The problem is: your timer event handler is executed in a thread other then your UI thread. And this is actually a good thing.
You cannot call anything related to UI from non-UI thread. Instead, you need to use the method
Invoke
or
BeginInvoke
of
System.Windows.Threading.Dispatcher
(for both Forms or WPF) or
System.Windows.Forms.Control
(Forms only).
You will find detailed explanation of how it works and code samples in my past answers:
Control.Invoke() vs. Control.BeginInvoke()[
^],
Problem with Treeview Scanner And MD5[
^].
See also more references on threading:
How to get a keydown event to operate on a different thread in vb.net[
^],
Control events not firing after enable disable + multithreading[
^].
Also consider using my schema for polling a database by timer based on a combination of a timer and a thread; this solution removes some unpleasant hassles:
Polling Database with Timer[
^].
—SA