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Replace Multiple Characters in string

Hi all a have a program working but return string like ")Hi("
it should be "(Hi)" so i need to replace '(' with ')' and replace
')' with '('
it sounds easy
s.Replace('(',')').Replace(')','(')
the trick is that after the first replace the string change from
")Hi(" to "(Hi(" after the second the replace will change both characters back
the final will become ")Hi)"
Help Please

You could also use a regex replacement.
s = System.Text.RegularExpressions.Regex.Replace(s, #"[)](\w+)[(]", "($1)");

You cannot use Replace because it works its replacement operation on the whole string, not char by char.
A simple brute force solution could be this one
void Main()
{
// A dictionary where every key points to its replacement char
Dictionary<char, char> replacements = new Dictionary<char, char>()
{
{'(', ')'},
{')', '('},
};
string source = ")Hi(";
StringBuilder sb = new StringBuilder();
foreach (char c in source)
{
char replacement = c;
if(replacements.ContainsKey(c))
replacement = replacements[c];
sb.Append(replacement,1);
}
Console.WriteLine(sb.ToString());
}
You can transform this in an extension method adding to a static class
public static class StringExtensions
{
public static string ProgressiveReplace(this string source, Dictionary<char, char> replacements)
{
StringBuilder sb = new StringBuilder();
foreach (char c in source)
{
char replacement = c;
if (replacements.ContainsKey(c))
replacement = replacements[c];
sb.Append(replacement, 1);
}
return sb.ToString();
}
}
and call it from your code with
Dictionary<char, char> replacements = new Dictionary<char, char>()
{{'(', ')'},{')', '('}};
s = s.ProgressiveReplace(replacements);

var s = ")Hi(";
var sb = new StringBuilder();
foreach (var c in s)
if (c == ')')
sb.Append('(');
else if (c == '(')
sb.Append(')');
else
sb.Append(c);
s = sb.ToString();

Method 1)
Use the following:
var requiredString = string.Join(string.Empty, str.Select(x=>{if (x == '(') x = ')'; else if (x == ')') x = '('; return x;}));
Method 2)
Or you can Use following Extension Method:
public static class StringExtensions
{
public static string ReplaceMultiple(this string source, Dictionary<char, char> replacements)
{
return string.Join(string.Empty , source.Select(x=>Replace(x,replacements)));
}
private static char Replace(char arg, Dictionary<char, char> replacements)
{
if(replacements.ContainsKey(arg))
arg = replacements[arg];
return arg;
}
}
This method can be used as follows:
var rep = new Dictionary<char, char>
{
{ ')', '(' },
{ '(', ')' },
// { '#', '*' },
// { '*', '#' }
};
var c = str.ReplaceMultiple(rep);

Regex.Replace lets you process each Match (needs using System.Text.RegularExpressions;) :
string result = Regex.Replace(")Hi(", #"\(|\)", m => m.Value == "(" ? ")" : "(");
Alternative can be replacing one of the characters with something else:
string result = ")Hi(".Replace('(', '\0').Replace(')', '(').Replace('\0', ')');

Convert String To camelCase from TitleCase C#

I have a string that I converted to a TextInfo.ToTitleCase and removed the underscores and joined the string together. Now I need to change the first and only the first character in the string to lower case and for some reason, I can not figure out how to accomplish it.
class Program
{
static void Main(string[] args)
{
string functionName = "zebulans_nightmare";
TextInfo txtInfo = new CultureInfo("en-us", false).TextInfo;
functionName = txtInfo.ToTitleCase(functionName).Replace('_', ' ').Replace(" ", String.Empty);
Console.Out.WriteLine(functionName);
Console.ReadLine();
}
}
Results: ZebulansNightmare
Desired Results: zebulansNightmare
UPDATE:
class Program
{
static void Main(string[] args)
{
string functionName = "zebulans_nightmare";
TextInfo txtInfo = new CultureInfo("en-us", false).TextInfo;
functionName = txtInfo.ToTitleCase(functionName).Replace("_", string.Empty).Replace(" ", string.Empty);
functionName = $"{functionName.First().ToString().ToLowerInvariant()}{functionName.Substring(1)}";
Console.Out.WriteLine(functionName);
Console.ReadLine();
}
}
Produces the desired output.

