Mocking Extension Methods with Moq

Question

I have a preexisting Interface     public interface ISomeInterface       void SomeMethod        and I ve extended this intreface using a mixin     public static class SomeInterfaceExtensions       public static void AnotherMethod this ISomeInterface someInterface                   Implementation here           I have a class thats calling this which I want to test     public class Caller       private readonly ISomeInterface someInterface       public Caller ISomeInterface someInterface                this someInterface   someInterface             public void Main                 someInterface AnotherMethod              and a test where I d like to mock the interface and verify the call to the extension method          Test      public void Main BasicCall CallsAnotherMethod                    Arrange         var someInterfaceMock   new Mock lt ISomeInterface gt             someInterfaceMock Setup x   gt  x AnotherMethod    Verifiable             var caller   new Caller someInterfaceMock Object               Act         caller Main                Assert         someInterfaceMock Verify            Running this test however generates an exception     System ArgumentException  Invalid setup on a non-member method  x   gt  x AnotherMethod     My question is  is there a nice way to mock out the mixin call

User · Answer

You can easily mock an extension method with JustMock. The API is the same as mocking normal method. Consider the following

public static string Echo(this Foo foo, string strValue) 
{ 
    return strValue; 
}

To arrange and verify this method use the following:

string expected = "World";

var foo = new Foo();
Mock.Arrange(() => foo.Echo(Arg.IsAny<string>())).Returns(expected);

string result = foo.Echo("Hello");

Assert.AreEqual(expected, result);

Here is also a link to the documentation: Extension Methods Mocking

Disclaimer. I am one of the developers responsible for JustMock.

User · Answer

I have used a Wrapper to get around this problem  Create a wrapper object and pass your mocked method   See Mocking Static Methods for Unit Testing by Paul Irwin  it has nice examples

User · Answer

So if you are using Moq  and want to mock the result of an Extension method  then you can use SetupReturnsDefault lt ReturnTypeOfExtensionMethod gt  new ConcreteInstanceToReturn    on the instance of the mock class that has the extension method you are trying to mock    It is not perfect  but for unit testing purposes it works well

User · Answer

I found that I had to discover the inside of the extension method I was trying to mock the input for  and mock what was going on inside the extension   I viewed using an extension as adding code directly to your method   This meant I needed to mock what happens inside the extension rather than the extension itself

User · Answer

I like to use the wrapper  adapter pattern  when I am wrapping the object itself  I m not sure I d use that for wrapping an extension method  which is not part of the object   I use an internal Lazy Injectable Property of either type Action  Func  Predicate  or delegate and allow for injecting  swapping out  the method during a unit test       internal Func lt IMyObject  string  object gt  DoWorkMethod                ExcludeFromCodeCoverage          get   return  DoWorkMethod      DoWorkMethod    obj  val    gt    return obj DoWork val                 set    DoWorkMethod   value          private Func lt IMyObject  string  object gt   DoWorkMethod    Then you call the Func instead of the actual method       public object SomeFunction                 var val    doesn t matter for this example           return DoWorkMethod Invoke MyObjectProperty  val           For a more complete example  check out http   www rhyous com 2016 08 11 unit-testing-calls-to-complex-extension-methods

User · Answer

You can t  quot directly quot  mock static method  hence extension method  with mocking framework  You can try Moles  http   research microsoft com en-us projects pex downloads aspx   a free tool from Microsoft that implements a different approach  Here is the description of the tool   Moles is a lightweight framework for test stubs and detours in  NET that is based on delegates  Moles may be used to detour any  NET method  including non-virtual static methods in sealed types   You can use Moles with any testing framework  it s independent about that

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