Using Moq to set indexers in C#

Question

I'm having trouble figuring out how to set indexers in C# with Moq. The Moq documentation is weak, and I've done a lot of searching... what I'd like to do is similar in the solution to How to Moq Setting an Indexed property:

var someClass = new Mock<ISomeClass>();
someClass.SetupSet(o => o.SomeIndexedProperty[3] = 25);

I want to modify the above to work for any index and any value so I can just do something like this:

someClass.Object.SomeIndexedProperty[1] = 5;

Currently I have the following, which works great for the indexed property getter, but if I ever set the value Moq ignores it:

var someValues = new int[] { 10, 20, 30, 40 };
var someClass = new Mock<ISomeClass>();
someClass.Setup(o => o.SomeIndexedProperty[It.IsAny<int>()])
    .Returns<int>(index => someValues[index]);

// Moq doesn't set the value below, so the Assert fails!
someClass.Object.SomeIndexedProperty[3] = 25;
Assert.AreEqual(25, someClass.Object.SomeIndexedProperty[3]);

Answer 1

In your test your only set expectations for the getter of the indexer - and this will always return the corresponding value from your array. The indexer doesn't behave like a real thing. You can try a few things to have it fixed:

1. You can check if you can stub this indexer. I am personally using RhinoMocks, so I have no idea if this is implemented

   someClass.SetupProperty(f => f.SomeIndexedProperty);

2. You can try to mock the setter as well and make it to change the underlying array. My first guess would be to try something along these lines (no guarantee it will work):

   someClass.Setup(o => o.SomeIndexedProperty[It.IsAny()] = It.IsAny()) .Callback((index, value) => someValues[index] = value);

3. Or if the ISomeClass interface is small enough, you could just created a stub implementation yourself (that would have the indexer implemented as a dictionary, array etc.)

Answer 2

I found a solution to my problem. It requires a shift in my thinking... I had been looking to use NUnit's Assert to verify that the indexer setter in my someClass mock was being called with the correct value.

It seems that with Moq the supported way to do this is to use the VerifySet function. A contrived example:

someClass.Object.SomeIndexedProperty[3] = 25;
someClass.VerifySet(mock => mock.SomeIndexedProperty[3] = 25);

The unit test passes since SomeIndexedProperty[3] is set to 25, but would fail otherwise.