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In Microsoft Windows applications you can call MessageBox to display a message to the user. Unfortunately,
there is no equivalent in web applications when you are executing server-side code. For example, if you write
a validation routine, you cannot simply call MessageBox to display an error message to the user.
If you want to give an error message to the user, you can use the standard <Validator>.ErrorMessage property to
set the value that will be displayed to the user. In some cases, you might want to have your own custom function.
So how can we do this?
In Microsoft .NET applications, data validation occurs in a postback. When the page is first loaded, the data
is displayed, and when you press the Submit button, a postback is triggered and your server-side code is called
to validate the data. This postback will eventually either re-display this page or redirect to another page.
If you want to display an error message, you need to do two things. First, register the alert message, and second,
cancel any redirection. Registering the alert message ensures that when the page is re-displayed as part of the
postback, any registered messages will be displayed to the user. If you do not cancel the redirection to another
page, your registered alert message will never be displayed.
Use the RegisterJScirptAlert method to register the alert message:
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BaseClasses.Utils.MiscUtils.RegisterJScriptAlert(Me, "MyAlertName", "My Alert Text")
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The RegisterJScriptAlert method can be used to display a message in user defined methods, BaseClasses methods,
or in events.
In the code below, override the OnApplicationEvent() method in the TableControl class and handle the DeleteRecord
event within it. If the UnitPrice of a selected record for the Order_Details table is greater than $10, the
RegisterJScriptAlert method is used to display a "Cannot delete record" message.
For .NET Framework 1.1, add the code below in ShowOrder_DetailsTablePage.aspx.cs, located in
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...\<App Folder>\Order_Details\ShowOrder_DetailsTablePage.aspx.cs
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For .NET Framework 2.0, add the code below in ShowOrder_DetailsTablePage.Controls.cs, located in
...\<App
Folder>\App_Code\Order_Details\ShowOrder_DetailsTablePage.Controls.cs
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C#:
public override void OnApplicationEvent(BaseClasses. ApplicationEventArgs args)
{
if(args.EventType == BaseClasses. ApplicationEventArgs.EventTypes.DeleteRecord)
{
Order_DetailsRecord selectedRecord = this.GetSelectedRecord(true);
decimal myUnitPrice = selectedRecord.UnitPrice;
if(myUnitPrice >10)
{
BaseClasses.Utils.MiscUtils.RegisterJScriptAlert(this,"CANNOT_ DISPLAY_MESSAGE","Cannot delete record");
return;
}
}
base.OnApplicationEvent(args);
}
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Visual Basic .NET
Public Overrides Sub OnApplicationEvent(ByVal args As BaseClasses.ApplicationEventArgs
If (args.EventType = BaseClasses. ApplicationEventArgs.EventTypes.DeleteRecord) Then
Dim selectedRecord As Order_DetailsRecord = Me.GetSelectedRecord(True)
Dim myUnitPrice As Decimal = selectedRecord.UnitPrice
If (myUnitPrice > 10) Then
BaseClasses.Utils.MiscUtils.RegisterJScriptAlert(Me, " CANNOT_DELETE_MESSAGE", "Cannot delete the record")
Else
'Do not stop the deletion of record and call the base OnApplicationEvent which will delete the record.
MyBase.OnApplicationEvent(args)
End If
Else
MyBase.OnApplicationEvent(args)
End If
End Sub
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