You just need to lower the first char in the array. See this answer
Char.ToLowerInvariant(name[0]) + name.Substring(1)
As a side note, seeing as you are removing spaces you can replace the underscore with an empty string.
.Replace("_", string.Empty)

If you're using .NET Core 3 or .NET 5, you can call:
System.Text.Json.JsonNamingPolicy.CamelCase.ConvertName(someString)
Then you'll definitely get the same results as ASP.NET's own JSON serializer.

Implemented Bronumski's answer in an extension method (without replacing underscores).
public static class StringExtension
{
public static string ToCamelCase(this string str)
{
if(!string.IsNullOrEmpty(str) && str.Length > 1)
{
return char.ToLowerInvariant(str[0]) + str.Substring(1);
}
return str.ToLowerInvariant();
}
}
//Or
public static class StringExtension
{
public static string ToCamelCase(this string str) =>
string.IsNullOrEmpty(str) || str.Length < 2
? str.ToLowerInvariant()
: char.ToLowerInvariant(str[0]) + str.Substring(1);
}
and to use it:
string input = "ZebulansNightmare";
string output = input.ToCamelCase();

Here is my code, in case it is useful to anyone
// This converts to camel case
// Location_ID => locationId, and testLEFTSide => testLeftSide
static string CamelCase(string s)
{
var x = s.Replace("_", "");
if (x.Length == 0) return "null";
x = Regex.Replace(x, "([A-Z])([A-Z]+)($|[A-Z])",
m => m.Groups[1].Value + m.Groups[2].Value.ToLower() + m.Groups[3].Value);
return char.ToLower(x[0]) + x.Substring(1);
}
If you prefer Pascal-case use:
static string PascalCase(string s)
{
var x = CamelCase(s);
return char.ToUpper(x[0]) + x.Substring(1);
}

The following code works with acronyms as well. If it is the first word it converts the acronym to lower case (e.g., VATReturn to vatReturn), and otherwise leaves it as it is (e.g., ExcludedVAT to excludedVAT).
name = Regex.Replace(name, #"([A-Z])([A-Z]+|[a-z0-9_]+)($|[A-Z]\w*)",
m =>
{
return m.Groups[1].Value.ToLower() + m.Groups[2].Value.ToLower() + m.Groups[3].Value;
});

Example 01
public static string ToCamelCase(this string text)
{
return CultureInfo.CurrentCulture.TextInfo.ToTitleCase(text);
}
Example 02
public static string ToCamelCase(this string text)
{
return string.Join(" ", text
.Split()
.Select(i => char.ToUpper(i[0]) + i.Substring(1)));
}
Example 03
public static string ToCamelCase(this string text)
{
char[] a = text.ToLower().ToCharArray();
for (int i = 0; i < a.Count(); i++)
{
a[i] = i == 0 || a[i - 1] == ' ' ? char.ToUpper(a[i]) : a[i];
}
return new string(a);
}

Adapted from Leonardo's answer:
static string PascalCase(string str) {
TextInfo cultInfo = new CultureInfo("en-US", false).TextInfo;
str = Regex.Replace(str, "([A-Z]+)", " $1");
str = cultInfo.ToTitleCase(str);
str = str.Replace(" ", "");
return str;
}
Converts to PascalCase by first adding a space before any group of capitals, and then converting to title case before removing all the spaces.

Here's my code, includes lowering all upper prefixes:
public static class StringExtensions
{
public static string ToCamelCase(this string str)
{
bool hasValue = !string.IsNullOrEmpty(str);
// doesn't have a value or already a camelCased word
if (!hasValue || (hasValue && Char.IsLower(str[0])))
{
return str;
}
string finalStr = "";
int len = str.Length;
int idx = 0;
char nextChar = str[idx];
while (Char.IsUpper(nextChar))
{
finalStr += char.ToLowerInvariant(nextChar);
if (len - 1 == idx)
{
// end of string
break;
}
nextChar = str[++idx];
}
// if not end of string
if (idx != len - 1)
{
finalStr += str.Substring(idx);
}
return finalStr;
}
}
Use it like this:
string camelCasedDob = "DOB".ToCamelCase();

