C++ dll sends utf8 as const char*, c# needs to do... what?

Question

The dll is written in c++ and sends text as utf8 through a const char* callback.

First, is this the correct way to declare the callback?

[UnmanagedFunctionPointer( CallingConvention.StdCall )]
public delegate void TextCallback( string sText );

[DllImport(
    "cppLib.dll",
    CharSet = CharSet.Ansi,
    CallingConvention = CallingConvention.Cdecl )]

public static extern void GetText(
    [MarshalAs( UnmanagedType.FunctionPtr )] TextCallback textCallback );

private TextCallback m_TextCallback;

Native callback:

typedef void ( __stdcall * TextCallback )( const char* szText );

If so, how do I handle the utf8 text once it arrives?

I'm sending it to a RichTextBox and the utf8 chars come out as garbage (the ones that happen to be ascii print fine).

Thank you all.

ANSWER

In the comments below, TheUndeadFish provided a link with an answer that works. It is also explained to a degree. Those interested should take a look. I'm just replicating the answer here as it applies to the code I posted.

Only this modification is needed:

[UnmanagedFunctionPointer( CallingConvention.StdCall )]
public delegate void TextCallback( IntPtr ipText );

The delegate reinterprets the int pointer (pointing to a utf8 string from the c++ dll) as follows:

m_TextCallback = ( ipText ) =>
{
     var data = new System.Collections.Generic.List<byte>();

     var off = 0;

     while( true )
     {
         var ch = Marshal.ReadByte( ipText, off++ );

        if( ch == 0 )
        {
            break;
        }

        data.Add( ch );
    }

    string sptr = Encoding.UTF8.GetString( data.ToArray() );
};

Answer 1

You should use CharSet.Unicode (if the string is a wchar*, 2 bytes per char) or CharSet.Ansi (if the string is 1 byte per char).

Since your string is in UTF8, you should convert by hand. None of the default conversion fit your problem.