How Can I Mock Extension Methods Using Moq?

Use Moq to simulate extension methods: encapsulate Mixin calls

When testing classes that rely on extension methods, it is not trivial to mock these methods using Moq. This article will solve this problem and provide a solution using wrapper objects.

Consider the following code, where an interface is extended using a mixin, and a class calls this extension method:

public interface ISomeInterface
{
    void SomeMethod();
}

public static class SomeInterfaceExtensions
{
    public static void AnotherMethod(this ISomeInterface someInterface)
    {
        // 实现代码
    }
}

public class Caller
{
    private readonly ISomeInterface someInterface;

    public Caller(ISomeInterface someInterface)
    {
        this.someInterface = someInterface;
    }

    public void Main()
    {
        someInterface.AnotherMethod();
    }
}

In the test method we try to mock the interface and verify the call to the extension method:

    [Test]
    public void Main_BasicCall_CallsAnotherMethod()
    {
        // Arrange
        var someInterfaceMock = new Mock<ISomeInterface>();
        someInterfaceMock.Setup(x => x.AnotherMethod()).Verifiable(); // 此处会报错

        var caller = new Caller(someInterfaceMock.Object);

        // Act
        caller.Main();

        // Assert
        someInterfaceMock.Verify();
    }

However, this test will fail with an ArgumentException, indicating that the mixin call cannot be mocked directly.

To overcome this problem, you can create a wrapper object to encapsulate the mixin call. This allows us to mock wrapper methods instead of extension methods.

For example:

public class SomeInterfaceWrapper
{
    private readonly ISomeInterface someInterface;

    public SomeInterfaceWrapper(ISomeInterface someInterface)
    {
        this.someInterface = someInterface;
    }

    public void AnotherMethod()
    {
        someInterface.AnotherMethod();
    }
}

In the test method we can mock the wrapper method:

    [Test]
    public void Main_BasicCall_CallsAnotherMethod()
    {
        // Arrange
        var wrapperMock = new Mock<SomeInterfaceWrapper>();
        wrapperMock.Setup(x => x.AnotherMethod()).Verifiable();

        var caller = new Caller(new SomeInterfaceWrapper(wrapperMock.Object)); // 注意此处修改

        // Act
        caller.Main();

        // Assert
        wrapperMock.Verify();
    }

This approach provides a convenient way to simulate extension method calls using Moq. By encapsulating the mixin calls in a separate object, we can easily simulate the behavior of the extension method without modifying the original code. Note that the constructor of the Caller class needs to pass in a SomeInterfaceWrapper instance instead of directly passing in the Mock object.

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