Many, if not most, Windows Forms samples you can find on the 'net include one or more calls to unmanaged code in Windows DLLs, often in the form of calls to the SendMessage API methods to fix some of the (very few) missing features of .NET controls. The problem is, such a call to unmanaged code creates a problem when the program runs as a ClickOnce application, because it requires higher CAS privileges.

Even though this problem doesn't have a generic solution, when you just need to send a message to the control you are inheriting from, you can avoid an explicit call to SendMessage by invoking the protected DefWndProc method instead. For example, let's say that you are writing an enhanced ComboBox that exposes the TopIndex property, which can set or return the index of the first visible item in the list area. These two operations can be implemented by sending the control the CB_SETTOPINDEX or CB_GETTOPINDEX message, respectively. Here's how you can use the DefWndProc method instead of SendMessage:

Public Class ComboBoxEx
   Inherits System.Windows.Forms.ComboBox

   Public Property TopIndex() As Integer
      Get
         Const CB_GETTOPINDEX As Int32 = &H15B
         Dim m As New Message()
         m.HWnd = Me.Handle
         m.Msg = CB_GETTOPINDEX
         Me.DefWndProc(m)
         Return m.Result.ToInt32()
      End Get
      Set(ByVal value As Integer)
         Const CB_SETTOPINDEX As Int32 = &H15C
         Dim m As New Message()
         m.HWnd = Me.Handle
         m.Msg = CB_SETTOPINDEX
         m.WParam = New IntPtr(value)
         Me.DefWndProc(m)
      End Set
   End Property

End Class