MOQ 创建抽象类的 Mock 返回 null

Question

Firstly, I understand that the title of my question doesn't really suit the question; I just wasn't sure what the title should've been.

It is to the best of my knowledge that when creating a mock of an abstract class with MOQ if the creation of the object fails (due to exception handling or something else) the creation of the mock will simply return null.

In my case, I wish to check if an ArgumentNullException has been thrown within the constructor. But instead my test fails due to what I've stated above and the actual result is null , not ArgumentNullException

public abstract class SomeClass
{
    public SomeClass(ISomeDependency someDependency)
    {
        if (someDependency == null)
        {
            throw new ArgumentNullException(nameof(someDependency));
        }

        this.someDependency = someDependency;
    }
}

public sealed class LogHandlerTests
{
    [Test]
    public void Constructor_Test_Should_Throw_ArgumentNulLException_When_SomeDependency_Is_Null()
    {
            Assert.Throws<ArgumentNullException>(() => new Mock<SomeClass>(null));
    }
}

Answer 1

No real need to use MOQ here.

Sometimes it helps to keep it simple and create a derived class to satisfy the desired behavior.

For example

public sealed class LogHandlerTests {

    private class Subject: SomeClass {
        public Subject(): base (null) { //<-- SHOULD THROW
            //...
        }
    }

    [Test]
    public void Constructor_Test_Should_Throw_ArgumentNulLException_When_SomeDependency_Is_Null() {
        Assert.Throws<ArgumentNullException>(() => new Subject());
    }
}

Answer 2

Instead of using new Mock<SomeClass>(null) you want to use new Mock<SomeClass>(null).Object because the constructor is only invoked the first time you use the mocked object. You'll then end up having a TargetInvocationException because Moq uses reflection to achieve Mocking. This means you probably need a code similar to

[TestClass]
public class Test
{
    [TestMethod]
    public void MyTestMethod()
    {
        Exception caughtException = null;
        try
        {
            var test = new Mock<SomeClass>(null).Object;
        }
        catch (Exception ex)
        {
            caughtException = ex;
        }

        Assert.IsInstanceOfType(caughtException.InnerException, typeof(ArgumentNullException));
    }
}

public abstract class SomeClass
{
    ISomeDependency someDependency;

    public SomeClass(ISomeDependency someDependency)
    {
        if (someDependency == null)
        {
            throw new ArgumentNullException(nameof(someDependency));
        }

        this.someDependency = someDependency;
    }
}