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Why Random generates different numbers in loop?

Hello there I want to generate random numbers, so I created method in my class
private readonly Random _rand = new Random(); //Private property of class
public void GenerateRandomNumber()
{
//For loop executes 10 times
for (int i = 1; i < 11; i++)
{
Console.WriteLine(_rand.Next(0, 10));
}
}
When I call this from Main I create new instance of my class and then call it. It works properly but I want to know why does this generate different numbers each time in for loop and also each time I run the program?
That's interesting for me because I know that Random can generate same numbers but in my case it generates different ones.
How will it affect if I add static modifier to private property?

In C# your random number generator will automatically set the seed to a value based on your system clock, thus generating pseudo random numbers. If you would like your random numbers to be in the same order every time you run the program you can manually set the seed using:
seed = 999
private readonly Random _rand = new Random(seed)

The Random object is not what's generating the numbers. When calling _rand.Next(0, 10), it's generating a random number between 0 and 9 which is why you are getting new numbers each time. See the documentation for Random.Next(). If you only want one random number, you will need to call this method outside of the loop and store the result in a variable.

Random number behaves weird, not fully random

In my .NET game my rand function that are determining how much damage each out of the players five characters should take, however the 1st one always seems to be at the bottom of the scale and the last one at the top. So in my Character[0] the damage is rarely more than 1 more than the minimum rand value, and for each Character on higher index the damage taken is only randomized from higher up the scale.
public int GetDamage(int low, int high)
{
Random r = new Random();
int rand = r.Next(low, high);
return rand;
}
This is the randomizer I use. Then I update the health left like this:
int Damage = GetDamage(3, 10);
Characters[Target].Health = Characters[Target].Health - Damage;
In this example dmg is divided like this:
Number 1: 3-4
Number 2: 4-6
Number 3: 5-7
Number 4: 7-8
Number 5: 8-9

You have to reuse the same random instance otherwise you won't get really random values since it is created with the current time as seed. If you call GetDamage very fast(e.g. in a loop) you will get always the same values.
So either use a field/property in the class of GetDamage or pass the random instance to the method.
private Random _rnd = new Random();
public int GetDamage(int low, int high)
{
int rand = _rnd.Next(low, high);
return rand;
}
MSDN
The random number generation starts from a seed value. If the same
seed is used repeatedly, the same series of numbers is generated. One
way to produce different sequences is to make the seed value
time-dependent, thereby producing a different series with each new
instance of Random. By default, the parameterless constructor of the
Random class uses the system clock to generate its seed value, while
its parameterized constructor can take an Int32 value based on the
number of ticks in the current time. However, because the clock has
finite resolution, using the parameterless constructor to create
different Random objects in close succession creates random number
generators that produce identical sequences of random numbers. This
problem can be avoided by creating a single Random object rather than
multiple ones.

You need to make your Random instance static, this seeds it once and thereafter you will get a more random looking number.
static Random r = new Random();
public int GetDamage(int low, int high)
{
int rand = r.Next(low, high);
return rand;
}

You need to seed the random number generator.
See: How do I seed a random class to avoid getting duplicate random values
Literally hundreds of this question on here, have a look around.

Random number generator always picks the same value when run inside a loop

The problem with the code is, when I try to generate a number, if the spin is equal 1 it generates values inside range (1,2,3) if if try to use the loop to sum random values inside the same range the random number gerated is always the same while in loop,
example, if I run the loop with:
spind3 = 4 the values go from 4, 8, 12
spind3 = 5 the values go from 5, 10, 15
That means the first time the RandomNumber generates a value inside loop, it never change until the loop completes.
if (toggled3.Checked)
{
if (spind3.Value != 1)
{
for (int i = 1; i <= spind3.Value; i++)
{
diceCalc[1] += RandomNumber(1, 4);
}
}
else
diceCalc[1] = RandomNumber(1, 4);
}

You are probably creating a new Random object inside RandomNumber method. The default constructor for Random uses the system time as a seed. If you create multiple Random objects in a tight loop the time probably won't have changed between each call so they will all be initialized with the same seed.
To fix your code you should only create one Random object and reuse it.
From the documentation:
The default seed value is derived from the system clock and has finite resolution. As a result, different Random objects that are created in close succession by a call to the default constructor will have identical default seed values and, therefore, will produce identical sets of random numbers. This problem can be avoided by using a single Random object to generate all random numbers. You can also work around it by modifying the seed value returned by the system clock and then explicitly providing this new seed value to the Random(Int32) constructor. For more information, see the Random(Int32) constructor.

