How to Mock Extension Methods in Unit Tests Using Moq?

Use Moq to mock extension methods

In unit testing scenarios, it may be necessary to mock extension methods applied to existing interfaces. Consider the following:

<code class="language-csharp">public interface ISomeInterface
{
    void SomeMethod();
}

public static class SomeInterfaceExtensions
{
    public static void AnotherMethod(this ISomeInterface someInterface)
    {
        // 方法实现
    }
}

public class Caller
{
    private readonly ISomeInterface someInterface;

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

    public void Main()
    {
        someInterface.AnotherMethod();
    }
}</code>

In order to test the Caller class, you need to mock the ISomeInterface interface and verify calls to the AnotherMethod extension methods. However, trying to simulate x => x.AnotherMethod() directly produces the exception:

<code class="language-csharp">[Test]
public void Main_BasicCall_CallsAnotherMethod()
{
    // Arrange
    var someInterfaceMock = new Mock<ISomeInterface>();
    someInterfaceMock.Setup(x => x.AnotherMethod()).Verifiable(); // 这里会抛出异常

    var caller = new Caller(someInterfaceMock.Object);

    // Act
    caller.Main();

    // Assert
    someInterfaceMock.Verify();
}</code>

Exception information:

<code>System.ArgumentException: Invalid setup on a non-member method:
x => x.AnotherMethod()</code>

To solve this problem, one way is to create a wrapper object for the extension method:

<code class="language-csharp">public class AnotherMethodWrapper
{
    public static void CallAnotherMethod(ISomeInterface someInterface)
    {
        someInterface.AnotherMethod();
    }
}</code>

In tests, wrapper methods can be mocked:

<code class="language-csharp">[Test]
public void Main_BasicCall_CallsAnotherMethod()
{
    // Arrange
    var wrapperMock = new Mock<AnotherMethodWrapper>();
    wrapperMock.Setup(x => x.CallAnotherMethod(It.IsAny<ISomeInterface>())).Verifiable();

    var someInterfaceMock = new Mock<ISomeInterface>();

    //  这里需要调整Caller的构造函数，注入wrapperMock.Object
    //  这部分需要根据实际情况修改，可能需要修改Caller类或者使用其他方法注入依赖
    var caller = new Caller(someInterfaceMock.Object); // 此处可能需要修改，注入wrapperMock.Object

    // Act
    caller.Main();

    // Assert
    wrapperMock.Verify();
}</code>

By using wrappers, extension method calls can be mocked and unit tested. It should be noted that the Caller class in this example needs to be modified to use AnotherMethodWrapper. This depends on the specific implementation of the Caller class, which may require dependency injection or other methods to inject AnotherMethodWrapper into Caller.

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