Message box in Asp.Net

Is it possible to create a message box in ASP.Net in this code:

Code:

---


sub test()
dim tester as single
try
  tester = Csng(txtBox1.text)
catch
  ((Show message box with text: "Please enter a number"))
end try

---

Maybe I forgot to make some Imports System.Web.?????

messagebox in ASP.NET isn't possible, but you can create a javascript alert to run if a button is clicked.

VB Code:

Private Sub btnUpload_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnUpload.Click
        If (myExpression <> True) Then
            Dim strScript As String = ""
 
            strScript = "<script>"
            strScript = strScript & "alert('You must select a file to upload');"
            strScript = strScript & "</script>"
 
            'Allows the alert box to be shown without wiping
            'the controls off the page.
            Page.RegisterStartupScript("ClientScript", strScript)
     End If
End Sub