Assigning out parameters in Moq for methods that return void

Question

In this question, I found an this answer that seems to be the best way to solve the problem to me.

The provided code assumes that the function that is mocked returns a value:

bool SomeFunc(out ISomeObject o);

However, the object I want to mock has an out function as follows:

void SomeFunc(out ISomeObject o);

The relevant code fragment from the mentioned answer:

public delegate void OutAction<TOut>(out TOut outVal);

public static IReturnsThrows<TMock, TReturn> OutCallback<TMock, TReturn, TOut>(
    this ICallback<TMock, TReturn> mock, OutAction<TOut> action)
    where TMock : class
{
    // ...
}

Void is not a valid type for TReturn. So I believe I would have to somehow adapt this code to get it to work with methods returning void. But how?

Solution

Maybe you simply need this:

ISomeObject so = new SomeObject(...);
yourMock.Setup(x => x.SomeFunc(out so));

Then when you use yourMock.Object in the code you test, the so instance will "magically" come out as the out parameter.

It is a little non-intuitive ("out is in"), but it works.

---

Addition: Not sure I understand the scenario. The following complete program works fine:

static class Program
{
  static void Main()
  {
    // test the instance method from 'TestObject', passing in a mock as 'mftbt' argument
    var testObj = new TestObject();

    var myMock = new Mock<IMyFaceToBeTested>();
    IMyArgFace magicalOut = new MyClass();
    myMock.Setup(x => x.MyMethod(out magicalOut)).Returns(true);

    testObj.TestMe(myMock.Object);
  }
}

class TestObject
{
  internal void TestMe(IMyFaceToBeTested mftbt)
  {
    Console.WriteLine("Now code to be tested is running. Calling the method");
    IMyArgFace maf; // not assigned here, out parameter
    bool result = mftbt.MyMethod(out maf);
    Console.WriteLine("Method call completed");
    Console.WriteLine("Return value was: " + result);
    if (maf == null)
    {
      Console.WriteLine("out parameter was set to null");
    }
    else
    {
      Console.WriteLine("out parameter non-null; has runtime type: " + maf.GetType());
    }
  }
}

public interface IMyFaceToBeTested
{
  bool MyMethod(out IMyArgFace maf);
}
public interface IMyArgFace
{
}
class MyClass : IMyArgFace
{
}

Please indicate how your situation is different, by using the names of the classes and interfaces from my example.