Modifying an array of string values (default marshaling)

Define some functions in native DLL (let say it “Win32Native.dll”) as shown below. this function receives array of char pointers (string).

extern "C" __declspec(dllexport) void ModifyStringArrayValues( int nCount, char* ppStrArray[] )

Implementing Functions

extern "C" __declspec(dllexport) void ModifyStringArrayValues( int nCount, char* ppStrArray[] ) 
{ 
    size_t cchDest = 40; 
    const size_t alloc_size = sizeof(char) * 40; 
    for ( int nI = 0; nI < nCount; nI++ ) 
    { 
        char *pszFormat = "<<from DLL>> [Modified String %2d]"; 
        STRSAFE_LPSTR temp = (STRSAFE_LPSTR)CoTaskMemAlloc( alloc_size ); 
        StringCchPrintfA(temp, cchDest, pszFormat, nI); 
        CoTaskMemFree( ppStrArray[nI] ); 
        ppStrArray[nI] = (char *) temp; 
   } 
}

Point 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.
• StringCchPrintfA printf used to print a formatted output to a string buffer.

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

See the code snippet below.

using System; 
using System.Runtime.InteropServices; 
using System.Text;
namespace MarshallingTest 
{ 
    class Program 
    { 
        [DllImport("Win32Native.dll")] 
        public static extern double ModifyStringArrayValues(int nSize, [In, Out] String[] strArr); 
     
        static void Main(string[] args) 
        { 
            int nSize = 5; 
            string[] arrStr = new string[nSize]; 
            for (int nI = 0; nI < nSize; nI++) arrStr[nI] = "String " + (nI + 1).ToString();
            
            Console.WriteLine("String Values in Array, before calling function"); 
            for (int i = 0; i < nSize; i++) 
            { 
                Console.WriteLine(string.Format("Array[{0:D2}] : {1}", i, arrStr[i])); 
            }
            ModifyStringArrayValues(nSize, arrStr);
            Console.WriteLine("String Values in Array, after calling function"); 
            for (int i = 0; i < nSize; i++) 
            { 
                Console.WriteLine(string.Format("Array[{0:D2}] : {1}", i, arrStr[i])); 
            } 
        } 
    } 
}

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 and you will get following output.