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

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.

Here is an example of doing this through C#. I am calling Native function GetWindowText through C# by pInvoking. GetWindowText returns the caption of the window whose handle is passed to it.

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);
    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    static extern int GetWindowTextLength(IntPtr hWnd);

    public static string GetText(IntPtr hWnd)
    {
        // Allocate correct string length first
        int length = GetWindowTextLength(hWnd);
        StringBuilder sb = new StringBuilder(length + 1);
        GetWindowText(hWnd, sb, sb.Capacity);
        return sb.ToString();
    }        

    private void button1_Click(object sender, EventArgs e)
    {
        string str = GetText(this.Handle);
    }

Here is a topic where string marshaling has been discussed.

It's needed to mark parameter with attribute

[MarshalAs(UnmanagedType.LPSTR)]