(C#) MessageBox.Show ใช้กับ Web form ไม่ได้หรอครับ ? --ต้อง using อะไรก่อนครับ

        If MessageBox.Show("Are you sure to exit?", "Confirm.", MessageBoxButtons.YesNo, MessageBoxIcon.Question, MessageBoxDefaultButton.Button1) = DialogResult.Yes Then
            Application.Exit()
        End If

MessageBox.Show("Please input (Name)")

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

using System.Collections;
using System.Text;

/// <summary>
/// Summary description for MessageBox
/// </summary>
public class MessageBox
{
    private static Hashtable m_executingPages = new Hashtable();
    
    public MessageBox()
	{
		//
		// TODO: Add constructor logic here
		//
	}
 
    public static void Show(string sMessage)
    {
        // If this is the first time a page has called this method then
        if(!m_executingPages.Contains(HttpContext.Current.Handler))
        {
            // Attempt to cast HttpHandler as a Page.
            Page executingPage = HttpContext.Current.Handler as Page;

            if(executingPage != null)
            {
                // Create a Queue to hold one or more messages.
                Queue messageQueue = new Queue();
                // Add our message to the Queue
                messageQueue.Enqueue(sMessage);
                // Add our message queue to the hash table. Use our page reference
                // (IHttpHandler) as the key.
                m_executingPages.Add(HttpContext.Current.Handler, messageQueue);
                // Wire up Unload event so that we can inject
                // some JavaScript for the alerts.
                executingPage.Unload += new EventHandler(ExecutingPage_Unload);
            }  
        }
        else
        {
            // If were here then the method has allready been
            // called from the executing Page.
            // We have allready created a message queue and stored a
            // reference to it in our hastable.
            Queue queue = (Queue) m_executingPages[HttpContext.Current.Handler];
            // Add our message to the Queue
            queue.Enqueue(sMessage);
        }
    }

    // Our page has finished rendering so lets output the
    // JavaScript to produce the alert's
    private static void ExecutingPage_Unload(object sender, EventArgs e)
    {
        // Get our message queue from the hashtable
        Queue queue = (Queue) m_executingPages[HttpContext.Current.Handler];

        if(queue != null)
        {
            StringBuilder sb = new StringBuilder();
            // How many messages have been registered?
            int iMsgCount = queue.Count;
            // Use StringBuilder to build up our client slide JavaScript.
            sb.Append("<script language='javascript'>");
            // Loop round registered messages
            string sMsg;

            while(iMsgCount-- > 0)
            {
                sMsg = (string) queue.Dequeue();
                sMsg = sMsg.Replace( "\n", "\\n" );
                sMsg = sMsg.Replace( "\"", "'" );
                sb.Append( @"alert( """ + sMsg + @""" );" );
            }
            // Close our JS
            sb.Append(@"</script>");
            // Were done, so remove our page reference from the hashtable
            m_executingPages.Remove(HttpContext.Current.Handler);
            // Write the JavaScript to the end of the response stream.
            HttpContext.Current.Response.Write(sb.ToString());
        }
    }
}

lberror.Text = "กรุณาตรวจสอบข้อมูล";
                    lberror.Visible = true;

protected void Button1_Click(object sender, EventArgs e)
{
   MessageBox.Show("แสดง alert");
}