Mocking (stubbing) async calls (async/await)

Published by Carlos Blé on 10/02/2015

.Net 4.5 came out with a really handy built-in asynchronous mechanism, async and await. However the method signature of a void method is a bit strange:

public async Task SomeAction(){...}

view raw codesai_carlos_mocking_stubbing_async_calls_1.cs hosted with ❤ by GitHub

It is possible to use async void but it’s not recommended unless we are talking about event handlers. It’s also a bit strange the fact that the type specified in the signature is not the one returned:

	public async Task<string> SomeQuery(){
	.....
	return someString;
	}

view raw codesai_carlos_mocking_stubbing_async_calls_2.cs hosted with ❤ by GitHub

But everything else is great and very handy.

Stubbing method calls can be hard though. You can get weird exceptions like System.AggregateException when running tests with NUnit. The problem comes up when awaiting for a

stubbed async method:

	// notice that async tests with NUnit require the method to be async Task rather than void

	[Test] public async Task
	filter_results_from_search(){
	var srv = new Mock<Service>(); // using Moq
	sut = new Sut(srv.Object);

	sut.someAction(); // Exception happens here

	SomeAssertionOverHere();
	}
	// Production code
	public class Service(){ // dependency
	public virtual async Task<string> SomeQuery(){ // virtual is mandatory in order to mock it
	return "someResult";
	}
	}
	public class Sut(){ // system under test
	Service srv;
	public Sut(Service srv){
	this.srv = srv;
	}
	public async Task SomeAction(){
	var result = await srv.SomeQuery(); // NullReferenceException in here
	....
	}
	}

view raw codesai_carlos_mocking_stubbing_async_calls_3.cs hosted with ❤ by GitHub

The problem is that Moq will make the stubbed method return null when invoked, because we are not specifying any return value. The default value for Task is null. We should tell Moq to return a proper Task:

	[Test] public async Task
	filter_results_from_search(){
	var srv = new Mock<Service>();
	srv.Setup(s => s
	.SomeQuery())
	.Returns(Task.Factory.StartNew(
	() => "someQuery")
	);
	sut = new Sut(srv.Object);

	sut.someAction();

	SomeAssertionOverHere();
	}

view raw codesai_carlos_mocking_stubbing_async_calls_4.cs hosted with ❤ by GitHub

The key is to return a new task: Task.Factory.StartNew(lambda)

Originally published in Carlos Blé's blog.

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