Gridview Textbox Validation?..

Question

Dear All,

                I had a requirement , I want to validate Gridview Textbox. I have A Grid and in that grid , I update the one base on the textbox value ,

   Before i  upDate , i wanna to check the texbox..

               I f it null i wanna throw the script Message for that..

             Guide Me !...

             Thanks in Advance.

 - Karthik .k