So how do I run a valid() method on a textbox as the user tries to tab out of it?

For instance, on one form I built over in my VFP app, when the users enters a material line item on the PO entry form, they charge each material item to a Job. They just type in the Job No. Well, I always do a valid() method on the Job No in the textbox.valid() to make sure the Job is still open, and that it is really a good Job No, and I throw up a messge right then to tell them if it is invalid, and I do not let them tab out of the textbox.


So how do I model this in my WPF textbox?