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 pas

Answer 1

Some additional information followup regarding @Rytmis's post.

From https://github.com/dotnet/runtime/blob/master/docs/coding-guidelines/interop-pinvokes.md:

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Guids are usable directly in signatures. When passed by ref they can either be passed by ref or with the [MarshalAs(UnmanagedType.LPStruct)] attribute.

[MarshalAs(UnmanagedType.LPStruct)] should only be used for by ref Guids.

Answer 2

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 3

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.