What happens on the stack with ref parameters in c#?

Question

Im reading some C# documentation about WCF and IDispatchMessageInspector and the interface defines a 'Message' object that is passed by reference so that it can be manipulated.

What actually happens on the stack when you pass something by ref as opposed to passing normally?

Answer 1

It's not the object that's passed by reference - it's the variable.

Basically it aliases the variable used as the argument from the calling side, and the parameter in the method that you call:

public void Foo()
{
    int x = 10;
    Bar(ref x);
    Console.WriteLine(x); // Prints 20
}

public void Bar(ref int y)
{
    y = 20;
}

Here, x and y are essentially the same variable - they refer to the same storage location. Changes made to x are visible via y and vice versa. (Note that in this case it's a local variable in the caller, but it doesn't have to be - if you'd passed an instance variable by reference, then Bar might call another method which changes the same variable, and then y would be seen to "magically" change...)

For more about parameter passing in C#, refer to my article on the subject.