How do you mock an IAsyncEnumerable?

Question

I want to unit test a method that calls another method of a service returning an IAsyncEnumerable. I have created a a mock of my service Mock<

Answer 1

One way of solving this is to use dedicated test classes that wrap an IEnumerable that is enumerated synchronously.

TestAsyncEnumerable.cs

internal class TestAsyncEnumerable : List, IAsyncEnumerable
{
   public TestAsyncEnumerable(IEnumerable enumerable) : base(enumerable) { }

   public IAsyncEnumerator GetAsyncEnumerator(CancellationToken cancellationToken = default) => new TestAsyncEnumerator(GetEnumerator());
}

internal class TestAsyncEnumerator : IAsyncEnumerator
{
   private readonly IEnumerator _inner;

   public TestAsyncEnumerator(IEnumerator inner)
   {
      _inner = inner;
   }

   public ValueTask MoveNextAsync() => new ValueTask(_inner.MoveNext());

   public T Current => _inner.Current;

   public ValueTask DisposeAsync()
   {
      _inner.Dispose();

      return new ValueTask(Task.CompletedTask);
   }
}

Usage:

[Fact]
public async Task MyTest() {
   var myItemRepository = A.Fake();

   A.CallTo(       () => myRepository.GetAll())
    .ReturnsLazily(() => new TestAsyncEnumerable(new List { new MyItem(), ... }));


   //////////////////
   /// ACT & ASSERT
   ////////
}