Intro

This is just a really quick post on how to write async test methods in C# with XUnit and NSubstitute. Thankfully its quite simple these days, and is very useful to know as async calls become more and more commonplace.

So how do you do it?

Say you have interface IA:

    public interface IA
    {
        Task<int> DoSomething();
    }

Which is called by:

    public class IAmUnderTest
    {
        public async Task<int> GetInt(IA a)
        {
            return await a.DoSomething();
        }
    }

Mocking

Because Async methods just return tasks, all you need to do to mock DoSomething() with NSubstitute is use Task.FromResult(<YourNumberHere>). Then the calling code gets a task back that it can still await and get back the integer result.

So mocking it with NSubstitute looks like this:

    public class ATest
    {
        [Fact]
        public void DoesSomething()
        {
            var dependency = Substitute.For<IA>();
            dependency.DoSomething().Returns(Task.FromResult(1));

            var target = new IAmUnderTest();
            var id = target.GetInt(dependency).Result;

            id.Should().Be(1);
        }
    }

So its not very different to usual, you just need that Task.FromResult around the returns method.

Async Test Methods

We can go one step further than this, and make the actual test async, because XUnit 1.9 + has support for this. This allows us to get rid of the .Result (shudder) from the tests and the method looks more like any other async method you might write:

    public class ATest
    {
        [Fact]
        public async Task DoesSomethingAsync()
        {
            var dependency = Substitute.For<IA>();
            dependency.DoSomething().Returns(Task.FromResult(1));

            var target = new IAmUnderTest();
            var id = await target.GetInt(dependency);

            id.Should().Be(1);
        }
    }

Both of these tests pass, and I think I favour the latter as its a bit more natural when testing async methods, and quite frankly seeing .Result is enough to give anyone nightmares whos come across the myriad of deadlocks it can cause.