DllImport and char Marshaling

Question

I'm newbie at C# and Marshaling. I need to use my C func in C#, but i have an incorrect return value from C func (or I don't know how to convert it to correct answer).

C source:

#include "main.h"

char *Ololo(char *arg, int &n3)
{
    char *szRet;
    szRet=(char*)malloc(strlen(arg)+1);
    strcpy(szRet,arg);
    n3 = strlen(szRet);
    return szRet;
}

C header:

extern "C" __declspec(dllexport) char *Ololo(char *arg, int &n3);

C# source:

class Program
{
    [DllImport(@"F:\Projects\service\dll\testDLL2.DLL", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Auto)]
    public static extern IntPtr Ololo([In] char[] arg, ref Int32 n3);

    static void Main(string[] args)
    {
        string n1 = "ololo";
        char[] chars = new char[n1.Length];
        chars = n1.ToCharArray();
        Int32 n3 = 0;
        IntPtr result;
        result = Ololo(chars, ref n3);
        string n4 = Marshal.PtrToStringUni(result,n3);
        Console.WriteLine(n4);
    }
}

I've got return something like "o?? ?"

Sorry for bad English

----------------------Solved-----------------------

class Program
    {
        [DllImport(@"F:\Projects\service\dll\testDLL2.DLL", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Auto)]
        public static extern IntPtr Ololo([MarshalAs(UnmanagedType.LPStr)]string arg, ref Int32 n3);

        static void Main(string[] args)
        {
            string n1 = "ololo";
            Int32 n3 = 0;
            int n2 = n1.Length;
            IntPtr result;
            result = Ololo(n1, ref n3);
            string n4 = Marshal.PtrToStringAnsi(result, n3);
            Console.WriteLine(n4);
        }
    }

That works fine. In n3 i ve got 5 and in n4 ololo! Thank s for quick answers!

Solution

public static extern IntPtr Ololo([In] char[] arg, ref Int32 n3);

IntPtr is the wrong return type, as essentially you want to return the string, not a pointer to the string. In C you can use a pointer to your string by using char*, the equivalent in .NET would be to use use this: [MarshalAs(UnmanagedType.LPStr)]string. This should marshall the char* to a string correctly.

IntPtr represents a pointer type, which to get your actual string is useless.

Also looks like you should be taking a StringBuilder into your Marshalled function, and not a char[]. Then at least you should be getting the correct string to your C function.

Comments

[Treno1]

I think IntPtr is correct because n3 returns me 1 and int IntPtr Ive got first letter (o). But why I have n3` = 1? I think it needs to be 5. Because of that I think [In] char[] arg` is incorrect. But what is correct?

[Tony The Lion]

@Treno1 your function in .NET will never give you the entire string back if you return IntPtr.

[Treno1]

VS 2010 think that public static extern [MarshalAs(UnmanagedType.LPStr)]string Ololo is incorrect because of [MarshalAs(UnmanagedType.LPStr)] cannot be in declaration of class , structure or interface.

[Puppy]

It isn't. You must have failed to copy the DLLImport attribute over, or placed the declaration in the wrong place.

[Treno1]

System.AccessViolationException if I try to public static extern StringBuilder Ololo(string arg, ref Int32 n3); and StringBuilder n5 = Ololo(n1, ref n3);