If you are Ok with the Newtonsoft.JSON dependency, the following string extension method will help. The advantage of this approach is the serialization will work on par with standard WebAPI model binding serialization with high accuracy.
public static class StringExtensions
{
private class CamelCasingHelper : CamelCaseNamingStrategy
{
private CamelCasingHelper(){}
private static CamelCasingHelper helper =new CamelCasingHelper();
public static string ToCamelCase(string stringToBeConverted)
{
return helper.ResolvePropertyName(stringToBeConverted);
}
}
public static string ToCamelCase(this string str)
{
return CamelCasingHelper.ToCamelCase(str);
}
}
Here is the working fiddle
https://dotnetfiddle.net/pug8pP

In .Net 6 and above
public static class CamelCaseExtension
{
public static string ToCamelCase(this string str) =>
char.ToLowerInvariant(str[0]) + str[1..];
}

public static string CamelCase(this string str)
{
TextInfo cultInfo = new CultureInfo("en-US", false).TextInfo;
str = cultInfo.ToTitleCase(str);
str = str.Replace(" ", "");
return str;
}
This should work using System.Globalization

var camelCaseFormatter = new JsonSerializerSettings();
camelCaseFormatter.ContractResolver = new CamelCasePropertyNamesContractResolver();
JsonConvert.SerializeObject(object, camelCaseFormatter));

Strings are immutable, but we can use unsafe code to make it mutable though.
The string.Copy insured that the original string stays as is.
In order for these codes to run you have to allow unsafe code in your project.
public static unsafe string ToCamelCase(this string value)
{
if (value == null || value.Length == 0)
{
return value;
}
string result = string.Copy(value);
fixed (char* chr = result)
{
char valueChar = *chr;
*chr = char.ToLowerInvariant(valueChar);
}
return result;
}
This version modifies the original string, instead of returning a modified copy. This will be annoying though and totally uncommon. So make sure the XML comments are warning users about that.
public static unsafe void ToCamelCase(this string value)
{
if (value == null || value.Length == 0)
{
return value;
}
fixed (char* chr = value)
{
char valueChar = *chr;
*chr = char.ToLowerInvariant(valueChar);
}
return value;
}
Why use unsafe code though? Short answer... It's super fast.

Here's my code which is pretty simple. My major objective was to ensure that camel-casing was compatible with what ASP.NET serializes objects to, which the above examples don't guarantee.
public static class StringExtensions
{
public static string ToCamelCase(this string name)
{
var sb = new StringBuilder();
var i = 0;
// While we encounter upper case characters (except for the last), convert to lowercase.
while (i < name.Length - 1 && char.IsUpper(name[i + 1]))
{
sb.Append(char.ToLowerInvariant(name[i]));
i++;
}
// Copy the rest of the characters as is, except if we're still on the first character - which is always lowercase.
while (i < name.Length)
{
sb.Append(i == 0 ? char.ToLowerInvariant(name[i]) : name[i]);
i++;
}
return sb.ToString();
}
}

/// <summary>
/// Gets the camel case from snake case.
/// </summary>
/// <param name="snakeCase">The snake case.</param>
/// <returns></returns>
private string GetCamelCaseFromSnakeCase(string snakeCase)
{
string camelCase = string.Empty;
if(!string.IsNullOrEmpty(snakeCase))
{
string[] words = snakeCase.Split('_');
foreach (var word in words)
{
camelCase = string.Concat(camelCase, Char.ToUpperInvariant(word[0]) + word.Substring(1));
}
// making first character small case
camelCase = Char.ToLowerInvariant(camelCase[0]) + camelCase.Substring(1);
}
return camelCase;
}

I use This method to convert the string with separated by "_" to Camel Case
public static string ToCamelCase(string? s)
{
var nameArr = s?.ToLower().Split("_");
var str = "";
foreach (var name in nameArr.Select((value, i) => new { value, i }))
{
if(name.i >= 1)
{
str += string.Concat(name.value[0].ToString().ToUpper(), name.value.AsSpan(1));
}
else
{
str += name.value ;
}
}
return str;
}
u can change the separated by "_" with any other you want.

