Cannot access a disposed object - How to fix

Question

In a VB NET WinForms project  I get an exception     Cannot access a disposed of object   when closing a form  It occurs very rarely and I cannot recreate it on demand  The stack trace looks like this   Cannot access a disposed object  Object name   dbiSchedule     at System Windows Forms Control CreateHandle     at System Windows Forms Control get Handle     at System Windows Forms Control PointToScreen Point p    at Dbi WinControl Schedule dbiSchedule a Boolean A 0    at Dbi WinControl Schedule dbiSchedule a Object A 0  EventArgs A 1    at System Windows Forms Timer OnTick EventArgs e    at System Windows Forms Timer TimerNativeWindow WndProc Message amp  m    at System Windows Forms NativeWindow Callback IntPtr hWnd  Int32 msg  IntPtr wparam  IntPtr lparam    The dbiSchedule is a schedule control from Dbi-tech  There is a timer on the form that updates the schedule on the screen every few minutes   Any ideas what is causing the exception and how I might go about fixing it  or even just being able to recreate it on demand     Hej  Thanks for all the answers  We do stop the Timer on the FormClosing event and we do check the IsDisposed property on the schedule component before using it in the Timer Tick event but it doesn t help   It s a really annoying problem because if someone did come up with a solution that worked - I wouldn t be able to confirm the solution because I cannot recreate the problem manually

User · Answer

Stopping the timer doesn t mean that it won t be called again  depending on when you stop the timer  the timer tick may still be queued on the message loop for the form   What will happen is that you ll get one more tick that you may not be expecting   What you can do is in your timer tick  check the Enabled property of your timer before executing the Timer Tick method

User · Answer

Try checking the IsDisposed property before accessing the control  You can also check it on the FormClosing event  assuming you re using the FormClosed event      We do stop the Timer on the   FormClosing event and we do check the   IsDisposed property on the schedule   component before using it in the Timer   Tick event but it doesn t help    Calling GC Collect before checking IsDisposed may help  but be careful with this  Read this article by Rico Mariani  When to call GC Collect

User · Answer

Looking at the error stack trace  it seems your timer is still active  Try to cancel the timer upon closing the form  i e  in the form s OnClose   method   This looks like the cleanest solution

User · Answer

It looks like a threading issue  Hypothesis  Maybe you have the main thread and a timer thread accessing this control  The main thread shuts down - calling Control Dispose   to indicate that I m done with this Control and I shall make no more calls to this  However  the timer thread is still active - a context switch to that thread  where it may call methods on the same control  Now the control says I m Disposed  already given up my resources  and I shall not work anymore  ObjectDisposed exception   How to solve this  In the timer thread  before calling methods properties on the control  do a check with   if ControlObject IsDisposed then return     or do whatever - but don t call control methods   OR stop the timer thread BEFORE disposing the object

User · Answer

because the solution folder was inside OneDrive folder   If you moving the solution folders out of the one drive folder made the errors go away   best

User · Answer

we do check the IsDisposed property on   the schedule component before using it   in the Timer Tick event but it doesn t   help    If I understand that stack trace  it s not your timer which is the problem  it s one in the control itself - it might be them who are not cleaning-up properly   Are you explicitly calling Dispose on their control

User · Answer

My Solution was to put a try catch   amp  is working fine     try      this Invoke new EventHandler DoUpdate         catch

User · Answer

Another place you could stop the timer is the FormClosing event - this happens before the form is actually closed  so is a good place to stop things before they might access unavailable resources

User · Answer

I had the same problem and solved it using a boolean flag that gets set when the form is closing  the System Timers Timer does not have an IsDisposed property    Everywhere on the form I was starting the timer  I had it check this flag   If it was set  then don t start the timer   Here s the reason   The Reason   I was stopping and disposing of the timer in the form closing event   I was starting the timer in the Timer Elapsed   event   If I were to close the form in the middle of the Timer Elapsed   event  the timer would immediately get disposed by the Form Closing   event   This would happen before the Timer Elapsed   event would finish and more importantly  before it got to this line of code    timer Start     As soon as that line was executed an ObjectDisposedException   would get thrown with the error you mentioned   The Solution   Private Sub myForm FormClosing ByVal sender As System Object  ByVal e As System Windows Forms FormClosingEventArgs  Handles MyBase FormClosing       set the form closing flag so the timer doesn t fire even after the form is closed       formIsClosing   True      timer Stop        timer Dispose   End Sub   Here s the timer elapsed event   Private Sub Timer Elapsed ByVal sender As System Object  ByVal e As System Timers ElapsedEventArgs  Handles  timer Elapsed       Don t want the timer stepping on itself  ie  the time interval elapses before the first call is done processing       timer Stop          do work here        Only start the timer if the form is open  Without this check  the timer will run even if the form is closed      If Not  formIsClosing Then          timer Interval    refreshInterval          timer Start     ObjectDisposedException   is thrown here unless you check the  formIsClosing flag      End If End Sub   The interesting thing to know  even though it would throw the ObjectDisposedException when attempting to start the timer  the timer would still get started causing it to run even when the form was closed  the thread would only stop when the application was closed

User · Answer

If this happens sporadically then my guess is that it has something to do with the timer   I m guessing  and this is only a guess since I have no access to your code  that the timer is firing while the form is being closed   The dbiSchedule object has been disposed but the timer somehow still manages to try to call it   This shouldn t happen  because if the timer has a reference to the schedule object then the garbage collector should see this and not dispose of it   This leads me to ask  are you calling Dispose   on the schedule object manually   If so  are you doing that before disposing of the timer   Be sure that you release all references to the schedule object before Disposing it  i e  dispose of the timer beforehand    Now I realize that a few months have passed between the time you posted this and when I am answering  so hopefully  you have resolved this issue   I m writing this for the benefit of others who may come along later with a similar issue   Hope this helps

User · Answer

You sure the timer isn t outliving the  dbiSchedule  somehow and firing after the  dbiSchedule  has been been disposed of   If that is the case you might be able to recreate it more consistently if the timer fires more quickly thus increasing the chances of you closing the Form just as the timer is firing

[.net] Cannot access a disposed object - How to fix?

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