Call function in C++ dll from C#

Question

I'm trying to call a function in a C++ dll, from C# code.

The C++ function :

#ifdef NT2000
      __declspec(dllexport)
    #endif

    void MyFunction
            (   long            *Code,
                long            Number,
                unsigned char   *Reference,
                unsigned char   *Result         ) ;

And my C# call is :

 [DllImport("mydll.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void MyFunction(ref IntPtr Code,
        int Number,
        string Reference,
        ref IntPtr Result);

This function is supposed to return "Code" and "Result"

The "Code" value that I get seems to be ok, but "Result" seems not ok, so I'm wondering if I'm getting the string value that I expect or the memory address ?

Thanks for your help.

Answer 1

long in C maps to int in C#. And char is ANSI, so you need to specify UnmanagedType.LPStr. The Result appears to be an out parameter, for which you will need to pre-assign a buffer (size unknown?) and pass it as StringBuilder

[DllImport("mydll.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void MyFunction(
        ref int Code,
        int Number,
        [MarshalAs(UnmanagedType.LPStr)]
        string Reference,
        [MarshalAs(UnmanagedType.LPStr), Out]
        StringBuilder Result);

Use it like this

var buffer = new StringBuilder(1024); // or some other size
MyFunction(ref myCode, myNumber, myReference, buffer);