Simple and easy in build c#
using System;
using System.Globalization;
public class SamplesTextInfo {
public static void Main() {
// Defines the string with mixed casing.
string myString = "wAr aNd pEaCe";
// Creates a TextInfo based on the "en-US" culture.
TextInfo myTI = new CultureInfo("en-US",false).TextInfo;
// Changes a string to lowercase.
Console.WriteLine( "\"{0}\" to lowercase: {1}", myString, myTI.ToLower( myString ) );
// Changes a string to uppercase.
Console.WriteLine( "\"{0}\" to uppercase: {1}", myString, myTI.ToUpper( myString ) );
// Changes a string to titlecase.
Console.WriteLine( "\"{0}\" to titlecase: {1}", myString, myTI.ToTitleCase( myString ) );
}
}
/*
This code produces the following output.
"wAr aNd pEaCe" to lowercase: war and peace
"wAr aNd pEaCe" to uppercase: WAR AND PEACE
"wAr aNd pEaCe" to titlecase: War And Peace
*/

public static class StringExtension
{
public static string ToCamelCase(this string str)
{
return string.Join(" ", str
.Split()
.Select(i => char.ToUpper(i[0]) + i.Substring(1).ToLower()));
}
}

I had the same issue with titleCase so I just created one, hope this helps this is an extension method.
public static string ToCamelCase(this string text)
{
if (string.IsNullOrEmpty(text))
return text;
var separators = new[] { '_', ' ' };
var arr = text
.Split(separators)
.Where(word => !string.IsNullOrWhiteSpace(word));
var camelCaseArr = arr
.Select((word, i) =>
{
if (i == 0)
return word.ToLower();
var characterArr = word.ToCharArray()
.Select((character, characterIndex) => characterIndex == 0
? character.ToString().ToUpper()
: character.ToString().ToLower());
return string.Join("", characterArr);
});
return string.Join("", camelCaseArr);
}

String formatting using C#

Is there a way to remove every special character from a string like:
"\r\n 1802 S St Nw<br>\r\n Washington, DC 20009"
And to just write it like:
"1802 S St Nw, Washington, DC 20009"

To remove special characters:
public static string ClearSpecialChars(this string input)
{
foreach (var ch in new[] { "\r", "\n", "<br>", etc })
{
input = input.Replace(ch, String.Empty);
}
return input;
}
To replace all double space with single space:
public static string ClearDoubleSpaces(this string input)
{
while (input.Contains(" ")) // double
{
input = input.Replace(" ", " "); // with single
}
return input;
}
You also may split both methods into a single one:
public static string Clear(this string input)
{
return input
.ClearSpecialChars()
.ClearDoubleSpaces()
.Trim();
}

two ways, you can use RegEx, or you can use String.Replace(...)

Use the Regex.Replace() method, specifying all of the characters you want to remove as the pattern to match.

You can use the C# Trim() method, look here:
http://msdn.microsoft.com/de-de/library/d4tt83f9%28VS.80%29.aspx

System.Text.RegularExpressions.Regex.Replace("\"\\r\\n 1802 S St Nw<br>\\r\\n Washington, DC 20009\"",
#"(<br>)*?\\r\\n\s+", "");

Maybe something like this, using ASCII int values. Assumes all html tags will be closed.
public static class StringExtensions
{
public static string Clean(this string str)
{
string[] split = str.Split(' ');
List<string> strings = new List<string>();
foreach (string splitStr in split)
{
if (splitStr.Length > 0)
{
StringBuilder sb = new StringBuilder();
bool tagOpened = false;
foreach (char c in splitStr)
{
int iC = (int)c;
if (iC > 32)
{
if (iC == 60)
tagOpened = true;
if (!tagOpened)
sb.Append(c);
if (iC == 62)
tagOpened = false;
}
}
string result = sb.ToString();
if (result.Length > 0)
strings.Add(result);
}
}
return string.Join(" ", strings.ToArray());
}
}

How to word by word iterate in string in C#?

