Application.ThreadException Event
Occurs when an untrapped thread exception is thrown.
Assembly: System.Windows.Forms (in System.Windows.Forms.dll)
This event allows your Windows Forms application to handle otherwise unhandled exceptions that occur in Windows Forms threads. Attach your event handlers to the ThreadException event to deal with these exceptions, which will leave your application in an unknown state. Where possible, exceptions should be handled by a structured exception handling block.
You can change whether this callback is used for unhandled Windows Forms thread exceptions by setting SetUnhandledExceptionMode. To catch exceptions that occur in threads not created and owned by Windows Forms, use the UnhandledException event handler.
Note: |
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To guarantee that no activations of this event are missed, you must attach a handler before you call Application.Run. |
Caution: |
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Because this is a static event, you must detach your event handlers when your application is disposed, or memory leaks will result. |
The following code example sets event handlers for exceptions that occur on Windows Forms threads and exceptions that occur on other threads. It sets SetUnhandledExceptionMode so that all exceptions are handled by the application, regardless of the settings in the application's user configuration file. It uses the ThreadException event to handle UI thread exceptions, and the UnhandledException event to handle non-UI thread exceptions. Since UnhandledException cannot prevent an application from terminating, the example simply logs the error in the application event log before termination.
This example assumes that you have defined two Button controls, button1 and button2, on your Form class.
Private newThread As Thread = Nothing
' Starts the application.
<SecurityPermission(SecurityAction.Demand, Flags:=SecurityPermissionFlag.ControlAppDomain)> _
Public Shared Sub Main()
' Add the event handler for handling UI thread exceptions to the event.
AddHandler Application.ThreadException, AddressOf ErrorHandlerForm.Form1_UIThreadException
' Set the unhandled exception mode to force all Windows Forms errors to go through
' our handler.
Application.SetUnhandledExceptionMode(UnhandledExceptionMode.CatchException)
' Add the event handler for handling non-UI thread exceptions to the event.
AddHandler AppDomain.CurrentDomain.UnhandledException, AddressOf CurrentDomain_UnhandledException
' Runs the application.
Application.Run(New ErrorHandlerForm())
End Sub
' Programs the button to throw an exception when clicked.
Private Sub button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles button1.Click
Throw New ArgumentException("The parameter was invalid")
End Sub
' Start a new thread, separate from Windows Forms, that will throw an exception.
Private Sub button2_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles button2.Click
Dim newThreadStart As New ThreadStart(AddressOf newThread_Execute)
newThread = New Thread(newThreadStart)
newThread.Start()
End Sub
' The thread we start up to demonstrate non-UI exception handling.
Sub newThread_Execute()
Throw New Exception("The method or operation is not implemented.")
End Sub
' Handle the UI exceptions by showing a dialog box, and asking the user whether
' or not they wish to abort execution.
Private Shared Sub Form1_UIThreadException(ByVal sender As Object, ByVal t As ThreadExceptionEventArgs)
Dim result As System.Windows.Forms.DialogResult = _
System.Windows.Forms.DialogResult.Cancel
Try
result = ShowThreadExceptionDialog("Windows Forms Error", t.Exception)
Catch
Try
MessageBox.Show("Fatal Windows Forms Error", _
"Fatal Windows Forms Error", MessageBoxButtons.AbortRetryIgnore, MessageBoxIcon.Stop)
Finally
Application.Exit()
End Try
End Try
' Exits the program when the user clicks Abort.
If result = DialogResult.Abort Then
Application.Exit()
End If
End Sub
' Handle the UI exceptions by showing a dialog box, and asking the user whether
' or not they wish to abort execution.
' NOTE: This exception cannot be kept from terminating the application - it can only
' log the event, and inform the user about it.
Private Shared Sub CurrentDomain_UnhandledException(ByVal sender As Object, _
ByVal e As UnhandledExceptionEventArgs)
Try
Dim ex As Exception = CType(e.ExceptionObject, Exception)
Dim errorMsg As String = "An application error occurred. Please contact the adminstrator " & _
"with the following information:" & ControlChars.Lf & ControlChars.Lf
' Since we can't prevent the app from terminating, log this to the event log.
If (Not EventLog.SourceExists("ThreadException")) Then
EventLog.CreateEventSource("ThreadException", "Application")
End If
' Create an EventLog instance and assign its source.
Dim myLog As New EventLog()
myLog.Source = "ThreadException"
myLog.WriteEntry((errorMsg + ex.Message & ControlChars.Lf & ControlChars.Lf & _
"Stack Trace:" & ControlChars.Lf & ex.StackTrace))
Catch exc As Exception
Try
MessageBox.Show("Fatal Non-UI Error", "Fatal Non-UI Error. Could not write the error to the event log. " & _
"Reason: " & exc.Message, MessageBoxButtons.OK, MessageBoxIcon.Stop)
Finally
Application.Exit()
End Try
End Try
End Sub
' Creates the error message and displays it.
Private Shared Function ShowThreadExceptionDialog(ByVal title As String, ByVal e As Exception) As DialogResult
Dim errorMsg As String = "An application error occurred. Please contact the adminstrator " & _
"with the following information:" & ControlChars.Lf & ControlChars.Lf
errorMsg = errorMsg & e.Message & ControlChars.Lf & _
ControlChars.Lf & "Stack Trace:" & ControlChars.Lf & e.StackTrace
Return MessageBox.Show(errorMsg, title, MessageBoxButtons.AbortRetryIgnore, MessageBoxIcon.Stop)
End Function
- SecurityPermission
for the immediate caller to call unmanaged code when adding a handler to this event. Associated enumeration: SecurityPermissionFlag.UnmanagedCode
Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98
The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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