使用异步方法上的Verifyable的UnitTest中的NullReferenceException [英] NullReferenceException in UnitTest using Verifyable on an async Method

问题描述

我只是在使用xUnit和Moq设置单元测试时遇到了问题.

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

方法TestMethod()包含我要测试的逻辑.它是异步的，尽管返回Task

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
    }
}

界面IDoSomething看起来像这样

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

单元测试

现在，我要像这样设置我的单元测试.

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);
    }
}

为什么会出现此异常?

推荐答案

mock.Setup()缺少返回值并返回null.因此，在TestMethod()中，由_doSomething.DoAsync(true)返回的Task是null. null等待着，当然会导致System.NullReferneceException.

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.

要解决此问题，我建议将.Returns(Task.CompletedTask)添加到Mock-Setup中.

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();

随着await使用Awaiter-Pattern进行编译，现在有一个任务可以调用.GetAwaiter()来返回TaskAwaiter.这些原则在这里有更好的解释:

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的博客
• Marko Papic的博客

• dixin's Blog
• Marko Papic's Blog

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