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Preventing crashes using UnhandledExceptionHandler

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0

Preface: I'm perfectly aware of how bad doing this is, I'm just curious as to why it works

I saw this piece of code while browsing questions related to UnhandledException and wondered why calling Thread.CurrentThread.Join() in the handler prevents the app from crashing, and what does a thread calling Join() on itself even mean in this scenario?

AddHandler AppDomain.CurrentDomain.UnhandledException, AddressOf UnhandledException

Public Sub UnhandledException(ByVal sender As Object, ByVal e As UnhandledExceptionEventArgs)
 Console.WriteLine($"[ERROR] DirectCast(e.ExceptionObject, Exception).Message")
 'Thread.CurrentThread.IsBackground = True
 Thread.CurrentThread.Join()
End Sub

I also noticed that that while this handler does indeed prevent crashing the app it seems to leave threads in some unknown/running state (I mean, everything is in an undefined state at this point anyway; but still) which means uncommenting the Thread.CurrentThread.IsBackground = True allows the app to exit when the main thread finishes.

Public Sub Main()
 Application.EnableVisualStyles()
 Application.SetCompatibleTextRenderingDefault(False)

 Application.SetUnhandledExceptionMode(UnhandledExceptionMode.CatchException)
 AddHandler AppDomain.CurrentDomain.UnhandledException, AddressOf UnhandledException

 Application.Run(New Form1) ' Form1 has a button that starts a thread which throws an exception
 ' This is only reached when Form1 is closed, but if there is
 ' no Thread.CurrentThread.IsBackground = True the app keeps running
 ' in the background because the other threads are still running or in
 ' an alive-ish state
End Sub

tl;dr: Why does calling Thread.CurrentThread.Join() in the UnhandledExceptionHandler prevent an app from crashing?

.net vb.net multithreading unhandled-exception

share|improve this question

edited Mar 9 at 7:41

John Mark Gabriel Caguicla

asked Mar 9 at 7:32

John Mark Gabriel CaguiclaJohn Mark Gabriel Caguicla

144

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The documentation explicitly states that you should not call Join on the current thread because it will prevent the thread terminating, so that is obviously the intention here. The thread on which the exception was thrown will never terminate of its own accord but, because it is a background thread, will not stop the application quitting. That all looks rather dodgy to me. Given that the application framework provides an UnhandledException event that allows you to prevent the application closing, I wonder whether it does that same thing.
  
  – jmcilhinney
  Mar 9 at 7:46
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @jmcilhinney Ah that makes sense, the self-Join essentially prevents the thread from exiting the handler and reaching .NET's actual Unhandled Exception handler (which presumably terminates the app) by causing the thread to hang.
  
  – John Mark Gabriel Caguicla
  Mar 9 at 8:15
  
  

  
  
  
  

add a comment | 

0

Preface: I'm perfectly aware of how bad doing this is, I'm just curious as to why it works

I saw this piece of code while browsing questions related to UnhandledException and wondered why calling Thread.CurrentThread.Join() in the handler prevents the app from crashing, and what does a thread calling Join() on itself even mean in this scenario?

AddHandler AppDomain.CurrentDomain.UnhandledException, AddressOf UnhandledException

Public Sub UnhandledException(ByVal sender As Object, ByVal e As UnhandledExceptionEventArgs)
 Console.WriteLine($"[ERROR] DirectCast(e.ExceptionObject, Exception).Message")
 'Thread.CurrentThread.IsBackground = True
 Thread.CurrentThread.Join()
End Sub

I also noticed that that while this handler does indeed prevent crashing the app it seems to leave threads in some unknown/running state (I mean, everything is in an undefined state at this point anyway; but still) which means uncommenting the Thread.CurrentThread.IsBackground = True allows the app to exit when the main thread finishes.

Public Sub Main()
 Application.EnableVisualStyles()
 Application.SetCompatibleTextRenderingDefault(False)

 Application.SetUnhandledExceptionMode(UnhandledExceptionMode.CatchException)
 AddHandler AppDomain.CurrentDomain.UnhandledException, AddressOf UnhandledException

 Application.Run(New Form1) ' Form1 has a button that starts a thread which throws an exception
 ' This is only reached when Form1 is closed, but if there is
 ' no Thread.CurrentThread.IsBackground = True the app keeps running
 ' in the background because the other threads are still running or in
 ' an alive-ish state
End Sub

tl;dr: Why does calling Thread.CurrentThread.Join() in the UnhandledExceptionHandler prevent an app from crashing?

