Generating Random Numbers in C#

Syntax

  • Random()

  • Random(int Seed)

  • int Next()

  • int Next(int maxValue)

  • int Next(int minValue, int maxValue)

Parameters

ParametersDetails
SeedA value for generating random numbers. If not set, the default value is determined by the current system time.
minValueGenerated numbers won't be smaller than this value. If not set, the default value is 0.
maxValueGenerated numbers will be smaller than this value. If not set, the default value is Int32.MaxValue.
return valueReturns a number with random value.

Remarks

The random seed generated by the system isn't the same in every different run.

Seeds generated in the same time might be the same.

Create multiple random class with different seeds simultaneously

Two Random class created at the same time will have the same seed value.

Using System.Guid.NewGuid().GetHashCode() can get a different seed even in the same time.

Random rnd1 = new Random();
Random rnd2 = new Random();
Console.WriteLine("First 5 random number in rnd1");
for (int i = 0; i < 5; i++)
    Console.WriteLine(rnd1.Next());

Console.WriteLine("First 5 random number in rnd2");
for (int i = 0; i < 5; i++)
    Console.WriteLine(rnd2.Next());

rnd1 = new Random(Guid.NewGuid().GetHashCode());
rnd2 = new Random(Guid.NewGuid().GetHashCode());
Console.WriteLine("First 5 random number in rnd1 using Guid");
for (int i = 0; i < 5; i++)
    Console.WriteLine(rnd1.Next());
Console.WriteLine("First 5 random number in rnd2 using Guid");
for (int i = 0; i < 5; i++)
    Console.WriteLine(rnd2.Next());

Another way to achieve different seeds is to use another Random instance to retrieve the seed values.

Random rndSeeds = new Random();
Random rnd1 = new Random(rndSeeds.Next());
Random rnd2 = new Random(rndSeeds.Next());

This also makes it possible to control the result of all the Random instances by setting only the seed value for the rndSeeds. All the other instances will be deterministically derived from that single seed value.

Generate a number that is a percentage of a max value

A common need for random numbers it to generate a number that is X% of some max value. this can be done by treating the result of NextDouble() as a percentage:

var rnd = new Random();
var maxValue = 5000;
var percentage = rnd.NextDouble();
var result = maxValue * percentage; 
//suppose NextDouble() returns .65, result will hold 65% of 5000: 3250.

Generate a random character

Generate a random letter between a and z by using the Next() overload for a given range of numbers, then converting the resulting int to a char

Random rnd = new Random();
char randomChar = (char)rnd.Next('a','z'); 
//'a' and 'z' are interpreted as ints for parameters for Next()

Generate a Random double

Generate a random number between 0 and 1.0. (not including 1.0)

Random rnd = new Random();
var randomDouble = rnd.NextDouble();

Generate a random int

This example generates random values between 0 and 2147483647.

Random rnd = new Random();
int randomNumber = rnd.Next();

Generate a random int in a given range

Generate a random number between minValue and maxValue - 1.

Random rnd = new Random();
var randomBetween10And20 = rnd.Next(10, 20);

Generating the same sequence of random numbers over and over again

When creating Random instances with the same seed, the same numbers will be generated.

int seed = 5;
for (int i = 0; i < 2; i++)
{
   Console.WriteLine("Random instance " + i);
   Random rnd = new Random(seed);
   for (int j = 0; j < 5; j++)
   {
      Console.Write(rnd.Next());
      Console.Write(" ");
   }

   Console.WriteLine();
}

Output:

Random instance 0
726643700 610783965 564707973 1342984399 995276750
Random instance 1
726643700 610783965 564707973 1342984399 995276750