RNGCryptoServiceProvider class if you need a strong random number generator.)

UPDATED for .NET 6. RNGCryptoServiceProvider is marked as obsolete. Instead, call RandomNumberGenerator.Create(). The code in the answer has been updated accordingly.
UPDATED based on comments. The original implementation generated a-h ~1.95% of the time and the remaining characters ~1.56% of the time. The update generates all characters ~1.61% of the time.
FRAMEWORK SUPPORT - .NET Core 3 (and future platforms that support .NET Standard 2.1 or above) provides a cryptographically sound method RandomNumberGenerator.GetInt32() to generate a random integer within a desired range.
Unlike some of the alternatives presented, this one is cryptographically sound.
using System;
using System.Security.Cryptography;
using System.Text;
namespace UniqueKey
{
public class KeyGenerator
{
internal static readonly char[] chars =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
public static string GetUniqueKey(int size)
{
byte[] data = new byte[4*size];
using (var crypto = RandomNumberGenerator.Create())
{
crypto.GetBytes(data);
}
StringBuilder result = new StringBuilder(size);
for (int i = 0; i < size; i++)
{
var rnd = BitConverter.ToUInt32(data, i * 4);
var idx = rnd % chars.Length;
result.Append(chars[idx]);
}
return result.ToString();
}
public static string GetUniqueKeyOriginal_BIASED(int size)
{
char[] chars =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
byte[] data = new byte[size];
using (RNGCryptoServiceProvider crypto = new RNGCryptoServiceProvider())
{
crypto.GetBytes(data);
}
StringBuilder result = new StringBuilder(size);
foreach (byte b in data)
{
result.Append(chars[b % (chars.Length)]);
}
return result.ToString();
}
}
}
Based on a discussion of alternatives here and updated/modified based on the comments below.
Here's a small test harness that demonstrates the distribution of characters in the old and updated output. For a deep discussion of the analysis of randomness, check out random.org.
using System;
using System.Collections.Generic;
using System.Linq;
using UniqueKey;
namespace CryptoRNGDemo
{
class Program
{
const int REPETITIONS = 1000000;
const int KEY_SIZE = 32;
static void Main(string[] args)
{
Console.WriteLine("Original BIASED implementation");
PerformTest(REPETITIONS, KEY_SIZE, KeyGenerator.GetUniqueKeyOriginal_BIASED);
Console.WriteLine("Updated implementation");
PerformTest(REPETITIONS, KEY_SIZE, KeyGenerator.GetUniqueKey);
Console.ReadKey();
}
static void PerformTest(int repetitions, int keySize, Func<int, string> generator)
{
Dictionary<char, int> counts = new Dictionary<char, int>();
foreach (var ch in UniqueKey.KeyGenerator.chars) counts.Add(ch, 0);
for (int i = 0; i < REPETITIONS; i++)
{
var key = generator(KEY_SIZE);
foreach (var ch in key) counts[ch]++;
}
int totalChars = counts.Values.Sum();
foreach (var ch in UniqueKey.KeyGenerator.chars)
{
Console.WriteLine($"{ch}: {(100.0 * counts[ch] / totalChars).ToString("#.000")}%");
}
}
}
}
Update 7/25/2022
Based on a question in the comments, I was curious whether the distribution is truly random.
I'm not a statistician, but I could probably play one on TV. If an actual statistician wants to chime in, that would be most welcome.
There are 62 possible output values (A-Za-Z0-9) and int.MaxValue numbers used to pick an array index. int.MaxValue % 62 is 1, so one character will be selected 1-in-4 billionish times more often than the others. We could further reduce that selection bias by randomly rotating the array of output values prior to indexing.
A T-test or other statistical measure would be the best approach to determine whether there is bias in the output results, but that's not something I can accomplish on my lunch break, so instead I leave you with a modification of the above code that measures deviation from expectation. Note that it tends toward zero.
using System.Security.Cryptography;
using System.Text;
const int REPETITIONS = 1_000_000;
const int KEY_SIZE = 32;
int TASK_COUNT = Environment.ProcessorCount - 1;
var expectedPercentage = 100.0 / KeyGenerator.chars.Length;
var done = false;
var iterationNr = 1;
var totalRandomSymbols = 0L;
var grandTotalCounts = new Dictionary<char, long>();
foreach (var ch in KeyGenerator.chars) grandTotalCounts.Add(ch, 0);
while (!done)
{
var experiments = Enumerable.Range(0, TASK_COUNT).Select(i => Task.Run(Experiment)).ToArray();
Task.WaitAll(experiments);
var totalCountsThisRun = experiments.SelectMany(e => e.Result)
.GroupBy(e => e.Key)
.Select(e => new { e.Key, Count = e.Select(_ => _.Value).Sum() })
.ToDictionary(e => e.Key, e => e.Count);
foreach (var ch in KeyGenerator.chars)
grandTotalCounts[ch] += totalCountsThisRun[ch];
var totalChars = grandTotalCounts.Values.Sum();
totalRandomSymbols += totalChars;
var distributionScores = KeyGenerator.chars.Select(ch =>
new
{
Symbol = ch,
OverUnder = (100.0 * grandTotalCounts[ch] / totalChars) - expectedPercentage
});
Console.WriteLine($"Iteration {iterationNr++}. Total random symbols: {totalRandomSymbols:N0}");
foreach (var chWithValue in distributionScores.OrderByDescending(c => c.OverUnder))
{
Console.WriteLine($"{chWithValue.Symbol}: {chWithValue.OverUnder:#.00000}%");
}
done = Console.KeyAvailable;
}
Dictionary<char, long> Experiment()
{
var counts = new Dictionary<char, long>();
foreach (var ch in KeyGenerator.chars) counts.Add(ch, 0);
for (int i = 0; i < REPETITIONS; i++)
{
var key = KeyGenerator.GetUniqueKey(KEY_SIZE);
foreach (var ch in key) counts[ch]++;
}
return counts;
}
public class KeyGenerator
{
internal static readonly char[] chars =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
public static string GetUniqueKey(int size)
{
byte[] data = new byte[4 * size];
using (var crypto = RandomNumberGenerator.Create())
{
crypto.GetBytes(data);
}
StringBuilder result = new StringBuilder(size);
for (int i = 0; i < size; i++)
{
var rnd = BitConverter.ToUInt32(data, i * 4);
var idx = rnd % chars.Length;
result.Append(chars[idx]);
}
return result.ToString();
}
}

