My application uses MVC validation explained here: http://blog.stevensanderson.com/2010/01/28/validating-a-variable-length-list-aspnet-mvc-2-style/
And it works fine with both server and client validation when the form is posted. However my problem is that I would like to have client validation on one of the fields before the form is posted. So when the TextBox loses focus it is validated directly.