how to require field validation not select value of dropdownlist in asp.net 4.0

From what I understand, you want to validate if a value has been selected on the Dropdownlist. In that case, you can use a RequiredFieldValidator[^]. There are plenty of examples if you just search. Here are just few of those I found useful.

http://forums.asp.net/t/1296491.aspx/1[^]
http://stackoverflow.com/questions/5310609/dropdownlist-validation-in-asp-net-using-required-field-validator[^]
http://asp-net-example.blogspot.com/2008/10/dropdownlist-example-how-to-validate.html[^]