After switching to VS2010, the managed debug assistant is displaying an error about an unbalanced stack from a call to an unmanaged C++ function from a C# application.
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You specify stdcall in C# but not in C++, a mismatch here will lead to both the function and the caller popping arguments off of the stack.
On the other hand there is a compiler switch that will turn on stdcall as the default calling convention, (-Gz) are you using that?
Or try this in your C++
short __stdcall SuperSpecialOpenFileFunc(SuperSpecialStruct * stuff);