CreateCompatibleBitmap returns NULL

[Guest]

I'm capturing the screen using the following code , but the problem i have
that after 43 calls of the functions the NULL reference object is returned ,


SIZE size;

//Here we get the handle to the desktop device context.
IntPtr hDC =
PlatformInvokeUSER32.GetDC(PlatformInvokeUSER32.GetDesktopWindow());
//Here we make a compatible device context in memory for screen device
context.
IntPtr hMemDC = PlatformInvokeGDI32.CreateCompatibleDC(hDC);
//We pass SM_CXSCREEN constant to GetSystemMetrics to get the X coordinates
of screen.
size.cx =
PlatformInvokeUSER32.GetSystemMetrics(PlatformInvokeUSER32.SM_CXSCREEN);
//We pass SM_CYSCREEN constant to GetSystemMetrics to get the Y coordinates
of screen.
size.cy =
PlatformInvokeUSER32.GetSystemMetrics(PlatformInvokeUSER32.SM_CYSCREEN);

//We create a compatible bitmap of screen size and using screen device
context.
m_HBitmap = PlatformInvokeGDI32.CreateCompatibleBitmap(hDC, size.cx, size.cy);

//Here m_HBitmap return be NULL(Zero) after 43 calls of the function...WHY?

//As m_HBitmap is IntPtr we can not check it against null. For this purspose
IntPtr.Zero is used.
if (m_HBitmap!=IntPtr.Zero)
{
//Here we select the compatible bitmap in memeory device context and keeps
the refrence to Old bitmap.
IntPtr hOld = (IntPtr) PlatformInvokeGDI32.SelectObject(hMemDC, m_HBitmap);
//We copy the Bitmap to the memory device context.
PlatformInvokeGDI32.BitBlt(hMemDC, 0, 0,size.cx,size.cy, hDC, 0, 0,
PlatformInvokeGDI32.SRCCOPY);
//We select the old bitmap back to the memory device context.
PlatformInvokeGDI32.SelectObject(hMemDC, hOld);
//We delete the memory device context.
PlatformInvokeGDI32.DeleteDC(hMemDC);
//We release the screen device context.
PlatformInvokeUSER32.ReleaseDC(PlatformInvokeUSER32.GetDesktopWindow(), hDC);
//Image is created by Image bitmap handle and returned.
return System.Drawing.Image.FromHbitmap(m_HBitmap);
}
//If m_HBitmap is null retunrn null.
return null;