Is C#'s Random.Next() Method Thread-Safe, and How Can I Make It So?

Thread safety of C# Random.Next() method

C#'s Random.Next() method is not thread-safe. Using the same Random instance from multiple threads may result in unpredictable behavior, or even returning a result of all 0's.

Solution

To create a thread-safe version of Random we can create local instances for each thread. The following code demonstrates this:

<code class="language-csharp">public class ThreadSafeRandom
{
    private static readonly Random _global = new Random();
    [ThreadStatic] private static Random _local;

    public int Next()
    {
        if (_local == null)
        {
            int seed;
            lock (_global)
            {
                seed = _global.Next();
            }
            _local = new Random(seed);
        }

        return _local.Next();
    }
}</code>

This class maintains a separate static Random variable (_local) for each thread. To prevent conflicts between instances created at the same time, use global static Random instances (_global) to seed thread-local instances.

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