Passing values by reference from a C# dll to a c++ dll

I had a managed c++ function that would take for one of its arguments a double value passed in as a reference. void MCppFunctions::ChangeValue (double &dValue) {...} So how does one call a function like this from c#. If you try the obvious method MCppFunctions.ChangeValue(ref dValue), you will get a compile error. To fix this problem, all you need to do is change the cpp function definition to the following: void MCppFunctions::ChangeValue(double __gc & dValue) Once thats done you can call the function from c# and pass in the value and qualifying the parameter with the ref keyword.