cs:

写在Page_Load 里面
Button1.Attributes.Add("onclick", "return fun(" + TextBox1.ClientID + "," + TextBox2.ClientID + ");");

aspx:
         <script language ="javascript">
                 function fun(tname,tpwd)
                 {
                     if(tname.value=="")
                     {
                           alert("请输入用户名!");
                           return false;                           
                       
                     }
                       if(tpwd.value=="")
                     {
                           alert("请输入密码!");
                           return false;                           
                       
                     }
                 
                 }
         </script>