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UnhandledExceptionMode Enumeration

Defines where a Windows Forms application should send unhandled exceptions.

Namespace:  System.Windows.Forms
Assembly:  System.Windows.Forms (in System.Windows.Forms.dll)

'Declaration
Public Enumeration UnhandledExceptionMode

Member nameDescription
AutomaticRoute all exceptions to the ThreadException handler, unless the application's configuration file specifies otherwise.
CatchExceptionAlways route exceptions to the ThreadException handler. Ignore the application configuration file.
ThrowExceptionNever route exceptions to the ThreadException handler. Ignore the application configuration file.

This enumeration is used by SetUnhandledExceptionMode.

NoteNote

In the .NET Framework version 2.0, the common language runtime behavior changed to allow unhandled exceptions to propagate up the call stack. This behavior can be disabled via the application configuration file. For more information, see Exceptions in Managed Threads.

The following code example sets event handlers for both errors that occur on Windows Forms threads and errors that occur on other threads. It sets SetUnhandledExceptionMode so that all errors are handled by the application, regardless of the settings in the application's user configuration file. It uses the ThreadException event for the former, and the UnhandledException event for the latter. Since UnhandledException cannot prevent an application from terminating, the example simply logs the error in the system Event Log prior to termination.

This sample 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

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1
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