Solution 1 - largest 'range' with most flexible length
string get_unique_string(int string_length) {
using(var rng = new RNGCryptoServiceProvider()) {
var bit_count = (string_length * 6);
var byte_count = ((bit_count + 7) / 8); // rounded up
var bytes = new byte[byte_count];
rng.GetBytes(bytes);
return Convert.ToBase64String(bytes);
}
}
This solution has more range than using a GUID because a GUID has a couple of fixed bits that are always the same and therefore not random, for example the 13 character in hex is always "4" - at least in a version 6 GUID.
This solution also lets you generate a string of any length.
Solution 2 - One line of code - good for up to 22 characters
Convert.ToBase64String(Guid.NewGuid().ToByteArray()).Substring(0, 8);
You can't generate strings as long as Solution 1 and the string doesn't have the same range due to fixed bits in GUID's, but in a lot of cases this will do the job.
Solution 3 - Slightly less code
Guid.NewGuid().ToString("n").Substring(0, 8);
Mostly keeping this here for historical purpose. It uses slightly less code, that though comes as the expense of having less range - because it uses hex instead of base64 it takes more characters to represent the same range compared the other solutions.
Which means more chance of collision - testing it with 100,000 iterations of 8 character strings generated one duplicate.

Here's an example that I stole from Sam Allen example at Dot Net Perls
If you only need 8 characters, then use Path.GetRandomFileName() in the System.IO namespace. Sam says using the "Path.GetRandomFileName method here is sometimes superior, because it uses RNGCryptoServiceProvider for better randomness. However, it is limited to 11 random characters."
GetRandomFileName always returns a 12 character string with a period at the 9th character. So you'll need to strip the period (since that's not random) and then take 8 characters from the string. Actually, you could just take the first 8 characters and not worry about the period.
public string Get8CharacterRandomString()
{
string path = Path.GetRandomFileName();
path = path.Replace(".", ""); // Remove period.
return path.Substring(0, 8); // Return 8 character string
}
PS: thanks Sam

The main goals of my code are:
The distribution of strings is almost uniform (don't care about minor deviations, as long as they're small)
It outputs more than a few billion strings for each argument set. Generating an 8 character string (~47 bits of entropy) is meaningless if your PRNG only generates 2 billion (31 bits of entropy) different values.
It's secure, since I expect people to use this for passwords or other security tokens.
The first property is achieved by taking a 64 bit value modulo the alphabet size. For small alphabets (such as the 62 characters from the question) this leads to negligible bias. The second and third property are achieved by using RNGCryptoServiceProvider instead of System.Random.
using System;
using System.Security.Cryptography;
public static string GetRandomAlphanumericString(int length)
{
const string alphanumericCharacters =
"ABCDEFGHIJKLMNOPQRSTUVWXYZ" +
"abcdefghijklmnopqrstuvwxyz" +
"0123456789";
return GetRandomString(length, alphanumericCharacters);
}
public static string GetRandomString(int length, IEnumerable<char> characterSet)
{
if (length < 0)
throw new ArgumentException("length must not be negative", "length");
if (length > int.MaxValue / 8) // 250 million chars ought to be enough for anybody
throw new ArgumentException("length is too big", "length");
if (characterSet == null)
throw new ArgumentNullException("characterSet");
var characterArray = characterSet.Distinct().ToArray();
if (characterArray.Length == 0)
throw new ArgumentException("characterSet must not be empty", "characterSet");
var bytes = new byte[length * 8];
var result = new char[length];
using (var cryptoProvider = new RNGCryptoServiceProvider())
{
cryptoProvider.GetBytes(bytes);
}
for (int i = 0; i < length; i++)
{
ulong value = BitConverter.ToUInt64(bytes, i * 8);
result[i] = characterArray[value % (uint)characterArray.Length];
}
return new string(result);
}

The simplest:
public static string GetRandomAlphaNumeric()
{
return Path.GetRandomFileName().Replace(".", "").Substring(0, 8);
}
You can get better performance if you hard code the char array and rely on System.Random:
public static string GetRandomAlphaNumeric()
{
var chars = "abcdefghijklmnopqrstuvwxyz0123456789";
return new string(chars.Select(c => chars[random.Next(chars.Length)]).Take(8).ToArray());
}
If ever you worry the English alphabets can change sometime around and you might lose business, then you can avoid hard coding, but should perform slightly worse (comparable to Path.GetRandomFileName approach)
public static string GetRandomAlphaNumeric()
{
var chars = 'a'.To('z').Concat('0'.To('9')).ToList();
return new string(chars.Select(c => chars[random.Next(chars.Length)]).Take(8).ToArray());
}
public static IEnumerable<char> To(this char start, char end)
{
if (end < start)
throw new ArgumentOutOfRangeException("the end char should not be less than start char", innerException: null);
return Enumerable.Range(start, end - start + 1).Select(i => (char)i);
}
The last two approaches looks better if you can make them an extension method on System.Random instance.

