Program crash after navigation

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Hi,

I have a program with button which will navigate to specific page once clicked. But if I click fasten on so many clicked on one button, program will crash with exception "Object reference not set to an instance of an object"..I do fasten click as on production it faced on public. So kids also touch the program and play. But as what I practice, the program crash. Code that faced error is on here.

VB

'Constant is placed on global Module
Public Const GoWelcome As String = "PageWelcome.xaml"
Public Const GoCategoryList As String = "PageCategoryList.xaml"

'Here goes after clicked
Private Sub go_completed(ByVal sender As Object, ByVal e As EventArgs)
    Try
        NavigationService.Navigate(New Uri(GoCategoryList , UriKind.Relative))
    Catch ex As Exception 'Goes to exception after fast clicking
        NavigationService.Navigate(New Uri(GoWelcome, UriKind.Relative))
    End Try
End Sub

go_completed is my storyboard finish play.

How to prevent from page navigation error?

Posted 30-May-15 22:08pm

Luiey Ichigo

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Sergey Alexandrovich Kryukov 31-May-15 13:45pm

In what line?
—SA

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Sergey Alexandrovich Kryukov · Answer 1 · 2015-05-31T08:27:00

You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object".

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx,
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx.

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx.

Good luck,

—SA