I want to iterate over string as word by word.
If I have a string "incidentno and fintype or unitno", I would like to read every word one by one as "incidentno", "and", "fintype", "or", and "unitno".

foreach (string word in "incidentno and fintype or unitno".Split(' ')) {
...
}

var regex = new Regex(#"\b[\s,\.-:;]*");
var phrase = "incidentno and fintype or unitno";
var words = regex.Split(phrase).Where(x => !string.IsNullOrEmpty(x));
This works even if you have ".,; tabs and new lines" between your words.

Slightly twisted I know, but you could define an iterator block as an extension method on strings. e.g.
/// <summary>
/// Sweep over text
/// </summary>
/// <param name="Text"></param>
/// <returns></returns>
public static IEnumerable<string> WordList(this string Text)
{
int cIndex = 0;
int nIndex;
while ((nIndex = Text.IndexOf(' ', cIndex + 1)) != -1)
{
int sIndex = (cIndex == 0 ? 0 : cIndex + 1);
yield return Text.Substring(sIndex, nIndex - sIndex);
cIndex = nIndex;
}
yield return Text.Substring(cIndex + 1);
}
foreach (string word in "incidentno and fintype or unitno".WordList())
System.Console.WriteLine("'" + word + "'");
Which has the advantage of not creating a big array for long strings.

Use the Split method of the string class
string[] words = "incidentno and fintype or unitno".Split(" ");
This will split on spaces, so "words" will have [incidentno,and,fintype,or,unitno].

Assuming the words are always separated by a blank, you could use String.Split() to get an Array of your words.

There are multiple ways to accomplish this. Two of the most convenient methods (in my opinion) are:
Using string.Split() to create an array. I would probably use this method, because it is the most self-explanatory.
example:
string startingSentence = "incidentno and fintype or unitno";
string[] seperatedWords = startingSentence.Split(' ');
Alternatively, you could use (this is what I would use):
string[] seperatedWords = startingSentence.Split(new char[] {' '}, StringSplitOptions.RemoveEmptyEntries);
StringSplitOptions.RemoveEmptyEntries will remove any empty entries from your array that may occur due to extra whitespace and other minor problems.
Next - to process the words, you would use:
foreach (string word in seperatedWords)
{
//Do something
}
Or, you can use regular expressions to solve this problem, as Darin demonstrated (a copy is below).
example:
var regex = new Regex(#"\b[\s,\.-:;]*");
var phrase = "incidentno and fintype or unitno";
var words = regex.Split(phrase).Where(x => !string.IsNullOrEmpty(x));
For processing, you can use similar code to the first option.
foreach (string word in words)
{
//Do something
}
Of course, there are many ways to solve this problem, but I think that these two would be the simplest to implement and maintain. I would go with the first option (using string.Split()) just because regex can sometimes become quite confusing, while a split will function correctly most of the time.

When using split, what about checking for empty entries?
string sentence = "incidentno and fintype or unitno"
string[] words = sentence.Split(new char[] { ' ', ',' ,';','\t','\n', '\r'}, StringSplitOptions.RemoveEmptyEntries);
foreach (string word in words)
{
// Process
}
EDIT:
I can't comment so I'm posting here but this (posted above) works:
foreach (string word in "incidentno and fintype or unitno".Split(' '))
{
...
}
My understanding of foreach is that it first does a GetEnumerator() and the calles .MoveNext until false is returned. So the .Split won't be re-evaluated on each iteration

public static string[] MyTest(string inword, string regstr)
{
var regex = new Regex(regstr);
var phrase = "incidentno and fintype or unitno";
var words = regex.Split(phrase);
return words;
}
? MyTest("incidentno, and .fintype- or; :unitno",#"[^\w+]")
[0]: "incidentno"
[1]: "and"
[2]: "fintype"
[3]: "or"
[4]: "unitno"

