Question

So I have this C .dll source code which I want to use in my C# application. Instead of doing bunch of DllImports I’ve decided to write a wrapper for it in C++/CLI.

My C function takes a pointer to a struct with 4 callback functions in it:

typedef struct
{
 PFN_IN readFp;
 PFN_OUT writeFp;
}OPEN_ARGS;

C++/CLI shares the same .h file, therefore uses the same typedef.

C# code has it’s own definition of this structure and delegates for CBs, because I can’t attach .h to C# project.

    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
    public delegate Int32 PFN_OUT(IntPtr arg, IntPtr pSrc, Int32 len);

    [StructLayout(LayoutKind.Sequential)]
    public struct OPEN_ARGS
    {
        public PFN_IN readFp;
        public PFN_OUT writeFp;
    };

So, when I add my C++/CLI dll explicitly to C# project references, the compliler wouldn’t accept calls to C++/CLI functions saying

"Error 2 Argument 2: cannot convert from 'WrapperTest.Program.OPEN_ARGS' to 'SAR_OPEN_ARGS'"

But if I include the C++/CLI dll implicitly like that

[DllImport("Wrapper.dll", CharSet = CharSet.Auto, EntryPoint = "?CLIOpen@@YAHHUOPEN_ARGS@@@Z")]
public static extern int CLIOpen(int a, OPEN_ARGS args);

It will work just fine.

So is there a way to tell C# compiler to ignore this type cast error, or may be other way to wrap C code functions?

EDIT: cleaned up variable names for better readabiltiy

Answer 1

What if you did this another way. Since you have a C++/CLI DLL handling interop duties between the C DLL and the C# assembly, you could expose an equivalent API, only using more .NET-like concepts.

For example, instead of exposing the struct with function pointers, you could expose a class that has three events. The C# assembly would add handlers for those events. Inside the C++ DLL, it would use the function pointers that the C DLL expects, but their implementation would fire the .NET events that the C# assembly is handling.

This would provide a much better experience using the DLL on the C# side, and likely get rid of the interop compiler errors that you’re encountering.