Unit testing with Moq - Returning different values from multiple calls to the same method in a loop, and then verifying multiple calls to another method.

I was doing some TDD the other day in a C# .net service and found myself wanting to write a loop that called a couple of methods and acted on them in different ways depending on the return values. I needed to mock the calls so they returned different data depending on how many times the methods were invoked.

This is the method under test that I ended up with after doing the TDD cycle. (I've stripped out all the exception handling and logging to keep this example clear)

using JourneyHeader.Domain.Entities;
namespace journeyMigration
{
  public class JourneyMigrator
  {
    private readonly JourneySource _journeySource;
    private readonly JourneyDestination _journeyDestination;

    public JourneyMigrator(JourneySource journeySource, JourneyDestination journeyDestination)
    {
      _journeySource = journeySource;
      _journeyDestination = journeyDestination;
    }

    public int JourneysProcessed { get; private set; }
    public int JourneysFailedProcessing { get; private set; }
    public JourneyHeader LastJourneyProcessed { get; private set; }

    public void Start()
    {
      JourneyHeader journeyHeader = _journeySource.GetNextJourney();
      while (journeyHeader != null)
      {
        _journeyDestination.Upload(journeyHeader);
        JourneysProcessed++;               
        LastJourneyProcessed = journeyHeader;   
        journeyHeader = _journeySource.GetNextJourney();
      }
    }
  }
}

The lines we really care about are 21 through 28. Notice that GetNextJourney() is getting called many times depending on the value returned last time. Also see that on line 24 the upload() method is called inside the loop, I want to verify I was passing the correct values through to it.

This is one of the tests I came up with whilst writing this code, its a good example of the 2 things I wanted to demonstrate here.

using System;
using System.Collections.Generic;
using FluentAssertions;
using JourneyHeader.Domain.Entities;
using journeyMigration;
using Moq;
using NUnit.Framework;

namespace journeyMigrationTests
{
  [TestFixture]
  public class JourneyMigratorTests
  {
    [Test]
    public void ShouldProcessTwoJourneys()
    {
      var journeyHeader1 = new JourneyHeader();
      var journeyHeader2 = new JourneyHeader();
      var queue = new Queue<JourneyHeader>(new [] {journeyHeader1, journeyHeader2, null});
      _journeyHeaderSource.Setup(x => x.GetNextJourney()).Returns(queue.Dequeue);
 
      _journeyMigrator.Start();
 
      _journeyMigrator.JourneysProcessed.Should().Be(2);
      _journeyMigrator.LastJourneyProcessed.Should().Be(journeyHeader2);
      _journeyDestination.Verify(x => x.Upload(journeyHeader1), Times.Exactly(1));
      _journeyDestination.Verify(x => x.Upload(journeyHeader2), Times.Exactly(1));
    }
  }
}

The interesting lines here are 16 through 20 where im setting up a queue that is used to return the values in the prescribed order. You have to do it this way because in Moq you cant do multiple Setups on a given classes method, the last one to be defined will win. In the following example journeyHeader2 is always returned.
_journeyHeaderSource.Setup(x => x.GetNextJourney()).Returns(journeyHeader1);
_journeyHeaderSource.Setup(x => x.GetNextJourney()).Returns(journeyHeader2);

The Returns method takes a value as above or a function that is run every time a return value is required, you could write it like this.
_journeyHeaderSource.Setup(x => x.GetNextJourney()).Returns(() => queue.Dequeue());
But the one I've used on line 20 is a lot clearer.

Finally lines 26 and 27 verify the method Upload was called correctly, once with the first journeyHeader and once with the second.

Appendix
Moq - A popular and friendly mocking framework for .NET