Just some performance comparisons of the various answers in this thread:
Methods & Setup
// what's available
public static string possibleChars = "abcdefghijklmnopqrstuvwxyz";
// optimized (?) what's available
public static char[] possibleCharsArray = possibleChars.ToCharArray();
// optimized (precalculated) count
public static int possibleCharsAvailable = possibleChars.Length;
// shared randomization thingy
public static Random random = new Random();
// http://stackoverflow.com/a/1344242/1037948
public string LinqIsTheNewBlack(int num) {
return new string(
Enumerable.Repeat(possibleCharsArray, num)
.Select(s => s[random.Next(s.Length)])
.ToArray());
}
// http://stackoverflow.com/a/1344258/1037948
public string ForLoop(int num) {
var result = new char[num];
while(num-- > 0) {
result[num] = possibleCharsArray[random.Next(possibleCharsAvailable)];
}
return new string(result);
}
public string ForLoopNonOptimized(int num) {
var result = new char[num];
while(num-- > 0) {
result[num] = possibleChars[random.Next(possibleChars.Length)];
}
return new string(result);
}
public string Repeat(int num) {
return new string(new char[num].Select(o => possibleCharsArray[random.Next(possibleCharsAvailable)]).ToArray());
}
// http://stackoverflow.com/a/1518495/1037948
public string GenerateRandomString(int num) {
var rBytes = new byte[num];
random.NextBytes(rBytes);
var rName = new char[num];
while(num-- > 0)
rName[num] = possibleCharsArray[rBytes[num] % possibleCharsAvailable];
return new string(rName);
}
//SecureFastRandom - or SolidSwiftRandom
static string GenerateRandomString(int Length) //Configurable output string length
{
byte[] rBytes = new byte[Length];
char[] rName = new char[Length];
SolidSwiftRandom.GetNextBytesWithMax(rBytes, biasZone);
for (var i = 0; i < Length; i++)
{
rName[i] = charSet[rBytes[i] % charSet.Length];
}
return new string(rName);
}
Results
Tested in LinqPad. For string size of 10, generates:
from Linq = chdgmevhcy [10]
from Loop = gtnoaryhxr [10]
from Select = rsndbztyby [10]
from GenerateRandomString = owyefjjakj [10]
from SecureFastRandom = VzougLYHYP [10]
from SecureFastRandom-NoCache = oVQXNGmO1S [10]
And the performance numbers tend to vary slightly, very occasionally NonOptimized is actually faster, and sometimes ForLoop and GenerateRandomString switch who's in the lead.
LinqIsTheNewBlack (10000x) = 96762 ticks elapsed (9.6762 ms)
ForLoop (10000x) = 28970 ticks elapsed (2.897 ms)
ForLoopNonOptimized (10000x) = 33336 ticks elapsed (3.3336 ms)
Repeat (10000x) = 78547 ticks elapsed (7.8547 ms)
GenerateRandomString (10000x) = 27416 ticks elapsed (2.7416 ms)
SecureFastRandom (10000x) = 13176 ticks elapsed (5ms) lowest [Different machine]
SecureFastRandom-NoCache (10000x) = 39541 ticks elapsed (17ms) lowest [Different machine]

One line of code Membership.GeneratePassword() does the trick :)
Here is a demo for the same.

The code written by Eric J. is quite sloppy (it is quite clear that it is from 6 years ago... he probably wouldn't write that code today), and there are even some problems.
Unlike some of the alternatives presented, this one is cryptographically sound.
Untrue... There is a bias in the password (as written in a comment), bcdefgh are a little more probable than the others (the a isn't because by the GetNonZeroBytes it isn't generating bytes with a value of zero, so the bias for the a is balanced by it), so it isn't really cryptographically sound.
This should correct all the problems.
public static string GetUniqueKey(int size = 6, string chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890")
{
using (var crypto = new RNGCryptoServiceProvider())
{
var data = new byte[size];
// If chars.Length isn't a power of 2 then there is a bias if
// we simply use the modulus operator. The first characters of
// chars will be more probable than the last ones.
// buffer used if we encounter an unusable random byte. We will
// regenerate it in this buffer
byte[] smallBuffer = null;
// Maximum random number that can be used without introducing a
// bias
int maxRandom = byte.MaxValue - ((byte.MaxValue + 1) % chars.Length);
crypto.GetBytes(data);
var result = new char[size];
for (int i = 0; i < size; i++)
{
byte v = data[i];
while (v > maxRandom)
{
if (smallBuffer == null)
{
smallBuffer = new byte[1];
}
crypto.GetBytes(smallBuffer);
v = smallBuffer[0];
}
result[i] = chars[v % chars.Length];
}
return new string(result);
}
}

My simple one line code works for me :)
string random = string.Join("", Guid.NewGuid().ToString("n").Take(8).Select(o => o));
Response.Write(random.ToUpper());
Response.Write(random.ToLower());
To expand on this for any length string
public static string RandomString(int length)
{
//length = length < 0 ? length * -1 : length;
var str = "";
do
{
str += Guid.NewGuid().ToString().Replace("-", "");
}
while (length > str.Length);
return str.Substring(0, length);
}

