how we can perform clientside validtion that user canot select more than five items in the list bosx(Client side only)

Check new username availability with ASP.NET without sending request to server.can you provide me the complete code

I am working on this problem a couple of days, I did not get solution. in my web page , there is a gridview with a checkbox templatefield I need to know if checkbox is checked, I use following way, Even though I saw the checkbox is checked, I get checkbox.check is false. protected void Button1_Click(object sender, EventArgs e) { string EmpId=""; //bool chkBox=false ; foreach (GridViewRow row in GridView1.Rows) //for(int row=0;row", EmpId); } }