C# - Assign Mocked dependencies that are Imported from MEF

Question

I'm using MEF for dependency injection and I'm at this point writing a unit test in which I'd have to inject a mocked dependency, but I have declared it as { get; protected set; } which renders me unable to do it.

The dependency is imported using the [Import] attribute, which means it isn't passed via constructor or anything similar.

What is the right approach, and by "right" I mean it doesn't violate any principle like SOLID and similar, to solve this inconvenient?

Code:

[Import]  // MEF is able to inject it even tho it's set accessor is protected
IMyDependencie MyDependency { get; protected set; }

Answer 1

Unless the class is sealed, you can create a class which inherits from it and sets the protected property:

public class TestClass : ClassToBeTested
{
    public TestClass(IMyDependencie mockDependency)
    {
        // Set protected property:
        MyDependency = mockDependency;
    }
}

Then in the setup of your unit test, create the object with a mocked dependency:

 // These will be fields in the test class.
 Mock<IMyDependencie> mockDependency = new Mock<IMyDependencie>();

 TestClass tc = new TestClass(mockDependency.Object);

An alternative - which you would have to use if the class under test is sealed - is to use a PrivateObject to set the property, or just just plain old reflection to do it.