抽象具有扩展方法的类的接口

Question

How do I replace a concrete class with its interface for the purpose of unit testing if I am using extension methods in that class?

I've got a method:

[HttpGet]
[Route("/yoyo/{yoyoId:int}/accounts")]
public ResponseYoyoEnvelope GetAccountsByYoyoId([FromBody] RequestYoyoEnvelope requestYoyoEnvelope, int yoyoId)
{
    var responseYoyoEnvelope = requestYoyoEnvelope.ToResponseYoyoEnvelope();

    // get our list of accounts
    // responseEnvelope.Data = //list of accounts
    return responseYoyoEnvelope;
}

I'd like to replace:

RequestYoyoEnvelope requestYoyoEnvelope

with an abstraction:

IRequestYoyoEnvelope requestYoyoEnvelope

however, ToResponseYoyoEnvelope is an extension method.

How do I replace a concrete class with its interface for the purpose of unit testing if I am using extension methods in that class?

Answer 1

You can write the extension method against the interface rather than the concrete class:

public static class Class2
{
    public static void Extension(this ITestInterface test)
    {
        Console.Out.WriteLine("This is allowed");
    }
}

Then you can do:

// "Test" is some class that implements the ITestInterface interface
ITestInterface useExtensionMethod = new Test();
useExtensionMethod.Extension();

Note too that this'll still work even if useExtensionMethod isn't explicitly of type ITestInterface :

Test useExtensionMethod = new Test();
useExtensionMethod.Extension();

There's controversy about whether this represents a Decorator pattern but keep in mind at a minimum that the Extension method isn't literally a part of the interface itself - it's still a static method "under the hood," it's just that the compiler's allowing you the convenience of treating this like an instance method.

Answer 2

Assuming

public class RequestYoyoEnvelope : IRequestYoyoEnvelope { ... }

Your extension method would need to target the interface

public static ResponseYoyoEnvelope ToResponseYoyoEnvelope(this IRequestYoyoEnvelope target) { ... }

Keep the action as is because the model binder will have issues binding the interface.

In your unit test you pass a concrete implementation of RequestYoyoEnvelope and the updated extension method should be able to be tested.

From your example you would not need an interface to test if that method is the method under test. just new up an instance of the model and pass it to the method during the unit test.

[TestMethod]
public void GetAccountsByYoyoIdTest() {
    //Arrange
    var controller = new YoyoController();
    var yoyoId = 123456;
    var model = new RequestYoyoEnvelope {
        //you populate properties for test
    };
    //Act
    var result = controller.GetAccountsByYoyoId(model, yoyoId);

    //Assert
    //...do your assertions.
}