C# Random

Last modified April 18, 2025

This tutorial explores how to generate random values in C# using the built-in Random class.

The Random class serves as a pseudo-random number generator, employing an algorithm to produce a sequence of numbers that satisfies specific statistical randomness criteria.

Random Number Generator

A Random Number Generator (RNG) produces a sequence of values that exhibit no discernible patterns. RNGs are divided into two types: hardware random-number generators and pseudo-random number generators. Hardware generators are considered to produce truly random numbers, while pseudo-random generators rely on software algorithms to create values that appear random. These pseudo-random values are deterministic and reproducible if the algorithm and initial conditions are known.

Random number generators are essential in applications such as gambling, gaming, simulations, and cryptography.

**Note: **For security-sensitive applications, cryptographically secure pseudo-random number generators are required.

To enhance the quality of pseudo-random number generators, operating systems collect environmental noise from sources like device drivers, user input latency, or hardware component jitter. This data forms the basis for cryptographically secure pseudo-random number generators.

The Seed

The seed is an initial value that initializes a random number generator. Random number generators operate by transforming a previous value through a specific algorithm. At the start, the seed provides the initial value for these operations. The most challenging aspect of random number generation is selecting a seed that closely approximates true randomness.

var rnd = new Random();

This constructor initializes a random number generator with a default seed.

**Note: **Since 2016, in .NET Core, the default seed has shifted from Environment.TickCount to Guid.NewGuid().GetHashCode(). This change ensures it is safe to create multiple random instances within a loop.

C# Random Numbers

In the following example, we demonstrate the generation of various random numbers.

Program.cs

var rand = new Random();

Console.WriteLine(rand.NextDouble()); Console.WriteLine(rand.NextInt64());

var buf = new byte[8]; rand.NextBytes(buf);

Console.WriteLine(string.Join(" “, buf));

This example generates and displays random doubles, integers, and bytes.

var rand = new Random();

A new instance of the Random class is instantiated.

Console.WriteLine(rand.NextDouble());

The NextDouble method produces a random floating-point number between 0.0 (inclusive) and 1.0 (exclusive).

Console.WriteLine(rand.NextInt64());

The NextInt64 method generates a non-negative random 64-bit integer.

var buf = new byte[8]; rand.NextBytes(buf);

The NextBytes method populates a specified byte array with random values.

$ dotnet run 0.0746532268944834 7374871010421669053 149 132 170 234 101 204 104 37

C# Random Next

The Next method generates a random integer, with options to define lower and upper bounds for the generated numbers.

There are three overloaded methods:

- Next - Returns a non-negative random integer.

- Next(Int32) - Returns a non-negative random integer less than the specified maximum.

- Next(Int32, Int32) - Returns a random integer within a specified range.

Program.cs

var rand = new Random();

Console.WriteLine(rand.Next()); Console.WriteLine(rand.Next(5)); Console.WriteLine(rand.Next(10, 20));

This example outputs three random integers using different Next method overloads.

$ dotnet run 741804443 3 11

C# Pick Random Element

The next example illustrates how to select a random element from a collection.

Program.cs

var rand = new Random();

List<int> vals = [1, 2, 3, 4, 5, 6, 7, 8];

var r1 = vals[rand.Next(vals.Count)]; var r2 = vals[rand.Next(vals.Count)];

Console.WriteLine(r1); Console.WriteLine(r2);

This code randomly selects and prints two elements from a list of integers. The upper bound of Next is exclusive, preventing out-of-range exceptions.

$ dotnet run 5 2

C# Shuffle List

The following example demonstrates how to shuffle the elements of a list.

Program.cs

var rng = new Random();

List<int> vals = [1, 2, 3, 4, 5, 6]; List<string> words = [“sky”, “blue”, “war”, “toy”, “tick”];

Shuffle(vals); Shuffle(words);

foreach (var e in vals) { Console.Write($"{e} “); }

Console.WriteLine("\n———————–”);

foreach (var e in words) { Console.Write($"{e} “); }

Console.WriteLine();

void Shuffle<T>(IList<T> vals) { int n = vals.Count;

while (n > 1)
{
    n--;
    int k = rng.Next(n + 1);

    (vals[n], vals[k]) = (vals[k], vals[n]);
}

}

This example defines a generic Shuffle method that randomly reorders the elements of a list using the Fisher-Yates shuffle algorithm.

$ dotnet run 2 1 3 4 6 5

blue war sky tick toy

C# Random String

The following example demonstrates how to generate a random string of characters.

Program.cs

var rand = new Random(); string chars = “ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789”; int length = 8;

char[] randomString = new char[length]; for (int i = 0; i < length; i++) { randomString[i] = chars[rand.Next(chars.Length)]; }

Console.WriteLine(new string(randomString));

This code creates a random string of 8 characters by selecting random characters from a predefined set of alphanumeric characters.

$ dotnet run K7mP9xT2

C# Random Boolean

The next example shows how to generate a random boolean value.

Program.cs

var rand = new Random();

bool randomBool = rand.Next(2) == 0;

Console.WriteLine(randomBool);

This code generates a random boolean by producing a random integer (0 or 1) and converting it to a boolean value.

$ dotnet run True

C# Random Enum Value

The following example illustrates how to select a random value from an enumeration.

Program.cs

var rand = new Random();

enum Colors { Red, Green, Blue, Yellow } var values = Enum.GetValues(typeof(Colors)); var randomColor = (Colors)values.GetValue(rand.Next(values.Length));

Console.WriteLine(randomColor);

This code randomly selects a value from the Colors enumeration by retrieving all enum values and picking one at random.

$ dotnet run Blue

Source

Random class - language reference

This tutorial has demonstrated various techniques for generating random values in C# using the Random class.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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