How Can I Effectively Mock Extension Methods with Moq?

Use Moq to mock extension methods

In the world of unit testing, mocking plays a vital role in isolating dependencies and ensuring test stability. However, when dealing with extension methods, traditional simulation techniques seem to fall short.

Moq, a popular mocking framework, does not directly support mocking static methods, which are the cornerstone of extension methods. The error "Invalid expectation on non-overridable member" aptly summarizes this limitation.

Consider the following example:

public class SomeType {
    public int Id { get; set; }
}

var ListMock = new Mock<List<SomeType>>();
ListMock.Setup(l => l.FirstOrDefault(st => st.Id == 5))
        .Returns(new SomeType { Id = 5 });

This test will fail because Moq cannot override the static method FirstOrDefault of the List class.

Some alternatives have been proposed:

• Mock type: Mocks a type that contains extension methods and redirects calls to the mocked implementation.
• Create a wrapper: Embed the extension type into a mockable wrapper class, separating the mockable implementation from the extension method.
• Replace calls: Use a tool like PostSharp or ILWeaving to replace extension methods with mockable implementations at runtime.

However, these methods introduce additional complexity and may be impractical in some cases. Therefore, be sure to consider the suitability of mocked extension methods in your testing strategy.

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