S
ilverlight 3 comes with built-in validation for many of the input
controls. Setting them up is fairly easy and they are styled well enough
that you could use them right out of the box.
If you would like to know how to set it up and also would like to
customize the look and feel of the fields and validation message, this
short tutorial will explain how to do just that.
See more here: Silverlight Validation & Tootip Styles...