Question

Using this function in my C# exe, I try to pass a Unicode string to my C++ DLL:

    [DllImport("Test.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.StdCall)]
    public static extern int xSetTestString(StringBuilder xmlSettings);

This is the function on the C++ DLL side:

__declspec(dllexport) int xSetTestString(char* pSettingsXML);

Before calling the function in C#, I do a MessageBox.Show(string) and it displays all characters properly. On the C++ side, I do: OutputDebugStringW((wchar_t*)pString);, but that shows that the non-ASCII characters were replaced by ‘?’.

Answer 1

Just change your export in native DLL to:

extern "C" __declspec(dllexport) int xSetTestString(wchar_t* pSettingsXML);

This will do the trick.

BTW – You cant simply do char* str1 = (wchar_t*)pSettingsXML; because it does not convert the string. You need to use wcstombs_s to convert from wchar_t* to char*. But in your case you don’t have to do it.

Notes: Best practice IMO is to use TCHAR* instead of wchar_t* directly, and set your native dll project General option Character Set to Use Unicode Character Set. This defines TCHAR* as wchar_t*.

Mirosoft natively uses two sets of functions: ANSI, using 1-byte chracter, marked as FunctionNameA and Unicode, using 2-bytes character, marked as FunctionNameW. This Unicode is in fact UTF-16.

UTF-8 is a multi-byte string that uses 1-byte for standard character and 2-bytes for non-standard characters. To convert UTF-8 to UTF-16 you can use MultiByteToWideChar function.