Mocking external responses with NSubstitute

Question

I have a method that calls various services and uses their output later. Something like this:

myOutput1 = await _apiCaller.Call(() => _myService.someCall(someData));
myOutput2 = await _apiCaller.Call(() => _myService.anotherCall(otherData));

I would like to just mock the responses to these with NSubstitute, without waiting for the calls.

However, it doesn't look like NSubstitute catches the internal function call (eg someCall , anotherCall ), so I have to mock the API caller. This is a problem in two ways: I can't seem to differentiate between the two function inputs, and more broadly, even properly mock the caller.

I am doing the following:

sub._apiCaller.Call(default).ReturnsForAnyArgs(Task.FromResult(myMockData));

But still get a null result in myOutput# .

What is the correct way to mock the results of such calls, and is there a way to differentiate the internal function call?

Answer 1

This is somewhat similar to what you are suggesting. The idea will be the same in terms of how you go about mocking the internal service.

[TestClass]
    public class UnitTest1
    {
        private IMyService _myService;
        private ApiCaller _apiCaller;

        [TestInitialize]
        public void Initialize()
        {
            _apiCaller = new ApiCaller();
            _myService = Substitute.For<IMyService>();
        }

        [TestMethod]
        public async Task TestMethod1()
        {
            _myService.SomeCall(Arg.Any<int>()).Returns(190);
            var output = await _apiCaller.Call(() => _myService.SomeCall(4));
            Assert.AreEqual(190, output);
        }
    }

    public class ApiCaller
    {
        public async Task<T> Call<T>(Func<T> service)
        {
            return await Task.Run(() => service.Invoke());
        }
    }

    public class MyService : IMyService
    {
        public int SomeCall(int x)
        {
            return x * 8;
        }
    }

    public interface IMyService
    {
        int SomeCall(int x);
    }

Because the mock is returning immediately, the call is not being actually awaited.