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Deterministic randomness

For snapshot tests, generated-id assertions, or reproducing a reported bug, you want the "random" values to be predictable. RandomProvider.Generate(...) lets you supply a generator for any of the primitive types Random produces:

IRandomProvider provider = RandomProvider.Generate(
    seed: 42,
    intGenerator: new[] { 1, 2, 3 },          // round-robins through the array
    guidGenerator: () => Guid.Parse("00000000-0000-0000-0000-000000000001"),
    doubleGenerator: 0.5);                    // always returns 0.5

MockRandomSystem randomSystem = new(provider);

randomSystem.Random.Shared.Next();   // 1
randomSystem.Random.Shared.Next();   // 2
randomSystem.Random.Shared.Next();   // 3
randomSystem.Random.Shared.Next();   // 1 (wraps around)
randomSystem.Guid.NewGuid();         // 00000000-0000-0000-0000-000000000001

Generate accepts generators for int, long, float, double, byte[] and Guid. Any generator left as null falls through to the seeded Random.

Building generators​

Generator<T> has implicit conversions and four factory methods so most call sites stay terse:

// From a single value (every call returns 42)
Generator<int> g1 = 42;

// From an array (round-robin)
Generator<int> g2 = new[] { 1, 2, 3 };

// From a callback
Generator<int> g3 = Generator.FromCallback(() => DateTime.Now.Millisecond);

// From any IEnumerable (loops once exhausted)
Generator<Guid> g4 = Generator.FromEnumerable(MyKnownGuids);

Generator<T> is IDisposable - when wrapping an IEnumerable<T>, dispose it (or rely on garbage collection in tests) to release the underlying enumerator.

Wiring it into the file system​

MockFileSystem uses the random system internally for things like Path.GetRandomFileName() and WithAFile(). Pass a deterministic provider through to keep file-system tests reproducible:

MockFileSystem fileSystem = new(o =>
    o.UseRandomProvider(RandomProvider.Generate(seed: 1234)));