Question

(Day 2 of learning c#) I am passing a buffer to a C dll from C#. The C func copies the string “text” into the buffer. Back in the C# code, I compare “text” with what’s in the buffer and it doesn’t compare equal. What am I missing?

    extern "C" __declspec( dllexport )      
    int  cFunction(char *plotInfo, int bufferSize) 
    {
        strcpy(plotInfo, "text");
        return(0);
    }

c#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;


namespace ConsoleApplication1
{
    class Program
    {
        [DllImport("mcDll.dll", CallingConvention = CallingConvention.Cdecl,
                                                      CharSet=CharSet.Ansi)]
        public static extern int cFunction(StringBuilder theString, int bufSize);

        static void Main(string[] args)
        {
            StringBuilder s = new StringBuilder(55);
            int result = cFunction(s, 55);
            Console.WriteLine(s);

            string zz = "text";
            if (s.Equals(zz))
                Console.WriteLine( "strings compare equal");
            else
                Console.WriteLine("not equal");

            Console.ReadLine();
        }
    }
}

Answer 1

s is a StringBuilder, while zz is a string.

Try comparing

s.ToString().Equals(zz);

Generally, Equals() performs a reference comparison for reference types. Some classes (such as String) override Equals() to allow for strings that contain the same characters to be considered equal (though for performance purposes, I believe the actual implementation first checks for reference equality, then compares the contents of each string).

Your current code is calling the .Equals() method of StringBuilder.