How to marshal a structure array using intptr.

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marshalling

I have a following code in a C++ DLL

typedef struct _MYPOINT
{
    int nX;
    int nY;
} MYPOINT;
 
extern "C"  __declspec(dllexport) void ModifyPointArray (int nSize, MYPOINT *ptArr[]) 
{ 
    for ( int nI = 0; nI < nSize; nI++ )
    {
	ptArr[nI]->nX +=5;
	ptArr[nI]->nY +=5;
    }
}

======================================

I have defined a Wrapper Class for the DLL something like this.

namespace MarshalDemo
{
    [StructLayout(LayoutKind.Sequential)]
    public class MyPoint
    {
        public int nX;
        public int nY;
        
        public MyPoint()
        {
            this.nX = 0;
            this.nY = 0;
        }
        public MyPoint(int x, int y)
        {
            this.nX = x;
            this.nY = y;
        }
    }
   
    public static class MarshalDllWrapper
    {
     
      [DllImport("MarshalDll.dll")]
      public static extern void ModifyPointArray(int nSize, IntPtr [] arrPts);
    }
}

What I have tried:

Now i am trying to call this function using Marshalling using IntPtr, my .NET client code (C#) is something like this.

MyPoint[] pointArr = { new MyPoint(5, 6), new MyPoint(7, 8), new MyPoint(9, 10) };
int nCount = pointArr.Length;
IntPtr[] ptrArr = new IntPtr[nCount];
for (int nI=0; nI< nCount; nI++)
{
     ptrArr[nI] = Marshal.AllocCoTaskMem(Marshal.SizeOf(typeof(MyPoint)));
     Marshal.StructureToPtr(pointArr[nI], ptrArr[nI], true);
}

MarshalDllWrapper.DisplayPointArray(nCount, ptrArr);
MyPoint[] pointArr2 = new MyPoint[nCount];
for (int nI = 0; nI < nCount; nI++)
{
     Marshal.PtrToStructure(ptrArr[nI], pointArr2[nI]);
     Marshal.FreeCoTaskMem(ptrArr[nI]);
}

I am getting an Error on line
//F:\VS2010\VCS2010\Marshalling\MarshalDll\MarshalDllClient\frmMarshalDemo.cs:line XXX

Marshal.PtrToStructure(ptrArr[nI], pointArr2[nI]);

with Error Message

System.ArgumentNullException: Value cannot be null.
Parameter name: structure
at System.Runtime.InteropServices.Marshal.PtrToStructureHelper(IntPtr ptr, Object structure, Boolean allowValueClasses)
at System.Runtime.InteropServices.Marshal.PtrToStructure(IntPtr ptr, Object structure)
at MarshalDemo.frmMarshalDemo.btnPointArray_Click(Object sender, EventArgs e) in F:\VS2010\VCS2010\Marshalling\MarshalDll\MarshalDllClient\frmMarshalDemo.cs:line 150


PS : I know if i write a Wrapper something like this.
public static extern void ModifyPointStructArray(int nSize, [In, Out] MyPoint[] arrStruc);

it works like a breeze, but i want to do with IntPtr, am i doing something conceptually wrong ?

What I am doing wrong ?
Any Help will will apreciated.

Thanks.

Praveen.