I'd like to add some information to JDunkerley's awnser.
You can easily make this method more reliable if you give a string or char parameter to search for.
public static IEnumerable<string> WordList(this string Text,string Word)
{
int cIndex = 0;
int nIndex;
while ((nIndex = Text.IndexOf(Word, cIndex + 1)) != -1)
{
int sIndex = (cIndex == 0 ? 0 : cIndex + 1);
yield return Text.Substring(sIndex, nIndex - sIndex);
cIndex = nIndex;
}
yield return Text.Substring(cIndex + 1);
}
public static IEnumerable<string> WordList(this string Text, char c)
{
int cIndex = 0;
int nIndex;
while ((nIndex = Text.IndexOf(c, cIndex + 1)) != -1)
{
int sIndex = (cIndex == 0 ? 0 : cIndex + 1);
yield return Text.Substring(sIndex, nIndex - sIndex);
cIndex = nIndex;
}
yield return Text.Substring(cIndex + 1);
}

I write a string processor class.You can use it.
Example:
metaKeywords = bodyText.Process(prepositions).OrderByDescending().TakeTop().GetWords().AsString();
Class:
public static class StringProcessor
{
private static List<String> PrepositionList;
public static string ToNormalString(this string strText)
{
if (String.IsNullOrEmpty(strText)) return String.Empty;
char chNormalKaf = (char)1603;
char chNormalYah = (char)1610;
char chNonNormalKaf = (char)1705;
char chNonNormalYah = (char)1740;
string result = strText.Replace(chNonNormalKaf, chNormalKaf);
result = result.Replace(chNonNormalYah, chNormalYah);
return result;
}
public static List<KeyValuePair<String, Int32>> Process(this String bodyText,
List<String> blackListWords = null,
int minimumWordLength = 3,
char splitor = ' ',
bool perWordIsLowerCase = true)
{
string[] btArray = bodyText.ToNormalString().Split(splitor);
long numberOfWords = btArray.LongLength;
Dictionary<String, Int32> wordsDic = new Dictionary<String, Int32>(1);
foreach (string word in btArray)
{
if (word != null)
{
string lowerWord = word;
if (perWordIsLowerCase)
lowerWord = word.ToLower();
var normalWord = lowerWord.Replace(".", "").Replace("(", "").Replace(")", "")
.Replace("?", "").Replace("!", "").Replace(",", "")
.Replace("<br>", "").Replace(":", "").Replace(";", "")
.Replace("،", "").Replace("-", "").Replace("\n", "").Trim();
if ((normalWord.Length > minimumWordLength && !normalWord.IsMemberOfBlackListWords(blackListWords)))
{
if (wordsDic.ContainsKey(normalWord))
{
var cnt = wordsDic[normalWord];
wordsDic[normalWord] = ++cnt;
}
else
{
wordsDic.Add(normalWord, 1);
}
}
}
}
List<KeyValuePair<String, Int32>> keywords = wordsDic.ToList();
return keywords;
}
public static List<KeyValuePair<String, Int32>> OrderByDescending(this List<KeyValuePair<String, Int32>> list, bool isBasedOnFrequency = true)
{
List<KeyValuePair<String, Int32>> result = null;
if (isBasedOnFrequency)
result = list.OrderByDescending(q => q.Value).ToList();
else
result = list.OrderByDescending(q => q.Key).ToList();
return result;
}
public static List<KeyValuePair<String, Int32>> TakeTop(this List<KeyValuePair<String, Int32>> list, Int32 n = 10)
{
List<KeyValuePair<String, Int32>> result = list.Take(n).ToList();
return result;
}
public static List<String> GetWords(this List<KeyValuePair<String, Int32>> list)
{
List<String> result = new List<String>();
foreach (var item in list)
{
result.Add(item.Key);
}
return result;
}
public static List<Int32> GetFrequency(this List<KeyValuePair<String, Int32>> list)
{
List<Int32> result = new List<Int32>();
foreach (var item in list)
{
result.Add(item.Value);
}
return result;
}
public static String AsString<T>(this List<T> list, string seprator = ", ")
{
String result = string.Empty;
foreach (var item in list)
{
result += string.Format("{0}{1}", item, seprator);
}
return result;
}
private static bool IsMemberOfBlackListWords(this String word, List<String> blackListWords)
{
bool result = false;
if (blackListWords == null) return false;
foreach (var w in blackListWords)
{
if (w.ToNormalString().Equals(word))
{
result = true;
break;
}
}
return result;
}
}

named String.Format, is it possible?