.net vb.net multithreading unhandled-exception

share|improve this question

edited Mar 9 at 7:41

John Mark Gabriel Caguicla

asked Mar 9 at 7:32

John Mark Gabriel CaguiclaJohn Mark Gabriel Caguicla

144

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The documentation explicitly states that you should not call Join on the current thread because it will prevent the thread terminating, so that is obviously the intention here. The thread on which the exception was thrown will never terminate of its own accord but, because it is a background thread, will not stop the application quitting. That all looks rather dodgy to me. Given that the application framework provides an UnhandledException event that allows you to prevent the application closing, I wonder whether it does that same thing.
  
  – jmcilhinney
  Mar 9 at 7:46
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @jmcilhinney Ah that makes sense, the self-Join essentially prevents the thread from exiting the handler and reaching .NET's actual Unhandled Exception handler (which presumably terminates the app) by causing the thread to hang.
  
  – John Mark Gabriel Caguicla
  Mar 9 at 8:15
  
  

  
  
  
  

add a comment | 

Preface: I'm perfectly aware of how bad doing this is, I'm just curious as to why it works

I saw this piece of code while browsing questions related to UnhandledException and wondered why calling Thread.CurrentThread.Join() in the handler prevents the app from crashing, and what does a thread calling Join() on itself even mean in this scenario?

AddHandler AppDomain.CurrentDomain.UnhandledException, AddressOf UnhandledException

Public Sub UnhandledException(ByVal sender As Object, ByVal e As UnhandledExceptionEventArgs)
 Console.WriteLine($"[ERROR] DirectCast(e.ExceptionObject, Exception).Message")
 'Thread.CurrentThread.IsBackground = True
 Thread.CurrentThread.Join()
End Sub

I also noticed that that while this handler does indeed prevent crashing the app it seems to leave threads in some unknown/running state (I mean, everything is in an undefined state at this point anyway; but still) which means uncommenting the Thread.CurrentThread.IsBackground = True allows the app to exit when the main thread finishes.

Public Sub Main()
 Application.EnableVisualStyles()
 Application.SetCompatibleTextRenderingDefault(False)

 Application.SetUnhandledExceptionMode(UnhandledExceptionMode.CatchException)
 AddHandler AppDomain.CurrentDomain.UnhandledException, AddressOf UnhandledException

 Application.Run(New Form1) ' Form1 has a button that starts a thread which throws an exception
 ' This is only reached when Form1 is closed, but if there is
 ' no Thread.CurrentThread.IsBackground = True the app keeps running
 ' in the background because the other threads are still running or in
 ' an alive-ish state
End Sub

tl;dr: Why does calling Thread.CurrentThread.Join() in the UnhandledExceptionHandler prevent an app from crashing?

.net vb.net multithreading unhandled-exception

share|improve this question

edited Mar 9 at 7:41

John Mark Gabriel Caguicla

asked Mar 9 at 7:32

John Mark Gabriel CaguiclaJohn Mark Gabriel Caguicla

144

AddHandler AppDomain.CurrentDomain.UnhandledException, AddressOf UnhandledException

Public Sub UnhandledException(ByVal sender As Object, ByVal e As UnhandledExceptionEventArgs)
 Console.WriteLine($"[ERROR] DirectCast(e.ExceptionObject, Exception).Message")
 'Thread.CurrentThread.IsBackground = True
 Thread.CurrentThread.Join()
End Sub

Public Sub Main()
 Application.EnableVisualStyles()
 Application.SetCompatibleTextRenderingDefault(False)

 Application.SetUnhandledExceptionMode(UnhandledExceptionMode.CatchException)
 AddHandler AppDomain.CurrentDomain.UnhandledException, AddressOf UnhandledException

 Application.Run(New Form1) ' Form1 has a button that starts a thread which throws an exception
 ' This is only reached when Form1 is closed, but if there is
 ' no Thread.CurrentThread.IsBackground = True the app keeps running
 ' in the background because the other threads are still running or in
 ' an alive-ish state
End Sub

share|improve this question

edited Mar 9 at 7:41

John Mark Gabriel Caguicla

asked Mar 9 at 7:32

John Mark Gabriel CaguiclaJohn Mark Gabriel Caguicla

144

share|improve this question

edited Mar 9 at 7:41

John Mark Gabriel Caguicla

asked Mar 9 at 7:32

John Mark Gabriel CaguiclaJohn Mark Gabriel Caguicla

144

asked Mar 9 at 7:32

John Mark Gabriel CaguiclaJohn Mark Gabriel Caguicla

144

asked Mar 9 at 7:32

John Mark Gabriel CaguiclaJohn Mark Gabriel Caguicla

144