How to show a message in C# timer event handler?

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MessageBox

timer

I created a timer using VC# which working seems fine. The problem is that I must show message at the time when timer event handler is called. I found the MessageBox.Show not working. Below is my modified code snippet, in which I simplified the event handler as just showing a message.
My window is a full screen and borderless. It seems the message shown is hiden by window's form.
Why can't MessageBox.Show() doesn't work at timer event handler? How can I do if I would like to prompt something when timer event occurres?

void OnGridTmEvt(object source, ElapsedEventArgs e)
{
    MessageBox.Show("Test end");
}
void createGridTmer()
{
    gridFlashTmer = new System.Timers.Timer();
    gridFlashTmer.Elapsed += new ElapsedEventHandler(OnGridTmEvt);
    gridFlashTmer.Interval = Constants.FLASH_MS_COUNT;
    gridFlashTmer.Enabled = true;
}