How to return text from Native (C++) code

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I’d do it with a BSTR since it means you don’t have to call into native twice per string, once to get the length and then once to get the contents.

With a BSTR the marshaller will take care of deallocating the BSTR with the right memory manager so you can safely pass it out of your C++ code.

C++

#include <comutil.h>
BSTR GetSomeText()
{
    return ::SysAllocString(L"Greetings from the native world!");
}

C#

[DllImport(@"test.dll", CallingConvention = CallingConvention.Cdecl)]
[return: MarshalAs(UnmanagedType.BStr)]
private static extern string GetSomeText();

There is one minor drawback of the BSTR, namely that it carries a UTF-16 payload but your source data may well be char*.

To overcome this you can wrap up the conversion from char* to BSTR like this:

BSTR ANSItoBSTR(const char* input)
{
    BSTR result = NULL;
    int lenA = lstrlenA(input);
    int lenW = ::MultiByteToWideChar(CP_ACP, 0, input, lenA, NULL, 0);
    if (lenW > 0)
    {
        result = ::SysAllocStringLen(0, lenW);
        ::MultiByteToWideChar(CP_ACP, 0, input, lenA, result, lenW);
    } 
    return result;
}

That’s the hardest one out of the way, and now it’s easy to add other wrappers to convert to BSTR from LPWSTR, std::string, std::wstring etc.

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