Call a wstring C++ method from C#

Question

I am using C# code and need to call this method inside a C++ dll.

static std::wstring DecryptData(const std::wstring& strKey);

I've read many things and my best guess would be to pass something that is easier to read for both languages, like char-array or even byte-array and build the wstring in C++ and the string in C# afterwards.

Did anyone do that already?

Edit:

I read the linkes Topic but none of the answers helps me: Using const didn't help.

This is what I have now: C#

 [DllImport(DLL_PATH, CharSet = CharSet.Unicode)]
        [return: MarshalAs(UnmanagedType.LPWStr)]
        public static extern string DecryptData([MarshalAs(UnmanagedType.LPWStr)]string strKey);

C++

extern "C" __declspec(dllexport) const std::wstring DecryptData(const std::wstring& strKey) {
    return WlanHelper::CWirelessHelper::DecryptData(strKey);
}

This gives me a PInvokeStackImbalance =/

Answer 1

You may find this question and this question to be relevant. There are two issues:

1. P/Invoke does not natively marshal std::string/std::wstring, and
2. Possible memory lifetime issues (depends on implementation of CWirelessHelper::DecryptData).

An approach is to copy the string to a plain wchar_t* buffer allocated using CoTaskMemAlloc (the framework will handle the string conversion and free the allocated memory).

On the unmanaged side, the code becomes:

extern "C" __declspec(dllexport) const wchar_t* DecryptData( wchar_t* strKey) {
    std::wstring retstr = WlanHelper::CWirelessHelper::DecryptData(std::wstring(strKey));
    const wchar_t* ret = retstr.c_str();
    size_t bufsize = wcslen(ret) + 1;
    wchar_t* buffer = (wchar_t*)CoTaskMemAlloc(bufsize * sizeof(wchar_t));
    wcscpy_s(buffer, bufsize, ret);
    return buffer;
}

And on the managed side:

[DllImport(DLL_PATH, 
    CharSet = CharSet.Unicode, 
    CallingConvention = CallingConvention.Cdecl)]
static extern string DecryptData(string strKey);