สอบถามวิธีการเขียน Code ให้กับ Event หลังจากมีการเลือก Dropdownlist ที่ 1 ครับ

    Protected Sub DropDownListPreDistrict_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DropDownListPreDistrict.SelectedIndexChanged

'ต้องการให้เมื่อ DropDownListPreDistrict นี้ถูกเลือก ให้แสดง MsgBox ข้อความครับ

    End Sub

Protected Sub DropDownListPreDistrict_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DropDownListPreDistrict.SelectedIndexChanged
   MessageBox.Show(DropDownListPreDistrict.SelectedItem.Value) 'MessageBox.vb ก็อปไว้ที่ app_code
End Sub

Imports System.Data
Imports System.Configuration
Imports System.Web
Imports System.Web.Security
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.Web.UI.WebControls.WebParts
Imports System.Web.UI.HtmlControls

Imports System.Collections
Imports System.Text

''' <summary>
''' Summary description for MessageBox
''' </summary>
Public Class MessageBox
	Private Shared m_executingPages As New Hashtable()

			'
			' TODO: Add constructor logic here
			'
	Public Sub New()
	End Sub

	Public Shared Sub Show(sMessage As String)
		' If this is the first time a page has called this method then
		If Not m_executingPages.Contains(HttpContext.Current.Handler) Then
			' Attempt to cast HttpHandler as a Page.
			Dim executingPage As Page = TryCast(HttpContext.Current.Handler, Page)

			If executingPage IsNot Nothing Then
				' Create a Queue to hold one or more messages.
				Dim messageQueue As 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.
				AddHandler executingPage.Unload, New EventHandler(AddressOf ExecutingPage_Unload)
			End If
		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.
			Dim queue As Queue = DirectCast(m_executingPages(HttpContext.Current.Handler), Queue)
			' Add our message to the Queue
			queue.Enqueue(sMessage)
		End If
	End Sub

	' Our page has finished rendering so lets output the
	' JavaScript to produce the alert's
	Private Shared Sub ExecutingPage_Unload(sender As Object, e As EventArgs)
		' Get our message queue from the hashtable
		Dim queue As Queue = DirectCast(m_executingPages(HttpContext.Current.Handler), Queue)

		If queue IsNot Nothing Then
			Dim sb As New StringBuilder()
			' How many messages have been registered?
			Dim iMsgCount As Integer = queue.Count
			' Use StringBuilder to build up our client slide JavaScript.
			sb.Append("<script language='javascript'>")
			' Loop round registered messages
			Dim sMsg As String

			While System.Math.Max(System.Threading.Interlocked.Decrement(iMsgCount),iMsgCount + 1) > 0
				sMsg = DirectCast(queue.Dequeue(), String)
				sMsg = sMsg.Replace(vbLf, "\n")
				sMsg = sMsg.Replace("""", "'")
				sb.Append("alert( """ & sMsg & """ );")
			End While
			' 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())
		End If
	End Sub
End Class