Cookbooks
C#
Chapter 3. Numbers
Generating random numbers

C#:
Generating random numbers

How to:

The most common way to generate random numbers in C# is using the System.Random class. Here’s a simple example demonstrating its usage:

using System;

public class RandomNumberExample
{
    static void Main(string[] args)
    {
        Random random = new Random();
        int randomNumber = random.Next(1, 100); // Generates a number between 1 and 99
        Console.WriteLine($"Random number: {randomNumber}");
    }
}

This will output a random number such as:

Random number: 42

For generating a random floating-point number between 0.0 and 1.0, you can use the NextDouble method:

double randomDouble = random.NextDouble();
Console.WriteLine($"Random double: {randomDouble}");

If you’re working on a security-sensitive application that requires cryptographic randomness, it’s better to use the RNGCryptoServiceProvider class found in System.Security.Cryptography:

using System;
using System.Security.Cryptography;

public class SecureRandomExample
{
    static void Main()
    {
        byte[] randomNumber = new byte[4]; // Creates a 4-byte long random number
        using (RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider())
        {
            rng.GetBytes(randomNumber);
        }
        int value = BitConverter.ToInt32(randomNumber, 0);
        Console.WriteLine($"Cryptographically secure random number: {value}");
    }
}

Deep Dive

Random number generation in C# has evolved over the years. Initially, the System.Random class was the go-to for generating pseudo-random numbers. It is pseudo-random because, given a specific seed value, it will produce the same sequence of numbers, which can be useful for debugging or repeatability of tests.

While sufficient for basic needs, System.Random is not thread-safe and can produce predictable outcomes, which is not suitable for security-dependent applications. This limitation led to the introduction of the RNGCryptoServiceProvider for cryptographic randomness, which is more secure but also more resource-intensive.

An alternative in .NET Core and .NET 5+ is the RandomNumberGenerator class in System.Security.Cryptography for generating random numbers securely, which is intended as a more modern and easy-to-use option compared to RNGCryptoServiceProvider.

Each method of generating random numbers in C# has its place depending on the requirements of the application. For most applications, System.Random suffices, but for those that require secure, unpredictable random numbers, the cryptographic classes provide a robust alternative.

Rounding numbers