Fetching an array of strings from a native DLL

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

extern "C" __declspec(dllexport) void FetchStringArray( int nCount, char* ppStrArray[]) 
{ 
    int result = 0; 
    STRSAFE_LPSTR temp; 
    size_t cchDest = 40;
    const size_t alloc_size = sizeof(char) * 40;
    for ( int nI = 0; nI < nCount; nI++ ) 
    { 
       char *pszFormat = "from DLL >> [returned String %d]"; 
       STRSAFE_LPSTR temp = (STRSAFE_LPSTR)CoTaskMemAlloc( alloc_size ); 
       StringCchPrintfA(temp, cchDest, pszFormat, nI);
       CoTaskMemFree( ppStrArray[nI] ); 
       ppStrArray[nI] = (char *) temp; 
    } 
}

Points of Interest

• CoTaskMemAlloc is used to allocated the memory required.
• CoTaskMemFree is used to free any previously allocated buffer, if null is passed then, CoTaskMemFree is not called.

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 based application:

using System; 
using System.Runtime.InteropServices; 
using System.Text;
namespace MarshallingTest 
{ 
   class Program 
   { 
       [DllImport("Win32Native.dll")] 
       public static extern void FetchStringArray(int nCount, [In, Out] String[] arrStr); 
       static void Main(string[] args) 
       { 
            int nSize = 10; 
            String[] arrStr = new String[nSize]; 
            FetchStringArray(nSize, arrStr); 
            Console.WriteLine("Returned String Array"); 
            for (int nI = 0; nI < nSize; nI++) 
            { 
                 Console.WriteLine(arrStr[nI]); 
            } 
       }
   }
}

Point 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.