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Calling a DLL with C# (C Sharp)

The following example demonstrates how to call a (C++) DLL from C Sharp. The code for the DLL is here.
call_dll.cs
// http://www.c-sharpcenter.com/Tutorial/UnManaged.htm


using System.Runtime.InteropServices;
using System;

class call_dll {

  [StructLayout(LayoutKind.Sequential, Pack=1)]
  private struct STRUCT_DLL {
    public Int32  count_int;
    public IntPtr ints;
  }

  [DllImport("mingw_dll.dll")]
  private static extern int func_dll(
      int an_int,
      [MarshalAs(UnmanagedType.LPArray)] byte[] string_filled_in_dll,
      ref STRUCT_DLL s
   );
  
  public static void Main() {

    byte[] string_filled_in_dll = new byte[21];


    STRUCT_DLL struct_dll = new STRUCT_DLL();
    struct_dll.count_int = 5;
    int[]  ia = new int[5];
    ia[0] = 2; ia[1] = 3; ia[2] = 5; ia[3] = 8; ia[4] = 13;

    GCHandle gch    = GCHandle.Alloc(ia);
    struct_dll.ints = Marshal.UnsafeAddrOfPinnedArrayElement(ia, 0);

    int ret=func_dll(5,string_filled_in_dll, ref struct_dll);
    
    Console.WriteLine("Return Value: " + ret);
    Console.WriteLine("String filled in DLL: " + System.Text.Encoding.ASCII.GetString(string_filled_in_dll));

  }
}