Using a MessageBox in ASP

using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;

/// 
/// Summary description for mbox
/// 
public class mijnmesbox
{
    public mijnmesbox()
 {
     }
        // for postback events
        public static void ShowMessageBox(string _message)   
        {       
            Page page = HttpContext.Current.Handler as Page;       
            page.RegisterStartupScript("key2", "");   
        }
   
        // for async postback events
        public static void ShowMessageBoxForAsync(string _message)   
        {       
            Page page = HttpContext.Current.Handler as Page;       
            ScriptManager.RegisterClientScriptBlock(page,page.GetType(), "key2", "alert('" + _message + "');", true);   
        }
       
}