Working with an out parameter in Moq

I had some legacy code which required that an int be reserved and a bool be returned if it failed. The int would be returned via an out parameter. For the purpose of the test I also wanted to increment the int every time it was requested.

Setup of a method with a method with an out parameter

It was tricky to achieve this with Moq, but I got there in the end. I had to pass the int using It.Ref<int>:

_mock.Setup(cir => cir.TryReserveIndex(out It.Ref<int>.IsAny))

Accepting the out parameter in a Returns callback

And then accept it in the Returns callback with a ref, after incrementing a local index that its value is assigned from:

.Callback(() => ++currentIndex)
.Returns((ref int index) =>
{
   index = currentIndex;
   return true;
});

The full example

var currentIndex = 19200;
_mock.Setup(cir => cir.TryReserveIndex(out It.Ref<int>.IsAny))
     .Callback(() => ++currentIndex)
     .Returns((ref int index) =>
     {
        index = currentIndex;
        return true;
     });

int i;
var indexReserved = _mock.Object.TryReserveIndex(var out i);

Assert.True(indexReserved);
Assert.Equal(19201, i)

Ideally I’d avoid out parameters, and I have since refactored this out, but I’m putting this here in case I need to do this with Moq again in the future.