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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