Instead of using {0} {1}, etc. I want to use {title} instead. Then fill that data in somehow (below I used a Dictionary). This code is invalid and throws an exception. I wanted to know if i can do something similar to what i want. Using {0 .. N} is not a problem. I was just curious.
Dictionary<string, string> d = new Dictionary<string, string>();
d["a"] = "he";
d["ba"] = "llo";
d["lol"] = "world";
string a = string.Format("{a}{ba}{lol}", d);

No, but this extension method will do it
static string FormatFromDictionary(this string formatString, Dictionary<string, string> valueDict)
{
int i = 0;
StringBuilder newFormatString = new StringBuilder(formatString);
Dictionary<string, int> keyToInt = new Dictionary<string,int>();
foreach (var tuple in valueDict)
{
newFormatString = newFormatString.Replace("{" + tuple.Key + "}", "{" + i.ToString() + "}");
keyToInt.Add(tuple.Key, i);
i++;
}
return String.Format(newFormatString.ToString(), valueDict.OrderBy(x => keyToInt[x.Key]).Select(x => x.Value).ToArray());
}

Check this one, it supports formating:
public static string StringFormat(string format, IDictionary<string, object> values)
{
var matches = Regex.Matches(format, #"\{(.+?)\}");
List<string> words = (from Match matche in matches select matche.Groups[1].Value).ToList();
return words.Aggregate(
format,
(current, key) =>
{
int colonIndex = key.IndexOf(':');
return current.Replace(
"{" + key + "}",
colonIndex > 0
? string.Format("{0:" + key.Substring(colonIndex + 1) + "}", values[key.Substring(0, colonIndex)])
: values[key] == null ? string.Empty : values[key].ToString());
});
}
How to use:
string format = "{foo} is a {bar} is a {baz} is a {qux:#.#} is a really big {fizzle}";
var dictionary = new Dictionary<string, object>
{
{ "foo", 123 },
{ "bar", true },
{ "baz", "this is a test" },
{ "qux", 123.45 },
{ "fizzle", DateTime.Now }
};
StringFormat(format, dictionary)

You can implement your own:
public static string StringFormat(string format, IDictionary<string, string> values)
{
foreach(var p in values)
format = format.Replace("{" + p.Key + "}", p.Value);
return format;
}

Phil Haack discussed several methods of doing this on his blog a while back: http://haacked.com/archive/2009/01/14/named-formats-redux.aspx. I've used the "Hanselformat" version on two projects with no complaints.

It's possible now
With Interpolated Strings of C# 6.0 you can do this:
string name = "John";
string message = $"Hi {name}!";
//"Hi John!"

static public class StringFormat
{
static private char[] separator = new char[] { ':' };
static private Regex findParameters = new Regex(
"\\{(?<param>.*?)\\}",
RegexOptions.Compiled | RegexOptions.Singleline);
static string FormatNamed(
this string format,
Dictionary<string, object> args)
{
return findParameters.Replace(
format,
delegate(Match match)
{
string[] param = match.Groups["param"].Value.Split(separator, 2);
object value;
if (!args.TryGetValue(param[0], out value))
value = match.Value;
if ((param.Length == 2) && (param[1].Length != 0))
return string.Format(
CultureInfo.CurrentCulture,
"{0:" + param[1] + "}",
value);
else
return value.ToString();
});
}
}
A little more involved than the other extension method, but this should also allow non-string values and formatting patterns used on them, so in your original example:
Dictionary<string, object> d = new Dictionary<string, object>();
d["a"] = DateTime.Now;
string a = string.FormatNamed("{a:yyyyMMdd-HHmmss}", d);
Will also work...

Since C# 6 released you are able to use String Interpolation feature
Code that solves your question:
string a = $"{d["a"]}{d["ba"]}{d["lol"]}";

