>How I can do to make the contents of a textbox flash
There is no built-in property to do that, so I guess you would have to use a Timer to change some property, probably the BackColor. Usually you wouldn't want to t keep it flashing continuously, or it will annoy the users.
Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire... (from Gulliver's Travels)