how to display alert by using serverside code in asp.net C#?

If you want an alert message box in your application which returns a value. Now i am using a code for an alertmessage which returns value.

Response.Write(""); 
*Please correct spelling alert, Session["UserID"] is a server side variable .

Or

 ClientScript.RegisterStartupScript(GetType(), "TestAlert", "alert(\'This is test alert.\');", true);

Do let me know your feedback, comments.......