Click here to Skip to main content
14,427,540 members

How to marshal a structure array using intptr.

Praveen Kumar Katiyar asked:

Open original thread
I have a following code in a C++ DLL

typedef struct _MYPOINT
    int nX;
    int nY;
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
    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
      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]);

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.


Tags: marshalling


When answering a question please:
  1. Read the question carefully.
  2. Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
  3. If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
  4. Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.
Let's work to help developers, not make them feel stupid.
Please note that all posts will be submitted under the The Code Project Open License (CPOL).

CodeProject, 503-250 Ferrand Drive Toronto Ontario, M3C 3G8 Canada +1 416-849-8900 x 100