Marshal Strukture med arrays og string

Jeg har en funktion, der får følgende struktur som input:

    [StructLayout(LayoutKind.Sequential, CharSet=DLL_CHAR_SET)]
    public struct WPGrabUrlResourceStruct
    {
      public int id;
      public int masterId;
      public int errCode;
      public string url;
      public string errMsg;
      public WPGRAB_STATUS_ENUM status;
    };

    public struct WPGrabResourceFoundStruct
    {
      public int masterId;
      public WPGrabUrlResourceStruct[] webpagesList;
      public WPGrabUrlResourceStruct[] imagesList;
      public WPGrabUrlResourceStruct[] otherList;
    };

Den skal jeg have konvertere til strukturen:

[StructLayout(LayoutKind.Sequential)]
    public struct WPGrabResourceFoundStructInt
    {
      public int masterId;
      public int webpageCount;
      public int imagesCount;
      public int otherCount;
      public IntPtr webpagePtr;
      public IntPtr imagePtr;
      public IntPtr otherPtr;
    };

For at kunne kalde en dll funktion, der har til opgave af frigøre hukommelse.

Jeg har forsøgt med (efter inspiration fra http://forums.msdn.microsoft.com/en-US/csharpgeneral/thread/a0cdf1eb-64a2-4893-9b52-14649006d5d8/):

    [DllImport(DLL_NAME, CallingConvention = DLL_CC, CharSet = DLL_CHAR_SET, EntryPoint = "WPGrabFreeFoundResourcesStruct")]
    private static extern void WPGrabFreeFoundResourcesStructInt(IntPtr resources);

    public static void WPGrabFreeFoundResourcesStruct(WPGrabResourceFoundStruct? resources)
    {
      if (resources != null)
      {
        WPGrabResourceFoundStructInt tmp = new WPGrabResourceFoundStructInt();
        int sizeOfStruct = 0;
        Int64 addr = 0;

        tmp.masterId = resources.Value.masterId;
        tmp.webpageCount = resources.Value.webpagesList.GetLength(0);

        if (tmp.webpageCount > 0)
        {
          sizeOfStruct = Marshal.SizeOf(resources.Value.webpagesList.GetValue(0)); //Marshal.SizeOf(resources.Value.webpagesList.GetValue(0));
          tmp.webpagePtr = Marshal.AllocHGlobal(sizeOfStruct  * tmp.webpageCount);
          addr = tmp.webpagePtr.ToInt64();

          for (int i = 0; i < tmp.webpageCount; i++)
          {
            Marshal.StructureToPtr(resources.Value.webpagesList[i], (IntPtr) (addr + i * sizeOfStruct), false);
          // tmp.webpagePtr[addr + i * sizeOfStruct] = (new StructToPointer<WPGrabUrlResourceStruct>(resources.Value.webpagesList(i)).ToPtr)
          }
        }
        else
        {
          tmp.webpagePtr = IntPtr.Zero;
        }

        tmp.imagesCount = 0;
        tmp.imagePtr = IntPtr.Zero;

        tmp.otherCount = 0;
        tmp.otherPtr = IntPtr.Zero;

        using (var t = new StructToPointer<WPGrabResourceFoundStructInt>(tmp))
        {
          WPGrabFreeFoundResourcesStructInt(t.ToPtr);
        }
      }
    }

StructToPointer kan findes her: http://www.eksperten.dk/spm/842083.

Mit problem er at når jeg køre programmet, så får jeg fint allokeret hukommelsen og i WPGrabFreeFoundResourcesStructInt kan jeg se de data jeg sender med, men når jeg via delete[] errMsg (fra WPGrabUrlResourceStruct), så får jeg assertion. Nogen ideer til hvad problemet er???