Why do I get the error message 'System.InvalidOperationException: It is invalid to show a modal dialog or form when the application is not running in UserInteractive mode. Specify the ServiceNotification or DefaultDesktopOnly style to display a ....'

You can’t use MsgBox or MessageBox.Show in ASP.NET WebForm.
You may use:

VB.NET

Response.Write('<script>alert(’Hello’);</script>')

Response.Write('<script>alert(’Hello’);</script>') ;

Why do I get the error message ‘System.InvalidOperationException: It is invalid to show a modal dialog or form when the application is not running in UserInteractive mode. Specify the ServiceNotification or DefaultDesktopOnly style to display a ….’