Dummy, Fake, Stub, Mock, Spy

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////////// Dummy
// It’s a placeholder required to pass the unit test.

[TestMethod]
public void PlayerRollDieWithMaxFaceValueStrictTest()
{
  var dummyBoard = new Mock<IBoard>(MockBehavior.Strict); //Ensure Board class is never invoked
  var player = new Player( dummyBoard.Object, new Die() );
  player.RollDie();
  Assert.AreEqual( 6, player.UnitsToMove );
}

////////// Fake
// Simplify a dependency so that unit test can pass easily.

public interface IProductRepository
{
  void AddProduct(IProduct product);
  IProduct GetProduct(int productId);
}

public class FakeProductRepository : IProductRepository
{
  List<IProduct> _products = new List<IProduct>();
  public void AddProduct(IProduct product)
  {  }
  public IProduct GetProduct(int productId)
  {  }
}

[TestMethod]
public void BillingManagerCalcuateTax()
{
  var fakeProductRepository = new FakeProductRepository();
  BillingManager billingManager = new BillingManager(fakeProductRepository);
  //...
}

////////// Stub
// Provide indirect inputs to the SUT coming from its collaborators / dependencies.

// The following example: use a Stub to return a fixed face value. This could simply our tests by taking out the randomness associated with rolling a Die.

[TestMethod]
public void PlayerRollDieWithMaxFaceValue()
{
  var stubDie = new Mock<IDie>();
  stubDie.Setup(d => d.GetFaceValue()).Returns(6).Verifiable();
  IDie die = stubDie.Object;
  Assert.AreEqual(6, die.GetFaceValue()); //Excercise the return value
}

////////// Mock
// For behavior verification. We can set expectations for SUT to exhibit the right behavior during its interactions with collaborators. Classic example of this is logging.

[TestMethod]
public void ModuleThrowExceptionInvokesLogger()
{
  var mock = new Mock<ILogger>();
  Module module = new Module();
  ILogger logger = mock.Object;
  module.SetLogger(logger);
  module.ThrowException("Catch me if you can");
  mock.Verify( m => m.Log( "Catch me if you can" ) );
}

////////// Spy
// A variation of behavior verification. Instead of setting up behavior expectations, Spy records calls made to the collaborator.

[TestMethod]
public void ModuleThrowExceptionInvokesLoggerOnlyOnce()
{
  var spyLogger = new Mock<ILogger>();
  Module module = new Module();
  ILogger logger = spyLogger.Object;
  module.SetLogger( logger );
  module.ThrowException( "Catch me if you can" );
  module.ThrowException( "Catch me if you can" );
  spyLogger.Verify( m => m.Log( It.IsAny<string>()), Times.Exactly(2) );
}

Other references: martinflower.com: mock aren't stubs.