NullReferenceException in UnitTest using Verifyable on an async Method [duplicate]

Question

This question already has answers here:

How can I tell Moq to return a Task? (4 answers)

Using Moq to mock an asynchronous method for a unit test (3 answers)

Closed 2 years ago.

I just had a problem with setting up a Unit Test using xUnit and Moq.

My Testee

The Method TestMethod() contains the Logic, I want to test. It is async and though returns a Task

public class Testee
{
    private readonly IDoSomething _doSomething;

    public Testee(IDoSomething doSomething)
    {
        _doSomething = doSomething;
    }

    public async Task TestMethod()
    {
        await _doSomething.DoAsync(true); // throws NullReferenceException in Unit-Test
    }
}

The Interface IDoSomething looks like this

public interface IDoSomething
{
    Task DoAsync(bool aboolean);
}

The Unit-Test

Now im Setting up my Unit-Test like this.

public class TesteeTest
{
    [Fact]
    public async Task MyTest()
    {
        var mock = new Mock<IDoSomething>();
        mock.Setup(x => x.DoAsync(It.IsAny<bool>())).Verifiable();

        var testee = new Testee(mock.Object);

        await testee.TestMethod(); // Test breaks here because it throws an exception

        mock.Verify(x => x.DoAsync(true), Times.Once);
        mock.Verify(x => x.DoAsync(false), Times.Never);
    }
}

Why am I getting this exception?

Answer 1

The mock.Setup() is missing a return value and returns null. So in the TestMethod() the Task returned by _doSomething.DoAsync(true) is null. null gets awaited which of course results in a System.NullReferneceException.

To Fix this, I'd recommend adding .Returns(Task.CompletedTask) to the Mock-Setup.

mock.Setup(x => x.DoAsync(It.IsAny<bool>())).Returns(Task.CompletedTask).Verifiable();

As await gets compiled using the Awaiter-Pattern, there's now a Task to call .GetAwaiter() on it, to return a TaskAwaiter. These principles are better explained here:

• dixin's Blog
• Marko Papic's Blog