How do I marshal a structure as a pointer to a structure?

Question

I am trying to pass a structure from C# into C++ library. I pass structure as an object, and C++ function expects it as a pointer (void *).

I am having problem passing the structure.

[DllImport("MockVadavLib.dll", CharSet = CharSet.Ansi)]
public static extern IntPtr TheFunction([MarshalAs(UnmanagedType.LPStruct)] UserRec userRec);

Here is the run-time exception text I get:

"Cannot marshal 'parameter #1': Invalid managed/unmanaged type combination (this value type must be paired with Struct)."

Though I found an MSDN article that uses LPStruct in exactly this context.

This is my structure I'm trying to marshal:

[StructLayout(LayoutKind.Sequential)]
public struct UserRec {
    [MarshalAs(UnmanagedType.I4)]
    public int userParam1;
}

This is C++ function:

MOCKVADAVLIB_API tVDACQ_CallBackRec * TheFunction(void * userParams) {...

Answer 1

Try passing the structure as a ref parameter.

[DllImport("MockVadavLib.dll", CharSet = CharSet.Ansi)]
public static extern IntPtr TheFunction(ref UserRec userRec);

When you use a ref combined with a structure, it conceptually passes the address.

Answer 2

Incidentally, UnmanagedType.LPStruct is rarely, if ever, the correct MarshalAs argument. A quote from Adam Nathan who is a Microsoft employee:

UnmanagedType.LPStruct is only supported for one specific case: treating a System.Guid value type as an unmanaged GUID with an extra level of indirection.