Support ref returns (c# 7)

[tzachshabtay]

My interface which I'm trying to mock has a ref return in it:

ref ModelMatrices GetModelMatrices();

The code for the interface is here:

https://github.com/tzachshabtay/MonoAGS/blob/734b13f467d33b5099b81d52f3d391448b41b9eb/Source/AGS.API/Objects/IModelMatrixComponent.cs#L36

If I just create my mock without overriding the behavior and then call GetModelMatrices on it, it crashes with:

System.InvalidProgramException : Invalid IL code in Castle.Proxies.IObjectProxy:GetModelMatrices (): IL_0030: ret

And I don't see how I can setup the return?
If I do:
_myMock.Setup(m => m.GetModelMatrices()).Returns(ref something);

I get a compile error: Error CS8153: An expression tree lambda may not contain a call to a method, property, or indexer that returns by reference (CS8153) (Tests)

[stakx]

Hi @tzachshabtay, thank you for reporting this. Quite frankly, my initial gut feeling is that it will be impossible to realize this feature in Moq. First, there are two roadblocks to be overcome before we could even start work in Moq:

Road block 1: DynamicProxy does not currently appear to support ref returns.

If I just create my mock without overriding the behavior and then call GetModelMatrices on it, it crashes with:

System.InvalidProgramException : Invalid IL code in Castle.Proxies.IObjectProxy:GetModelMatrices (): IL_0030: ret

This is an issue with Castle DynamicProxy (upon which Moq relies to generate mock proxy types), which apparently does not currently support ref returns.

We could raise an issue over at https://github.com/castleproject/Core/issues to discuss this feature, and possibly submit a PR with the necessary changes. (However I'm not certain at this point whether it would even be possible for DynamicProxy to properly support ref returns.)

Roadblock 2: The C# compiler does not allow ref returns in LINQ expression trees.

And I don't see how I can setup the return?
If I do:
_myMock.Setup(m => m.GetModelMatrices()).Returns(ref something);

I get a compile error: Error CS8153: An expression tree lambda may not contain a call to a method, property, or indexer that returns by reference (CS8153) (Tests)

This in turn is a current limitation of the C# language / compiler. There's not much we can do in Moq. I suggest you add your vote in dotnet/csharplang#158 if you want this changed.

[tzachshabtay]

@stakx can you open an issue for DynamicProxy then?
Thanks.

[stakx]

@tzachshabtay: Perhaps I will, just to get confirmation what I already suspect (with a fairly high degree of confidence): that DynamicProxy won't be able to support ref returns, due to the way how its central IInvocation and IInterceptor abstractions are designed.

Moq 5 just might be able to support ref returns (@kzu - correct?), but don't get your hopes up for Moq 4. :-/

[kzu]

Will keep this in mind for v5