Why a Dictionary? It's unnecessary and overly complicated. A simple 2 dimensional array of name/value pairs would work just as well:
public static string Format(this string formatString, string[,] nameValuePairs)
{
if (nameValuePairs.GetLength(1) != 2)
{
throw new ArgumentException("Name value pairs array must be [N,2]", nameof(nameValuePairs));
}
StringBuilder newFormat = new StringBuilder(formatString);
int count = nameValuePairs.GetLength(0);
object[] values = new object[count];
for (var index = 0; index < count; index++)
{
newFormat = newFormat.Replace(string.Concat("{", nameValuePairs[index,0], "}"), string.Concat("{", index.ToString(), "}"));
values[index] = nameValuePairs[index,1];
}
return string.Format(newFormat.ToString(), values);
}
Call the above with:
string format = "{foo} = {bar} (really, it's {bar})";
string formatted = format.Format(new[,] { { "foo", "Dictionary" }, { "bar", "unnecessary" } });
Results in: "Dictionary = unnecessary (really, it's unnecessary)"

public static string StringFormat(this string format, IDictionary<string, object> values)
{
return Regex.Matches(format, #"\{(?!\{)(.+?)\}")
.Select(m => m.Groups[1].Value)
.Aggregate(format, (current, key) =>
{
string[] splits = key.Split(":");
string replacement = splits.Length > 1
? string.Format($"{{0:{splits[1]}}}", values[splits[0]])
: values[key].ToString();
return Regex.Replace(current, "(.|^)("+ Regex.Escape($"{{{key}}}")+")(.|$)",
m => m.Groups[1].ToString() == "{" && m.Groups[3].ToString() == "}"
? m.Groups[2].ToString()
: m.Groups[1] + replacement + m.Groups[3]
);
});
}
This is similar to another answer, but it considers escaping with {{text}}.

I took #LPCRoy's answer and refactored for generic dictionary types, added parameter validation, it only replaces fields it can find, and attempts to solve the "double curly brace" issue by escaping them first, then replacing them at the end with single braces in the same way that string.Format() does.
public static string FormatFromDictionary<K, T>(this string formatString, IDictionary<K, T> valueDict)
{
if (string.IsNullOrWhiteSpace(formatString)) return formatString;
if (valueDict == null || !valueDict.Any()) return formatString;
bool escapedDoubleCurlyBraces = false;
if (formatString.Contains("{{") || formatString.Contains("}}"))
{
formatString = formatString.Replace("{{", "\\{\\{").Replace("}}", "\\}\\}");
escapedDoubleCurlyBraces = true;
}
int i = 0;
StringBuilder newFormatString = new StringBuilder(formatString);
Dictionary<K, int> keyToInt = new Dictionary<K, int>();
string field;
//StringBuilder.Replace() is faster than string.Replace().
foreach (var kvp in valueDict)
{
field = "{" + kvp.Key.ToString() + "}";
if (formatString.Contains(field))
{
newFormatString = newFormatString.Replace(field, "{" + i.ToString() + "}");
keyToInt.Add(kvp.Key, i);
i++;
}
}
//Any replacements to make?
if (keyToInt.Any())
{
formatString = string.Format(
newFormatString.ToString(),
keyToInt.OrderBy(kvp => kvp.Value).Select(kvp => (object)valueDict[kvp.Key]).ToArray()
);
}
if (escapedDoubleCurlyBraces)
{
//Converts "{{" and "}}" to single "{" and "}" for the final result just like string.format().
formatString = formatString.Replace("\\{\\{", "{").Replace("\\}\\}", "}");
}
return formatString;
}

Here is a nice solution that is very useful when formatting emails: http://www.c-sharpcorner.com/UploadFile/e4ff85/string-replacement-with-named-string-placeholders/
Edited:
public static class StringExtension
{
public static string Format( this string str, params Expression<Func<string,object>>[] args)
{
var parameters = args.ToDictionary( e=>string.Format("{{{0}}}",e.Parameters[0].Name), e=>e.Compile()(e.Parameters[0].Name));
var sb = new StringBuilder(str);
foreach(var kv in parameters)
{
sb.Replace( kv.Key, kv.Value != null ? kv.Value.ToString() : "");
}
return sb.ToString();
}
}
Example usage:
public string PopulateString(string emailBody)
{
User person = _db.GetCurrentUser();
string firstName = person.FirstName; // Peter
string lastName = person.LastName; // Pan
return StringExtension.Format(emailBody.Format(
firstname => firstName,
lastname => lastName
));
}

(your Dictionary + foreach + string.Replace) wrapped in a sub-routine or extension method?
Obviously unoptimized, but...