i meeted the same question a few days before, i know i can't use messagebox directly, so i used a row of code like this in my .aspx.vb file:
button1.Attributes.Add("onclick", "window.alert('The username can't be null!');")
but the question is i don't know how to control it under a condition of "If userName.Text = Nothing", please help me, thanks

Do better than yesterday!

or how to realize the alert message with javascript in the .aspx.vb file?

here is my code in my .aspx.vb file, i want to alert user if he don't input his name in the login page:
Private Sub btnEnter_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnEnter.Click
        If txtName.Text = Nothing Then
            btnEnter.Attributes.Add("onclick", "window.alert('The username can't be NULL!');")
            Exit Sub
        End If
End Sub
but it can't work as i expected, but i don't know how to modify it, how to touch off the attribute under the assigned condition?

add a function in ut <script> & call it validating()
then set ur last code as
button1.Attributes.Add("onclick", "validating();")
then u can control all u want there & aleret in the case want!

HTH.

Always:),
Hovik Melkomian.

after i add the attributes, only when i click my left button at the second time, it can work to alert, how can make it work at the first click?

Ensure u have added it in ur PostBack, check that & set it in first line of page_load.

Always:),
Hovik Melkomian.

You are missing a fundamental concept. This is code-behind code. "btnEnter_Click()" will only run when the page is posted back. You must add the attribute to the button unconditionally (i.e. in page_load or similar). Then you need to do your text test in javascript, not in the code behind.

Try this:

btnEnter.Attributes.Add("onclick", String.Format("if(document.getElementById('{0}').v alue==''){window.alert('The username can't be NULL!');return false;}else{return true;}", txtName.ClientId))

This will test the value of the textbox for '' and put up the alert, then return false if it's empty. If not, it will return true. The end result is that if the test passes, the button should fire the postback. If the textbox is empty, the button should NOT fire the postback. (Please note, I have not tested this exact code.)

i add your code in my sub Page_Load, as follow:
btnEnter.Attributes.Add("onclick", String.Format("if(document.getElementById('{0}').v alue==''){window.alert('The username can't be NULL!');return false;}else{return true;}", txtName.Text))
when i run, i get the error: System.FormatException: format of input string is not correct.
i think i can't accurately understand your "getElementById" and "txtName.ClientId".

if u ask me I'll call only a JavaScript-function by clicking button from Behinde page & then use my all conditions & ... from there, it'll help u to see where is the problem from.

Always:),
Hovik Melkomian.

i know ur JavaScript-function , but i don't know how to use it.i develop my page with .net,the code is in a code-behind ".vb" file. <script></script> is not allowed, i can alert my user with a label control, but i prefer to the "window.alert".

Do better than yesterday!