Interoperability returning a string from a Win32 DLL

Introduction

Define  a function in native DLL (let's call it “Win32Native.dll”)  as shown below.

extern "C" __declspec(dllexport)  char * GetStringFromDLL() 
{ 
    const size_t alloc_size = 128; 
    STRSAFE_LPSTR result=(STRSAFE_LPSTR)CoTaskMemAlloc(alloc_size); 
    STRSAFE_LPCSTR teststr = "This is return string From Native DLL"; 
    StringCchCopyA ( result, alloc_size, teststr ); 
    return result; 
}

Points of Interest

• STRSAFE_LPSTR is a typedef of char *
• StringCchCopy is a replacement for strcpy (with safety).  The size, in characters, of the destination buffer is provided to the function to ensure that StringCchCopyb does not write past the end of this buffer. has two variants.
  • StringCchCopyA (ANSI Version)
  • StringCchCopyW (Unicode/Wide character Version)
  • If you want to use a heap that is shared between native and managed, it is more common to use the COM heap.
    • On the native side use CoTaskMemAlloc() and CoTaskMemFree().
    • On the managed side use Marshal.AllocCoTaskMem() and Marshal.FreeCoTaskMem().

Writing the client code (the managed part)

We can simply create a console base application which can use this DLL. Let’s name it MarshallingTest.

See the code snippet below.

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Runtime.InteropServices; 
using System.Text;
namespace MarshallingTest 
{ 
    class Program 
    { 
       [DllImport("Win32Native.dll")] public static extern String GetStringFromDLL();
        static void Main(string[] args) 
        { 
            Console.WriteLine("Line displayed below is returned from a Native DLL"); 
            string strData = GetStringFromDLL(); 
            Console.WriteLine ( "Returned value [" + strData +"]" ); 
        } 
    } 
}

Points of Interest

• namespace System.Runtime.InteropServices; defines the declarations necessary for Interop operations, like DllImport.
• DllImport defines the DLL entry point.

Compile and execute you will get following output.