c# - 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

If you don’t want to do anything special, e.g. a delayed return which is usually the point of async enumerables, then you can just create a generator function that returns the values for you.

public static async IAsyncEnumerable<string> GetTestValues()
{
    yield return "foo";
    yield return "bar";

    await Task.CompletedTask; // to make the compiler warning go away
}

With that, you can simply create a mock for your service and test your object:

var serviceMock = new Mock<IMyService>();
serviceMock.Setup(s => s.CallSomethingReturningAsyncStream()).Returns(GetTestValues);

var thing = new Thing(serviceMock.Object);
var result = await thing.MyMethodIWantToTest();
Assert.Equal("foo", result[0]);
Assert.Equal("bar", result[1]);

Of course, since you are now using a generator function, you can also make this more complicated and add actual delays, or even include some mechanism to control the yielding.