We also use custom string random but we implemented is as a string's helper so it provides some flexibility...
public static string Random(this string chars, int length = 8)
{
var randomString = new StringBuilder();
var random = new Random();
for (int i = 0; i < length; i++)
randomString.Append(chars[random.Next(chars.Length)]);
return randomString.ToString();
}
Usage
var random = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".Random();
or
var random = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789".Random(16);

A slightly cleaner version of DTB's solution.
var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
var random = new Random();
var list = Enumerable.Repeat(0, 8).Select(x=>chars[random.Next(chars.Length)]);
return string.Join("", list);
Your style preferences may vary.

public static string RandomString(int length)
{
const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
var random = new Random();
return new string(Enumerable.Repeat(chars, length).Select(s => s[random.Next(s.Length)]).ToArray());
}

Another option could be to use Linq and aggregate random chars into a stringbuilder.
var chars = "abcdefghijklmnopqrstuvwxyz123456789".ToArray();
string pw = Enumerable.Range(0, passwordLength)
.Aggregate(
new StringBuilder(),
(sb, n) => sb.Append((chars[random.Next(chars.Length)])),
  sb => sb.ToString());

Question: Why should I waste my time using Enumerable.Range instead of typing in "ABCDEFGHJKLMNOPQRSTUVWXYZ0123456789"?
using System;
using System.Collections.Generic;
using System.Linq;
public class Test
{
public static void Main()
{
var randomCharacters = GetRandomCharacters(8, true);
Console.WriteLine(new string(randomCharacters.ToArray()));
}
private static List<char> getAvailableRandomCharacters(bool includeLowerCase)
{
var integers = Enumerable.Empty<int>();
integers = integers.Concat(Enumerable.Range('A', 26));
integers = integers.Concat(Enumerable.Range('0', 10));
if ( includeLowerCase )
integers = integers.Concat(Enumerable.Range('a', 26));
return integers.Select(i => (char)i).ToList();
}
public static IEnumerable<char> GetRandomCharacters(int count, bool includeLowerCase)
{
var characters = getAvailableRandomCharacters(includeLowerCase);
var random = new Random();
var result = Enumerable.Range(0, count)
.Select(_ => characters[random.Next(characters.Count)]);
return result;
}
}
Answer: Magic strings are BAD. Did ANYONE notice there was no "I" in my string at the top? My mother taught me not to use magic strings for this very reason...
n.b. 1: As many others like #dtb said, don't use System.Random if you need cryptographic security...
n.b. 2: This answer isn't the most efficient or shortest, but I wanted the space to separate the answer from the question. The purpose of my answer is more to warn against magic strings than to provide a fancy innovative answer.

