Using Moq to verify calls are made in the correct order

Question

I need to test the following method:

CreateOutput(IWriter writer)
{
    writer.Write(type);
    writer.Write(id);
    writer.Write(sender);

    // many more Write()s...
}

I've created a Moq'd IWriter and I want to ensure that the Write() methods are called in the right order.

I have the following test code:

var mockWriter = new Mock<IWriter>(MockBehavior.Strict);
var sequence = new MockSequence();
mockWriter.InSequence(sequence).Setup(x => x.Write(expectedType));
mockWriter.InSequence(sequence).Setup(x => x.Write(expectedId));
mockWriter.InSequence(sequence).Setup(x => x.Write(expectedSender));

However, the second call to Write() in CreateOutput() (to write the id value) throws a MockException with the message "IWriter.Write() invocation failed with mock behavior Strict. All invocations on the mock must have a corresponding setup.".

I'm also finding it hard to find any definitive, up-to-date documentation/examples of Moq sequences.

Am I doing something wrong, or can I not set up a sequence using the same method? If not, is there an alternative I can use (preferably using Moq/NUnit)?

Solution 1

There is bug when using MockSequence on same mock. It definitely will be fixed in later releases of Moq library (you can also fix it manually by changing Moq.MethodCall.Matches implementation).

If you want to use Moq only, then you can verify method call order via callbacks:

int callOrder = 0;
writerMock.Setup(x => x.Write(expectedType)).Callback(() => Assert.That(callOrder++, Is.EqualTo(0)));
writerMock.Setup(x => x.Write(expectedId)).Callback(() => Assert.That(callOrder++, Is.EqualTo(1)));
writerMock.Setup(x => x.Write(expectedSender)).Callback(() => Assert.That(callOrder++, Is.EqualTo(2)));

Solution 2

I've managed to get the behaviour I want, but it requires downloading a 3rd-party library from http://dpwhelan.com/blog/software-development/moq-sequences/

The sequence can then be tested using the following:

var mockWriter = new Mock<IWriter>(MockBehavior.Strict);
using (Sequence.Create())
{
    mockWriter.Setup(x => x.Write(expectedType)).InSequence();
    mockWriter.Setup(x => x.Write(expectedId)).InSequence();
    mockWriter.Setup(x => x.Write(expectedSender)).InSequence();
}

I've added this as an answer partly to help document this solution, but I'm still interested in whether something similar could be achieved using Moq 4.0 alone.

I'm not sure if Moq is still in development, but fixing the problem with the MockSequence, or including the moq-sequences extension in Moq would be good to see.

Solution 3

I wrote an extension method that will assert based on order of invocation.

public static class MockExtensions
{
  public static void ExpectsInOrder<T>(this Mock<T> mock, params Expression<Action<T>>[] expressions) where T : class
  {
    // All closures have the same instance of sharedCallCount
    var sharedCallCount = 0;
    for (var i = 0; i < expressions.Length; i++)
    {
      // Each closure has it's own instance of expectedCallCount
      var expectedCallCount = i;
      mock.Setup(expressions[i]).Callback(
        () =>
          {
            Assert.AreEqual(expectedCallCount, sharedCallCount);
            sharedCallCount++;
          });
    }
  }
}

It works by taking advantage of the way that closures work with respect to scoped variables. Since there is only one declaration for sharedCallCount, all of the closures will have a reference to the same variable. With expectedCallCount, a new instance is instantiated each iteration of the loop (as opposed to simply using i in the closure). This way, each closure has a copy of i scoped only to itself to compare with the sharedCallCount when the expressions are invoked.

Here's a small unit test for the extension. Note that this method is called in your setup section, not your assertion section.

[TestFixture]
public class MockExtensionsTest
{
  [TestCase]
  {
    // Setup
    var mock = new Mock<IAmAnInterface>();
    mock.ExpectsInOrder(
      x => x.MyMethod("1"),
      x => x.MyMethod("2"));

    // Fake the object being called in order
    mock.Object.MyMethod("1");
    mock.Object.MyMethod("2");
  }

  [TestCase]
  {
    // Setup
    var mock = new Mock<IAmAnInterface>();
    mock.ExpectsInOrder(
      x => x.MyMethod("1"),
      x => x.MyMethod("2"));

    // Fake the object being called out of order
    Assert.Throws<AssertionException>(() => mock.Object.MyMethod("2"));
  }
}

public interface IAmAnInterface
{
  void MyMethod(string param);
}

Solution 4

The simplest solution would be using a Queue:

var expectedParameters = new Queue<string>(new[]{expectedType,expectedId,expectedSender});
mockWriter.Setup(x => x.Write(expectedType))
          .Callback((string s) => Assert.AreEqual(expectedParameters.Dequeue(), s));

Solution 5

Recently, I put together two features for Moq: VerifyInSequence() and VerifyNotInSequence(). They work even with Loose Mocks. However, these are only available in a moq repository fork:

https://github.com/grzesiek-galezowski/moq4

and await more comments and testing before deciding on whether they can be included in official moq releaase. However, nothing prevents you from downloading the source as ZIP, building it into a dll and giving it a try. Using these features, the sequence verification you need could be written as such:

var mockWriter = new Mock<IWriter>() { CallSequence = new LooseSequence() };

//perform the necessary calls

mockWriter.VerifyInSequence(x => x.Write(expectedType));
mockWriter.VerifyInSequence(x => x.Write(expectedId));
mockWriter.VerifyInSequence(x => x.Write(expectedSender));

(note that you can use two other sequences, depending on your needs. Loose sequence will allow any calls between the ones you want to verify. StrictSequence will not allow this and StrictAnytimeSequence is like StrictSequence (no method calls between verified calls), but allows the sequence to be preceeded by any number of arbitrary calls.

If you decide to give this experimental feature a try, please comment with your thoughts on: https://github.com/Moq/moq4/issues/21

Thanks!

View more solutions

Comments

[g t]

I've double-checked and the values are correct and as expected. Rolling my own mocks is something I try to avoid now I use Moq, but may have to resort to! And Verify() cannot be added after the setup, only Verifiable() - which sadly makes no difference. Thanks for the pointers though.

[John Nicholas]

shame you can't pm. Cool didn't know that existed :D. But you are really stubbing btw once you get into this territory which maybe why moq is a bit lacking in this area - and yeah the project seems very quiet. Looked at its issues and sequencing has been on there for 4 years outstanding. I also noticed the latest release does have final in its name.

[g t]

Thanks for the information, I've resorted to using this method. Hope there will be later releases of Moq! One of the best testing tools there is...

[Adam Naylor]

One word of warning here, if the Write() method is never invoked, none of these callbacks will have a chance to assert anything. Be sure to add a catch all verification that the method is invoked at least once.

[Ray]

There is ongoing discussion on github regarding sequencing, for anyone who wants to contribute / comment: github.com/moq/moq4/issues/75

[Auguste Van Nieuwenhuyzen]

This is a great idea, thanks! I actually prefer this, as it means I can overwrite the setups I had before. I did add .Verifiable to each setup, and a .Verify() on the end, to check that they were actually run, as well as being run in the right order :)