#nsubstitute

Mocking Generic Method with NSubstitute

The generic method:

public interface ICacheService { T Get<T>(string id); }

User object in NSubstitute to mock the generic method.

private ICacheService MockICacheService() { var service = Substitute.For(); // Return Models object returnForAny = null; // Mock service.Get(Arg.Any()).ReturnsForAnyArgs(returnForAny); return service; }

The usual way to stub a property in NSubstitute is something like: -

myMockObject.SomeProperty.Returns(value)

But what if you only have the mock object as an untyped object, but you want to stub a method on it anyway? The NSubstitute documentation doesn’t tell you how to do that, but it can be done.

First, you need to get hold of a PropertyInfo via reflection, then you invoke it on the mock object: -

myProperty.GetMethod.Invoke(myMockObject, null)

This sets it as the “last call” in NSubstitute. So now you can use the substitution context to set the return value: -

SubstitutionContext.Current.LastCallShouldReturn(new ReturnValue(value), MatchArgs.Any)

It’s important that the above call immediately follows the Invoke(). Any code in between can change the “last call” in NSubstitute.

The same kind of thing can be done for methods, as opposed to properties. I leave that as an exercise for the reader.

Return Null in NSubstitute Mock Framework

The API is not intuitive, at least not for me. So, I have to figure out how to return null with this mocking framework I have never used before.

var service = Substitute.For<ISomethingService>(); // Mock service.Get<IEnumerable<SomeDomain>>(Arg.Any<string>()).ReturnsForAnyArgs(x => null);

Normally, I would just to .ReturnsForAnyArgs(null), not the case with NSubstitute.

If we have a dependency that can be abstracted as a single Action<> or Func<> there is no point (at least more often than not) to create a specific interface.

How to spy on this dependency then?

With NSubstitute it is, it turns out, surprisingly easy:

setClipboard = Substitute.For<Action<string>>(); sut = new SomeSut(setClipboard); sut.CopyAsText(); setClipboard.Received().Invoke("some expected text");

As you would expect, the test will pass if SomeSut.CopyAsText does: