Question

I am trying to use a third party DLL that wants an int** as one of the parameters to the method. It describes the parameter as the address of the pointer that will point to the memory allocation.

Sorry for any confusion. The parameter is two-way I think. The DLL is for talking to an FPGA board and the method is setting up DMA transfer between the host PC and the PCI board.

Answer 1

Use a by-ref System.IntPtr.

 [DllImport('thirdparty.dll')]  static extern long ThirdPartyFunction(ref IntPtr arg);   long f(int[] array)   { long retval = 0;     int  size   = Marshal.SizeOf(typeof(int));     var  ptr    = IntPtr.Zero;      try       { ptr = Marshal.AllocHGlobal(size * array.Length);         for (int i= 0; i < array.Length; ++i)          { IntPtr tmpPtr = new IntPtr(ptr.ToInt64() + (i * size));           Marshal.StructureToPtr(array, tmpPtr, false);         }         retval = ThirdPartyFunction(ref ptr);      }     finally       { if (ptr != IntPtr.Zero) Marshal.FreeHGlobal(ptr);      }      return retval;   }