.NET: The Random Class is Not Thread-safe!

How not to use the Random class:

// This seems like it would work, but can generate duplicate sequences
Parallel.For(0, 10, i =>
{
	var localRandom = new Random(); // BAD! Could get same seed in different threads
	var number = localRandom.Next(1000);
	Console.WriteLine(number);
});

No, the C# Random class is not thread-safe. It maintains internal state that can be corrupted when accessed by multiple threads simultaneously. Here are three ways to make the Random class thread-safe:

// This demonstrates thread safety issues with Random
using System;
using System.Threading.Tasks;

class RandomThreadSafetyDemo
{
	// Shared Random instance - NOT thread-safe
	private static Random sharedRandom = new Random();

	// Thread-safe approach using ThreadLocal
	private static ThreadLocal<Random> threadLocalRandom = new ThreadLocal<Random>(() => new Random());

	// Another thread-safe approach using lock
	private static object lockObject = new object();

	public static void DemonstrateThreadSafetyIssues()
	{
		// This can produce duplicate numbers due to thread safety issues
		Parallel.For(0, 100, i =>
		{
			int number = sharedRandom.Next(1000);
			Console.WriteLine($"Unsafe number: {number}");
		});
	}

	public static void ThreadSafeApproach1()
	{
		// Using ThreadLocal - each thread gets its own Random instance
		Parallel.For(0, 100, i =>
		{
			int number = threadLocalRandom.Value.Next(1000);
			Console.WriteLine($"Thread-local number: {number}");
		});
	}

	public static void ThreadSafeApproach2()
	{
		// Using local copy of Random with unique seed
		Parallel.For(0, 100, i =>
		{
			var localRandom = new Random(Guid.NewGuid().GetHashCode()); // Unique seeds with local copy of Random
			var number = localRandom.Next(1000);
			Console.WriteLine($"Safe number: {number}");
		});
	}

	public static void ThreadSafeApproach3()
	{
		// Using lock - serializes access to shared Random
		Parallel.For(0, 100, i =>
		{
			int number;
			lock (lockObject) // May have concurrency issues and less performance, but yes, thread-safe
			{
				number = sharedRandom.Next(1000);
			}
			Console.WriteLine($"Locked number: {number}");
		});
	}
}

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