The problem is that you are creating random generators too close in time. The random generator constructor uses the current time to seed the generator, and when you create them too close in time they will all be seeded using the same time.
Create one random generator and use in the loop:
Random rnd = new Random();
for (int i = 1; i <= spind3.Value; i++) {
diceCalc[1] += rnd.Next(1, 4);
}

You need to initialize your Random object, then call Next() inside your loop.
i.e.
if (toggled3.Checked)
{
// initialize your total and the random number generator
int diceTotal = 0;
Random rand = new Random();
for (int i = 0; i < spind3.Value; i++)
{
// add the next random number between 1 and 3
diceTotal += rand.Next(1, 4);
}
}

You can use Random(seed) constructor.
Random rand = new Random(Guid.NewGuid().GetHashCode());

One trick is to manually create the seed by adding DateTime.Now.Ticks to the variable i:
Random r = new Random((int)DateTime.Now.Ticks + i);

How do I use random numbers in C#?

I'm working on Pong in C# w/ XNA.
I want to use a random number (within a range) to determine things such as whether or not the ball rebounds straight, or at an angle, and how fast the ball moves when it hits a paddle.
I want to know how to implement it.

Use the Random class. For example:
Random r = new Random();
int nextValue = r.Next(0, 100); // Returns a random number from 0-99

Unless you need cryptographically secure numbers, Random should be fine for you... but there are two gotchas to be aware of:
You shouldn't create a new instance every time you need one. If you create an instance without specifying a seed, it will use the current time as the seed - which means if you create several instances in quick succession, many of them will produce the same sequence of numbers. Typically you create a long-lasting instance of Random and reuse it.
It's not thread-safe. If you need to generate random numbers from multiple threads, you should think about having one instance per thread. Read this blog post for more information - but make sure you read the comments as well, as they have very useful information.

Random rnd = new Random();
rnd.Next(minValue, maxValue);
i.e.
rnd.Next(1,10);

Use the Random object's Next method that takes a min and max and returns a value in that range:
var random = new Random();
int randomNum = random.Next(min, max);

While you can use the Random class like all the other are suggesting, the Random class only uses psuedo-random number generation. The RandomNumberGenerator, which can be found in the System.Security.Cryptography namespace, creates actual random numbers.
How To Use:
RandomNumberGenerator rng = RandomNumberGenerator.Create();
byte[] rand = new byte[25]; //Set the length of this array to
// the number of random numbers you want
rng.GetBytes(rand);
More Info: http://msdn.microsoft.com/en-us/library/system.security.cryptography.randomnumbergenerator(v=VS.80).aspx

Here is my random generator
private static Random rnd = new Random(Environment.TickCount);
private int RandomNum(int Lower, int Upper)
{
return rnd.Next(Lower, Upper);//MyRandomNumber;
}

Random number in a loop [duplicate]

This question already has answers here:
Random number generator only generating one random number
(15 answers)
Closed 9 years ago.
having an issue generating random numbers in a loop. Can get around it by using Thread.Sleep but after a more elegant solution.
for ...
Random r = new Random();
string += r.Next(4);
Will end up with 11111... 222... etc.
Suggestions?

Move the declaration of the random number generator out of the loop.
The random number generation starts from a seed value. If the same seed is used repeatedly, the same series of numbers is generated. One way to produce different sequences is to make the seed value time-dependent, thereby producing a different series with each new instance of Random. By default, the parameterless constructor of the Random class uses the system clock to generate its seed value, ...
Source
By having the declaration in the loop you are effectively calling the constructor with the same value over and over again - hence you are getting the same numbers out.
So your code should become:
Random r = new Random();
for ...
string += r.Next(4);

Random r = new Random();
for ...
string += r.Next(4);
new Random() will initialize the (pseudo-)random number generator with a seed based on the current date and time. Thus, two instances of Random created at the same date and time will yield the same sequence of numbers.
You created a new random number generator in each iteration and then took the first value of that sequence. Since the random number generators were the same, the first value of their sequences was the same. My solution will create one random number generator and then return the first, the second, etc... value of the sequence (which will be different).

I found a page in Chinese that said the same with the time: http://godleon.blogspot.hk/2007/12/c.html, it said if you type like this:
Random random = new Random(Guid.NewGuid().GetHashCode());
you MAY get a random number even in a loop! It solved my question as well!

You should be using the same Random instance throughout instead of creating a new one each time.
As you have it:
for ...
Random r = new Random();
string += r.Next(4);
the seed value is the same for each (it defaults to the current timestamp) so the value returned is the same.
By reusing a single Random instance like so:
Random r = new Random()
for ...
string += r.Next(4);
Each time you call r.Next(4) the values are updated (basically a different seed for each call).

Move the Random r = new Random(); outside the loop and just call next inside the loop.