After reviewing the other answers and considering CodeInChaos' comments, along with CodeInChaos still biased (although less) answer, I thought a final ultimate cut and paste solution was needed. So while updating my answer I decided to go all out.
For an up to date version of this code, please visit the new Hg repository on Bitbucket: https://bitbucket.org/merarischroeder/secureswiftrandom. I recommend you copy and paste the code from: https://bitbucket.org/merarischroeder/secureswiftrandom/src/6c14b874f34a3f6576b0213379ecdf0ffc7496ea/Code/Alivate.SolidSwiftRandom/SolidSwiftRandom.cs?at=default&fileviewer=file-view-default (make sure you click the Raw button to make it easier to copy and make sure you have the latest version, I think this link goes to a specific version of the code, not the latest).
Updated notes:
Relating to some other answers - If you know the length of the output, you don't need a StringBuilder, and when using ToCharArray, this creates and fills the array (you don't need to create an empty array first)
Relating to some other answers - You should use NextBytes, rather than getting one at a time for performance
Technically you could pin the byte array for faster access.. it's usually worth it when your iterating more than 6-8 times over a byte array. (Not done here)
Use of RNGCryptoServiceProvider for best randomness
Use of caching of a 1MB buffer of random data - benchmarking shows cached single bytes access speed is ~1000x faster - taking 9ms over 1MB vs 989ms for uncached.
Optimised rejection of bias zone within my new class.
End solution to question:
static char[] charSet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".ToCharArray();
static int byteSize = 256; //Labelling convenience
static int biasZone = byteSize - (byteSize % charSet.Length);
public string GenerateRandomString(int Length) //Configurable output string length
{
byte[] rBytes = new byte[Length]; //Do as much before and after lock as possible
char[] rName = new char[Length];
SecureFastRandom.GetNextBytesMax(rBytes, biasZone);
for (var i = 0; i < Length; i++)
{
rName[i] = charSet[rBytes[i] % charSet.Length];
}
return new string(rName);
}
But you need my new (untested) class:
/// <summary>
/// My benchmarking showed that for RNGCryptoServiceProvider:
/// 1. There is negligable benefit of sharing RNGCryptoServiceProvider object reference
/// 2. Initial GetBytes takes 2ms, and an initial read of 1MB takes 3ms (starting to rise, but still negligable)
/// 2. Cached is ~1000x faster for single byte at a time - taking 9ms over 1MB vs 989ms for uncached
/// </summary>
class SecureFastRandom
{
static byte[] byteCache = new byte[1000000]; //My benchmark showed that an initial read takes 2ms, and an initial read of this size takes 3ms (starting to raise)
static int lastPosition = 0;
static int remaining = 0;
/// <summary>
/// Static direct uncached access to the RNGCryptoServiceProvider GetBytes function
/// </summary>
/// <param name="buffer"></param>
public static void DirectGetBytes(byte[] buffer)
{
using (var r = new RNGCryptoServiceProvider())
{
r.GetBytes(buffer);
}
}
/// <summary>
/// Main expected method to be called by user. Underlying random data is cached from RNGCryptoServiceProvider for best performance
/// </summary>
/// <param name="buffer"></param>
public static void GetBytes(byte[] buffer)
{
if (buffer.Length > byteCache.Length)
{
DirectGetBytes(buffer);
return;
}
lock (byteCache)
{
if (buffer.Length > remaining)
{
DirectGetBytes(byteCache);
lastPosition = 0;
remaining = byteCache.Length;
}
Buffer.BlockCopy(byteCache, lastPosition, buffer, 0, buffer.Length);
lastPosition += buffer.Length;
remaining -= buffer.Length;
}
}
/// <summary>
/// Return a single byte from the cache of random data.
/// </summary>
/// <returns></returns>
public static byte GetByte()
{
lock (byteCache)
{
return UnsafeGetByte();
}
}
/// <summary>
/// Shared with public GetByte and GetBytesWithMax, and not locked to reduce lock/unlocking in loops. Must be called within lock of byteCache.
/// </summary>
/// <returns></returns>
static byte UnsafeGetByte()
{
if (1 > remaining)
{
DirectGetBytes(byteCache);
lastPosition = 0;
remaining = byteCache.Length;
}
lastPosition++;
remaining--;
return byteCache[lastPosition - 1];
}
/// <summary>
/// Rejects bytes which are equal to or greater than max. This is useful for ensuring there is no bias when you are modulating with a non power of 2 number.
/// </summary>
/// <param name="buffer"></param>
/// <param name="max"></param>
public static void GetBytesWithMax(byte[] buffer, byte max)
{
if (buffer.Length > byteCache.Length / 2) //No point caching for larger sizes
{
DirectGetBytes(buffer);
lock (byteCache)
{
UnsafeCheckBytesMax(buffer, max);
}
}
else
{
lock (byteCache)
{
if (buffer.Length > remaining) //Recache if not enough remaining, discarding remaining - too much work to join two blocks
DirectGetBytes(byteCache);
Buffer.BlockCopy(byteCache, lastPosition, buffer, 0, buffer.Length);
lastPosition += buffer.Length;
remaining -= buffer.Length;
UnsafeCheckBytesMax(buffer, max);
}
}
}
/// <summary>
/// Checks buffer for bytes equal and above max. Must be called within lock of byteCache.
/// </summary>
/// <param name="buffer"></param>
/// <param name="max"></param>
static void UnsafeCheckBytesMax(byte[] buffer, byte max)
{
for (int i = 0; i < buffer.Length; i++)
{
while (buffer[i] >= max)
buffer[i] = UnsafeGetByte(); //Replace all bytes which are equal or above max
}
}
}
For history - my older solution for this answer, used Random object:
private static char[] charSet =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".ToCharArray();
static rGen = new Random(); //Must share, because the clock seed only has Ticks (~10ms) resolution, yet lock has only 20-50ns delay.
static int byteSize = 256; //Labelling convenience
static int biasZone = byteSize - (byteSize % charSet.Length);
static bool SlightlyMoreSecurityNeeded = true; //Configuration - needs to be true, if more security is desired and if charSet.Length is not divisible by 2^X.
public string GenerateRandomString(int Length) //Configurable output string length
{
byte[] rBytes = new byte[Length]; //Do as much before and after lock as possible
char[] rName = new char[Length];
lock (rGen) //~20-50ns
{
rGen.NextBytes(rBytes);
for (int i = 0; i < Length; i++)
{
while (SlightlyMoreSecurityNeeded && rBytes[i] >= biasZone) //Secure against 1/5 increased bias of index[0-7] values against others. Note: Must exclude where it == biasZone (that is >=), otherwise there's still a bias on index 0.
rBytes[i] = rGen.NextByte();
rName[i] = charSet[rBytes[i] % charSet.Length];
}
}
return new string(rName);
}
Performance:
SecureFastRandom - First single run = ~9-33ms. Imperceptible. Ongoing: 5ms (sometimes it goes up to 13ms) over 10,000 iterations, With a single average iteration= 1.5 microseconds.. Note: Requires generally 2, but occasionally up to 8 cache refreshes - depends on how many single bytes exceed the bias zone
Random - First single run = ~0-1ms. Imperceptible. Ongoing: 5ms over 10,000 iterations. With a single average iteration= .5 microseconds.. About the same speed.
Also check out:
https://bitbucket.org/merarischroeder/number-range-with-no-bias/src
https://stackoverflow.com/a/45118325/887092
These links are another approach. Buffering could be added to this new code base, but most important was exploring different approaches to removing bias, and benchmarking the speeds and pros/cons.

