.Net custom control locks up when losing focus

Today I had to solve an interesting problem.

If a DataGridView, has the focus and is embedded in a custom control which is hosted by a native windows or MFC dialog, and the Dialog loses focus then the app locks up.

This happens because the focus management system is looking for the control which is losing the focus but can't find it.

The symptoms are 100% CPU usage (on one core) and if you use Spy or Spy++ you will see a flood of WM_GETDLGCODE messages to the control.

The cure is to set the WS_EX_CONTROLPARENT extended style on the DataGridView. To do this handle the HandleCreated event for the DataGridView and add the WS_EX_CONTROLPARENT extended style. Eg

void Grid_HandleCreated(object sender, EventArgs e)
{
   Control grid = sender as Control;
   if (grid != null && grid.Handle != IntPtr.Zero)
      NativeWindowAPI.ModifyStyleEx(grid.Handle, 0, NativeWindowAPI.WS_EX_CONTROLPARENT);
}

This uses my NativeWindowAPI class which I use to wrap Windows APIs.

class NativeWindowAPI
{
   [DllImport("User32.dll", CharSet = CharSet.Ansi)]
   extern static Int32 SetWindowLong(IntPtr hWnd, int nIndex, Int32 dwNewLong);
 
   [DllImport("User32.dll", CharSet = CharSet.Ansi)]
   extern static Int32 GetWindowLong(IntPtr hWnd, int nIndex);
 
   const Int32 GWL_EXSTYLE = -20;
 
   public const Int32 WS_EX_DLGMODALFRAME = 0x00000001;
   public const Int32 WS_EX_NOPARENTNOTIFY = 0x00000004;
   public const Int32 WS_EX_TOPMOST = 0x00000008;
   public const Int32 WS_EX_ACCEPTFILES = 0x00000010;
   public const Int32 WS_EX_TRANSPARENT = 0x00000020;
   public const Int32 WS_EX_MDICHILD = 0x00000040;
   public const Int32 WS_EX_TOOLWINDOW = 0x00000080;
   public const Int32 WS_EX_WINDOWEDGE = 0x00000100;
   public const Int32 WS_EX_CLIENTEDGE = 0x00000200;
   public const Int32 WS_EX_CONTEXTHELP = 0x00000400;
   public const Int32 WS_EX_RIGHT = 0x00001000;
   public const Int32 WS_EX_LEFT = 0x00000000;
   public const Int32 WS_EX_RTLREADING = 0x00002000;
   public const Int32 WS_EX_LTRREADING = 0x00000000;
   public const Int32 WS_EX_LEFTSCROLLBAR = 0x00004000;
   public const Int32 WS_EX_RIGHTSCROLLBAR = 0x00000000;
   public const Int32 WS_EX_CONTROLPARENT = 0x00010000;
   public const Int32 WS_EX_STATICEDGE = 0x00020000;
   public const Int32 WS_EX_APPWINDOW = 0x00040000;
   public const Int32 WS_EX_OVERLAPPEDWINDOW = (0x00000100 | 0x00000200);
   public const Int32 WS_EX_PALETTEWINDOW = (0x00000100 | 0x00000080 | 0x00000008);
   public const Int32 WS_EX_LAYERED = 0x00080000;
   public const Int32 WS_EX_NOINHERITLAYOUT = 0x00100000;
   public const Int32 WS_EX_LAYOUTRTL = 0x00400000;
   public const Int32 WS_EX_NOACTIVATE = 0x08000000;
 
   public static void ModifyStyleEx(IntPtr hWnd, Int32 remove, Int32 add)
   {
      Int32 res = GetWindowLong(hWnd, GWL_EXSTYLE);
 
      res &= ~remove;
      res |= add;
 
      SetWindowLong(hWnd, GWL_EXSTYLE, res);
   }
}