Displaying Alert Message Boxes
Besides having prompting the user to confirm that they want to perform an action, we might also want to notify the
user of some server-side behavior. For example, if there was some server-side error and the data entered wasn't correctly
saved, we might want to use an alert message box to inform the user. Or perhaps when attempting to add the data provided
into the database, we deduced that the provided data was duplicate data.
The question becomes, coming back from the server to the client, how would we alert the user with a message box?
Traditionally we would do this with JavaScript in the html of the page by creating an onload event that
generated a message box. However like the previous case, ASP.NET provides us additional methods to handle all this
from the code-behind without having to hard code HTML ourselves.
What we want to do is register a client script block with the ASP.NET Web page. More specifically, we want this to
run when the page starts up on the client. Again it's our lucky day, as the ASP.NET Page class provides
a RegisterStartupScript() method for that very purpose.
Typically what will happen is the user will submit his data, we'll do some processing, and then realize that we need
to alert the user of some unexpected behavior. Therefore, we'll probably need to add this RegisterStartupScript()
method call in the Button Web control's server-side Click event handler. The code for this might look
something like:
Private Sub BtnSave_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles BtnSave.Click
'Generates the message:
Dim strMessage As String
If (Me.CheckBox1.Checked()) Then
strMessage = "The data was saved."
Else
strMessage = "The data was NOT saved."
End If
'finishes server processing, returns to client.
Dim strScript As String = "<script language=JavaScript>"
strScript += "alert(""" & strMessage & """);"
strScript += "</script>"
If (Not Page.IsStartupScriptRegistered("clientScript")) Then
Page.RegisterStartupScript("clientScript", strScript)
End If
End Sub
This code first generates the message by checking field values (in this case if a CheckBox1 CheckBox Web control was
checked or not) and building the message appropriately. However note that the message could be generated from any
server-side process, such as the return value of a business object, or based upon the results of a stored procedure
or database query.
Once we have the message, we can build the client-side JavaScript script block.
The RegisterStartupScript() method takes two string inputs: a key, identifying the script block being
registered, and the actual script itself. Notice that the second input parameter, the client-side script, includes
both the <script> tag, and the JavaScript code to run (alert('...');).
Before calling RegisterStartupScript() it is prudent to first check to make sure that the script block
hasn't already been registered. (In our simple example above, it clearly couldn't have already been registered, but
if you are registering a script block based on, say, values in a DataGrid, there might be multiple records that would
cause the script block to be rendered. A check to IsStartupScriptRegistered() quickly determines if
a startup script has already been registered or not.)
In the screen below, the user has selected the checkbox and then clicked the save button. The server then generates
the necessary JavaScript block, registers it, and displays it upon returning the page to the client.
Like the previous sample, we can also abstract this functionality to a Utilities class.
For this method, we need to pass in a Page class instance (since the RegisterStartupScript()
and IsStartupScriptRegistered() methods are methods of the Page class), along with the
string to display in the alert message box and the key name by which to register the script.
Public Class Utilities
Public Shared Sub CreateMessageAlert(ByRef aspxPage As System.Web.UI.Page, _
ByVal strMessage As String, ByVal strKey As String)
Dim strScript As String = "<script language=JavaScript>alert('" _
& strMessage & "')</script>"
If (Not aspxPage.IsStartupScriptRegistered(strKey)) Then
aspxPage.RegisterStartupScript(strKey, strScript)
End If
End Sub
End Class
The original page could then call the CreateMessageAlert() method like so:
Private Sub BtnSaveUtil_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles BtnSaveUtil.Click
'Generates the message:
Dim strMessage As String
If (Me.CheckBox1.Checked()) Then
strMessage = "The data was saved."
Else
strMessage = "The data was NOT saved."
End If
'Call separate class, passing page reference, to register Client Script:
Utilities.CreateMessageAlert(Me, strMessage, "strKey1")
End Sub
Notice that when calling the CreateMessageAlert() method we pass in Me for the Page class
instance. This is because the code-behind class is inherited from the Page class, so we can just
pass in the code-behind class reference. The second parameter is the message to display in the alert message box and
the third input parameter provides the key name for the script block.
Conclusion
In this article we looked at how to add client-side message boxes to your ASP.NET application. Adding confirm message
boxes is as simple as adding an onclick attribute to the Button Web control's Attributes collection.
Displaying an alert message box on page load involves calling the Page.RegisterStartupScript() method, passing
in the complete JavaScript block that should be performed on page load.
