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` } 

Answer 2

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.