Question

I have a simple managed C++ assembly which I am providing unmanaged wrappers for some static methods I have in a C# assembly. One of these methods returns a string which I have converted to a “const char*” type in the C++ assembly. The trouble I am having is that when I load this dll from an unmanaged source, I get bad data back in the pointer. What can I do about this? I have simplified my testcase down to this:

managed assembly (Test.dll compiled with /clr; full code follows):

extern "C" {
    __declspec(dllexport) const char* GetValue(const char* s) {
        return s;
    }
}

unmanaged console win32 app:

#include "stdafx.h"
#include <stdio.h>
#include <tchar.h>
#include <windows.h>

typedef const char* (*GetValueFunc)(const char* s);

int _tmain(int argc, _TCHAR* argv[]) {
    wprintf(L"Hello from unmanaged code\n");
    HMODULE hDll = LoadLibrary(L"Test.dll");
    if (!hDll)
        return GetLastError();

    wprintf(L"library found and loaded\n");

    GetValueFunc getValue = (GetValueFunc)GetProcAddress(hDll, "GetValue");
    if(!getValue)
        return GetLastError();
    wprintf(L"%s\n", getValue("asdf"));

    return 0;
}

I get the first two lines just fine, but the third line that comes out is garbage. It also doesn’t matter if I make the dll have “#pragma unmanaged” at the top of the cpp file or not (same results).

Answer 1

wprintf with a %s format specifier will interpret your first parameter as a “const wchar_t*”, while you’re passing a “const char*” to it. Try to use wchar_t in your GetValue function.