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.

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.

C# Random Numbers

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

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.

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.

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.

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.

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.

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.

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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