Simplest and most flexible solution with cryptographic security (.NET Core 3.0+):
If you're using .NET Core 3.0 or above, you can use the new static GetInt32 method (which is cryptographically secure) on the RandomNumberGenerator class to generate a random index for a given character set and populate the result that way quite easily.
This approach is far simpler than, for instance, the one proposed in this answer; and it also provides complete flexibility since you can pass in any character set you want.
public static string GenerateRandomString(int length, IEnumerable<char> charSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")
{
var charArray = charSet.Distinct().ToArray();
char[] result = new char[length];
for (int i = 0; i < length; i++)
result[i] = charArray[RandomNumberGenerator.GetInt32(charArray.Length)];
return new string(result);
}
Usage:
string randomAlphanumericString = GenerateRandomString(length: 10);

A simple and highly secure way might be generating the cryptography Aes key.
public static string GenerateRandomString()
{
using Aes crypto = Aes.Create();
crypto.GenerateKey();
return Convert.ToBase64String(crypto.Key);
}

I was looking for a more specific answer, where I want to control the format of the random string and came across this post. For example: license plates (of cars) have a specific format (per country) and I wanted to created random license plates.
I decided to write my own extension method of Random for this. (this is in order to reuse the same Random object, as you could have doubles in multi-threading scenarios).
I created a gist (https://gist.github.com/SamVanhoutte/808845ca78b9c041e928), but will also copy the extension class here:
void Main()
{
Random rnd = new Random();
rnd.GetString("1-###-000").Dump();
}
public static class RandomExtensions
{
public static string GetString(this Random random, string format)
{
// Based on http://stackoverflow.com/questions/1344221/how-can-i-generate-random-alphanumeric-strings-in-c
// Added logic to specify the format of the random string (# will be random string, 0 will be random numeric, other characters remain)
StringBuilder result = new StringBuilder();
for(int formatIndex = 0; formatIndex < format.Length ; formatIndex++)
{
switch(format.ToUpper()[formatIndex])
{
case '0': result.Append(getRandomNumeric(random)); break;
case '#': result.Append(getRandomCharacter(random)); break;
default : result.Append(format[formatIndex]); break;
}
}
return result.ToString();
}
private static char getRandomCharacter(Random random)
{
string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
return chars[random.Next(chars.Length)];
}
private static char getRandomNumeric(Random random)
{
string nums = "0123456789";
return nums[random.Next(nums.Length)];
}
}

Now in one-liner flavour.
private string RandomName()
{
return new string(
Enumerable.Repeat("ABCDEFGHIJKLMNOPQRSTUVWXYZ", 13)
.Select(s =>
{
var cryptoResult = new byte[4];
using (var cryptoProvider = new RNGCryptoServiceProvider())
cryptoProvider.GetBytes(cryptoResult);
return s[new Random(BitConverter.ToInt32(cryptoResult, 0)).Next(s.Length)];
})
.ToArray());
}

Try to combine two parts: unique (sequence, counter or date ) and random
public class RandomStringGenerator
{
public static string Gen()
{
return ConvertToBase(DateTime.UtcNow.ToFileTimeUtc()) + GenRandomStrings(5); //keep length fixed at least of one part
}
private static string GenRandomStrings(int strLen)
{
var result = string.Empty;
using (var gen = new RNGCryptoServiceProvider())
{
var data = new byte[1];
while (result.Length < strLen)
{
gen.GetNonZeroBytes(data);
int code = data[0];
if (code > 48 && code < 57 || // 0-9
code > 65 && code < 90 || // A-Z
code > 97 && code < 122 // a-z
)
{
result += Convert.ToChar(code);
}
}
return result;
}
}
private static string ConvertToBase(long num, int nbase = 36)
{
const string chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; //if you wish to make the algorithm more secure - change order of letter here
// check if we can convert to another base
if (nbase < 2 || nbase > chars.Length)
return null;
int r;
var newNumber = string.Empty;
// in r we have the offset of the char that was converted to the new base
while (num >= nbase)
{
r = (int)(num % nbase);
newNumber = chars[r] + newNumber;
num = num / nbase;
}
// the last number to convert
newNumber = chars[(int)num] + newNumber;
return newNumber;
}
}
Tests:
[Test]
public void Generator_Should_BeUnigue1()
{
//Given
var loop = Enumerable.Range(0, 1000);
//When
var str = loop.Select(x=> RandomStringGenerator.Gen());
//Then
var distinct = str.Distinct();
Assert.AreEqual(loop.Count(),distinct.Count()); // Or Assert.IsTrue(distinct.Count() < 0.95 * loop.Count())
}

