Example showing how to mock a method from an interface using Moq. From http://hjerpbakk.com/blog/2013/4/15/moq-mock-a-method-from-an-interface.html

MoqExample.cs

public class MyClass {
	private readonly ComplexClass complexClass;
	
	public MyClass(ComplexClass complexClass) {
		this.complexClass = complexClass;
	}
	
	public void Verify() {
		((IVerifiable)complexClass).Verify();
	}
	
	// Imagine many more uninteresting methods...  
	public void Uninteresting1() {
	}
}

public class ComplexClass : IVerifiable {
	public object NeededProperty { get; set; }
	
	// Verifies the state of the object
	public void Verify() {
		if(NeededProperty == null) {
			// Don't use this exception message in production :)
			throw new InvalidOperationException("Method is invalid for the current state of the object");
		}
	}
	
	// Imagine many more uninteresting methods...  
	public void Uninteresting2() {
	}
}

public interface IVerifiable {
	void Verify();
}

[TestClass]
public class MyClassTests {
	[TestMethod]
	public void Verify_ComplexObjectIsValid_ShouldNotThrowException() {
		var complexClass = new ComplexClass();
		var myClass = new MyClass(complexClass);
		
		myClass.Verify();
		
		// Test fails with exception :( Must be replaced with a working test.
	}
	
	[TestMethod]
	public void Verify_ComplexObjectIsValid_ShouldReallyNotThrowException() {
		var complexClassFake = new Mock<ComplexClass>();
		var verifiable = complexClassFake.As<IVerifiable>();
		verifiable.Setup(v => v.Verify()).Verifiable();
		var myClass = new MyClass(complexClassFake.Object);
		
		myClass.Verify();
		
		verifiable.Verify();
		// Test works as expected :)
	}
}