How do you mock an IAsyncEnumerable?

Question

I want to unit test a method that calls another method of a service returning an IAsyncEnumerable<T>. I have created a a mock of my service Mock<MyService> and I want to setUp this mock but I don't know how to do that. Is it possible ? Are there other ways of unit testing a method that calls something retuning an IAsyncEnumerable

public async Task<List<String>> MyMethodIWantToTest()
{
  var results = new List<string>();
  await foreach(var item in _myService.CallSomethingReturningAsyncStream())
  {
    results.Add(item);
  }
  return results;
}

Answer 1

I recommend using ToAsyncEnumerable from System.Linq.Async, as Jeroen suggested. It seems like you're using Moq, so this would look like:

async Task MyTest()
{
  var mock = new Mock<MyService>();
  var mockData = new[] { "first", "second" };
  mock.Setup(x => x.CallSomethingReturningAsyncStream()).Returns(mockData.ToAsyncEnumerable());

  var sut = new SystemUnderTest(mock.Object);
  var result = await sut.MyMethodIWantToTest();

  // TODO: verify `result`
}