For both crypto & noncrypto, efficiently:
public static string GenerateRandomString(int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890") =>
new Random().GenerateRandomString(length, charset);
public static string GenerateRandomString(this Random random, int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890") =>
RandomString(random.NextBytes, length, charset.ToCharArray());
public static string GenerateRandomCryptoString(int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890")
{
using (var crypto = new System.Security.Cryptography.RNGCryptoServiceProvider())
return crypto.GenerateRandomCryptoString(length, charset);
}
public static string GenerateRandomCryptoString(this RNGCryptoServiceProvider random, int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890") =>
RandomString(random.GetBytes, length, charset.ToCharArray());
private static string RandomString(Action<byte[]> fillRandomBuffer, int length, char[] charset)
{
if (length < 0)
throw new ArgumentOutOfRangeException(nameof(length), $"{nameof(length)} must be greater or equal to 0");
if (charset is null)
throw new ArgumentNullException(nameof(charset));
if (charset.Length == 0)
throw new ArgumentException($"{nameof(charset)} must contain at least 1 character", nameof(charset));
var maxIdx = charset.Length;
var chars = new char[length];
var randomBuffer = new byte[length * 4];
fillRandomBuffer(randomBuffer);
for (var i = 0; i < length; i++)
chars[i] = charset[BitConverter.ToUInt32(randomBuffer, i * 4) % maxIdx];
return new string(chars);
}
Using generators & LINQ. Not the fastest option (especially because it doesn't generate all the bytes in one go) but pretty neat & extensible:
private static readonly Random _random = new Random();
public static string GenerateRandomString(int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890") =>
new string(_random.GetGenerator().RandomChars(charset.ToCharArray()).Take(length).ToArray());
public static string GenerateRandomCryptoString(int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890")
{
using (var crypto = new System.Security.Cryptography.RNGCryptoServiceProvider())
return new string(crypto.GetGenerator().RandomChars(charset.ToCharArray()).Take(length).ToArray());
}
public static IEnumerable<char> RandomChars(this Func<uint, IEnumerable<uint>> randomGenerator, char[] charset)
{
if (charset is null)
throw new ArgumentNullException(nameof(charset));
if (charset.Length == 0)
throw new ArgumentException($"{nameof(charset)} must contain at least 1 character", nameof(charset));
return randomGenerator((uint)charset.Length).Select(r => charset[r]);
}
public static Func<uint, IEnumerable<uint>> GetGenerator(this Random random)
{
if (random is null)
throw new ArgumentNullException(nameof(random));
return GeneratorFunc_Inner;
IEnumerable<uint> GeneratorFunc_Inner(uint maxValue)
{
if (maxValue > int.MaxValue)
throw new ArgumentOutOfRangeException(nameof(maxValue));
return Generator_Inner();
IEnumerable<uint> Generator_Inner()
{
var randomBytes = new byte[4];
while (true)
{
random.NextBytes(randomBytes);
yield return BitConverter.ToUInt32(randomBytes, 0) % maxValue;
}
}
}
}
public static Func<uint, IEnumerable<uint>> GetGenerator(this System.Security.Cryptography.RNGCryptoServiceProvider random)
{
if (random is null)
throw new ArgumentNullException(nameof(random));
return Generator_Inner;
IEnumerable<uint> Generator_Inner(uint maxValue)
{
var randomBytes = new byte[4];
while (true)
{
random.GetBytes(randomBytes);
yield return BitConverter.ToUInt32(randomBytes, 0) % maxValue;
}
}
}
a simpler version using LINQ for only non-crypto strings:
private static readonly Random _random = new Random();
public static string RandomString(int length, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890") =>
new string(_random.GenerateChars(charset).Take(length).ToArray());
public static IEnumerable<char> GenerateChars(this Random random, string charset)
{
if (charset is null) throw new ArgumentNullException(nameof(charset));
if (charset.Length == 0) throw new ArgumentException($"{nameof(charset)} must contain at least 1 character", nameof(charset));
return random.Generator(charset.Length).Select(r => charset[r]);
}
public static IEnumerable<int> Generator(this Random random, int maxValue)
{
if (random is null) throw new ArgumentNullException(nameof(random));
return Generator_Inner();
IEnumerable<int> Generator_Inner() { while (true) yield return random.Next(maxValue); }
}

Horrible, I know, but I just couldn't help myself:
namespace ConsoleApplication2
{
using System;
using System.Text.RegularExpressions;
class Program
{
static void Main(string[] args)
{
Random adomRng = new Random();
string rndString = string.Empty;
char c;
for (int i = 0; i < 8; i++)
{
while (!Regex.IsMatch((c=Convert.ToChar(adomRng.Next(48,128))).ToString(), "[A-Za-z0-9]"));
rndString += c;
}
Console.WriteLine(rndString + Environment.NewLine);
}
}
}

A solution that includes all alphabet characters and numbers which you can change as you wish:
public static string RandomString(int length)
{
Random rand = new Random();
string charbase = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
return new string(Enumerable.Range(0,length)
.Select(_ => charbase[rand.Next(charbase.Length)])
.ToArray());
}
If you like one-liner methods ;)
public static Random rand = new Random();
public const string charbase = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
public static string RandomString(int length) =>
new string(Enumerable.Range(0,length).Select(_ => charbase[rand.Next(charbase.Length)]).ToArray());

A solution without using Random :
var chars = Enumerable.Repeat("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789", 8);
var randomStr = new string(chars.SelectMany(str => str)
.OrderBy(c => Guid.NewGuid())
.Take(8).ToArray());

Here is a variant of Eric J's solution, i.e. cryptographically sound, for WinRT (Windows Store App):
public static string GenerateRandomString(int length)
{
var chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
var result = new StringBuilder(length);
for (int i = 0; i < length; ++i)
{
result.Append(CryptographicBuffer.GenerateRandomNumber() % chars.Length);
}
return result.ToString();
}
If performance matters (especially when length is high):
public static string GenerateRandomString(int length)
{
var chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
var result = new System.Text.StringBuilder(length);
var bytes = CryptographicBuffer.GenerateRandom((uint)length * 4).ToArray();
for (int i = 0; i < bytes.Length; i += 4)
{
result.Append(BitConverter.ToUInt32(bytes, i) % chars.Length);
}
return result.ToString();
}

I know this one is not the best way. But you can try this.
string str = Path.GetRandomFileName(); //This method returns a random file name of 11 characters
str = str.Replace(".","");
Console.WriteLine("Random string: " + str);

I don't know how cryptographically sound this is, but it's more readable and concise than the more intricate solutions by far (imo), and it should be more "random" than System.Random-based solutions.
return alphabet
.OrderBy(c => Guid.NewGuid())
.Take(strLength)
.Aggregate(
new StringBuilder(),
(builder, c) => builder.Append(c))
.ToString();
I can't decide if I think this version or the next one is "prettier", but they give the exact same results:
return new string(alphabet
.OrderBy(o => Guid.NewGuid())
.Take(strLength)
.ToArray());
Granted, it isn't optimized for speed, so if it's mission critical to generate millions of random strings every second, try another one!
NOTE: This solution doesn't allow for repetitions of symbols in the alphabet, and the alphabet MUST be of equal or greater size than the output string, making this approach less desirable in some circumstances, it all depends on your use-case.

