Pass string to c# program from C dll

[Cyril B.]

I have read a lot of threads about passing strings from dlls to c# programs but a many of them are examples in C++ witch use a lot of C++ features.

I want to make a dll in C (only) and it should pass strings too C# program and vice versa.

I would appriciate any help and code examples.

Thank you.

[Krishnaa]

How are you calling a C# DLL ?

[Cyril B.]

With dllimport.

[Cyril B.]

I am calling the C dll from C# program. (NOT the C# dll from C program.)

[S_M_A]

I did it like this

[DllImport("My.dll", CharSet = CharSet.Unicode)]
public static extern void Read([MarshalAs(UnmanagedType.LPWStr)] out String str);

In the dll I converted the char array to unicode before returning but maybe this isn't neccessary with changed CharSet and LPWStr. This worked fine however so I didn't bother to test another way.

[Cyril B.]

I am trying this code but it doesn't work. What i am trying to do is basicaly trying to receive a string from the dll and desplay it in a textbox:

C part:

   __declspec(dllexport) void  GetStringFromDLL()
{
char string[5];
strcpy(string,"un");

return string;
}

C# part:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace dll_import
{
    public partial class Form1 : Form
   {

        [DllImport("dll.dll", CharSet = CharSet.Unicode)]
        public static extern void GetStringFromDLL([MarshalAs(UnmanagedType.LPWStr)] out String str);
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void textBox2_TextChanged(object sender, EventArgs e)
        {
            

        }

        private void textBox3_TextChanged(object sender, EventArgs e)
        {
            
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string m;
           GetStringFromDLL(out m);
           textBox3.Text = m;

        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }
    }

}

[creatorul]

I am trying this code but it doesn't work. What i am trying to do is basicaly trying to receive a string from the dll and desplay it in a textbox:

C part:

 __declspec(dllexport) void GetStringFromDLL()
{
char string[5];
strcpy(string,"un");
 
return string;
}

Does the C code compile ? I see that you return something but the method signature says something else ...and try with StringBuilder.

[S_M_A]

If you use my example the dll shall be as:

__declspec(dllexport) void Read(_TCHAR* sz)
{
  static _TCHAR string[100];
  _tcscpy(sz,_T("un"));
}

I think this works if dll is built as unicode if not string has to be converted to unicode before copying to buf.

[darwen]

I hate to say this but I believe using the

__declspec(dllexport)

will cause a DECORATED version of the method's name to be exported. Therefore DllImport won't be able to find it. I.e. it's got a lot of gunk surrounding the actual name.

Personally I don't do it this way : I use a .def file with __stdcall on all the exported methods instead to ensure that their names aren't decorated.

Of course I may be wrong...

Darwen.

[S_M_A]

I don't think that's neccessary darwen.

I checked my project again. I used __declspec( dllexport ) in code, nothing fancy in header a def file to be on the safe side but then without __stdcall.

Nevertheless, if things go wrong the OP now has another option as well.

[Cyril B.]

I use Dev C++ compiler for the dll.

When i try to compile the code S_M_A wrote i get various errors.

I don't think _TCHAR is a C language type. Or _tcscpy a C language function.

(Please remember i use C language not C++)

Thank you for your replies so far.

[S_M_A]

Sorry, _TCHAR & _tcscpy are MS way of making it possible to build both unicode and not from a single source.

Build your DLL the DevC++ way but remember that C# expects a unicode wide string so if the DLL not produce that you have to convert using before copying to return pointer. MultiByteToWideChar() should be helpful.