If your values are not completely random, but in fact may depend on something - you may compute an md5 or sha1 hash of that 'somwthing' and then truncate it to whatever length you want.
Also you may generate and truncate a guid.

Generate random string [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
c# random string generator
I need to generate a random string with a given length. This is my code so far. The problem is the random string is like "RRRRRR" or "SSSSS" The same letter every time. Just when i restart the app the letter change. I need something like "asrDvgDgREGd"
public string GenerateChar()
{
Random random = new Random();
return Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65))).ToString();
}
public string GenerateChar(int count)
{
string randomString = "";
for (int i = 0; i < count; i++)
{
nahodneZnaky += GenerateChar();
}
return randomString;
}

Try using the same random object for the whole string, rather than initializing one for each char.
The random object will generate a "pseudo-random" number, based on mathematical progressions starting from a "seed" number. You can actually get the same sequence of "random" numbers if you initialize the Random object to the same seed every time.
Now, when you initialize Random without specifying a seed, it'll take the computer's clock as seed. In this case, you're probably doing it fast enough that the clock hasn't changed from one initialization to another, and you get always get the same seed.
You're better off initializing the Random object in your function that generates the random string, and passing it as a parameter to the GenerateChar function, so that you're calling NextDouble() several times on the same instance of the Random object, instead of only once on different instances of it.

Don't create a new instance of Random on each iteration - that's going to seed each instance with the current time in milliseconds, which obviously isn't likely to change between iterations.
Create a single instance of Random, and pass it into the method. I'd also advise you not to concatenate strings like that in a loop, or even to create that many strings. Additionally, if you use Random.Next(int, int) to make your life a lot easier.
Try this:
public char GenerateChar(Random rng)
{
// 'Z' + 1 because the range is exclusive
return (char) (rng.Next('A', 'Z' + 1));
}
public string GenerateString(Random rng, int length)
{
char[] letters = new char[length];
for (int i = 0; i < length; i++)
{
letters[i] = GenerateChar(rng);
}
return new string(letters);
}
private static readonly Random SingleRandom = new Random();
public string GenerateStringNotThreadSafe(int length)
{
return GenerateString(SingleRandom, length);
}
Now it's worth being aware that Random isn't thread-safe, so if you've got multiple threads you shouldn't just have a single instance of Random in a static variable without locking. There are various ways around this - either create a subclass of Random which is thread-safe, or a set of static methods which do the same thing, or use thread-local variables to have one instance per thread.
I have a StaticRandom class as part of MiscUtil but these days I'm leaning towards the thread-local version and passing it down the chain where appropriate. One day I'll add that as another option to MiscUtil...

You could save yourself the hassle and use Membership.GeneratePassword method. It is essentially doing what you require.
Usage:
int passwordLength = 5;
int alphaNumericalCharsAllowed = 2;
string random = Membership.GeneratePassword(passwordLength, alphaNumericalCharsAllowed);
For readability aswell you could wrap this in a helper method:
private string GenerateRandomString(int length, int alphaNumericalChars)
{
return Membership.GeneratePassword(length, alphaNumericalChars);
}

Nothing special, but short. 32 and 127 are min and max range of chars you want to be generated.
public static string GetRandomString(int length)
{
var r = new Random();
return new String(Enumerable.Range(0, length).Select(n => (Char)(r.Next(32, 127))).ToArray());
}

I know this might not be the best solution, but I kind of like the idea to be able to specify "allowed" characters:
private const int _defaultNumberOfCharacters = 8;
private const int _defaultExpireDays = 10;
private static readonly string _allowedCharacters = "bcdfghjklmnpqrstvxz0123456789";
public static string GenerateKey(int numberOfCharacters)
{
const int from = 0;
int to = _allowedCharacters.Length;
Random r = new Random();
StringBuilder qs = new StringBuilder();
for (int i = 0; i < numberOfCharacters; i++)
{
qs.Append(_allowedCharacters.Substring(r.Next(from, to), 1));
}
return qs.ToString();
}

Maybe you can find the Path.GetRandomFileName method useful, depending on the exact characters you need in the string.

Try static
public string GenerateChar()
{
static Random random = new Random();
return Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65))).ToString();
}

http://msdn.microsoft.com/en-us/library/system.io.path.getrandomfilename.aspx
string randomName = Path.GetRandomFileName();
randomName = randomName.Replace(".", string.Empty);
// take substring...

Pass random from the calling method and initialise it once - you are reseeding the Random generator with the same seed each time....

You should initialize your random variable outside the method:
public Random random = new Random();
public string GenerateChar()
{
return Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65))).ToString();
}
...

Try this out.
public string GetRandomString()
{
int randomNumber = new Random().Next();
string RandomString = string.Empty;
for(int i = 0;i < randomNumber.ToString().Length;i++)
{
RandomString += ((char)Convert.ToUInt32(randomNumber.ToString()[i])).ToString();
}
return RandomString;
}

We Keep Coding

C# (C-Sharp) is a programming language developed by Microsoft that runs on the .NET Framework.

generating string from exited characters - c#

(C# example) Random rand = new Random(); char[] arr = new char[rand.Next(7,15)]; for(int i = 0 ; i < arr.Length ; i++) { arr[i] = pwdCharArray[rand.Next(pwdCharArray.Length)]; } string pwd = new string(arr);

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