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Application.ThreadException Event

Occurs when an untrapped thread exception is thrown.

Namespace:  System.Windows.Forms
Assembly:  System.Windows.Forms (in System.Windows.Forms.dll)
public static event ThreadExceptionEventHandler ThreadException

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.

NoteNote

To guarantee that no activations of this event are missed, you must attach a handler before you call Application.Run.

Caution noteCaution

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.

Thread newThread = null;

// Starts the application. 
[SecurityPermission(SecurityAction.Demand, Flags = SecurityPermissionFlag.ControlAppDomain)]
public static void Main(string[] args)
{
    // Add the event handler for handling UI thread exceptions to the event.
    Application.ThreadException += new ThreadExceptionEventHandler(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. 
    AppDomain.CurrentDomain.UnhandledException +=
        new UnhandledExceptionEventHandler(CurrentDomain_UnhandledException);

    // Runs the application.
    Application.Run(new ErrorHandlerForm());
}

// Programs the button to throw an exception when clicked. 
private void button1_Click(object sender, System.EventArgs e)
{
    throw new ArgumentException("The parameter was invalid");
}

// Start a new thread, separate from Windows Forms, that will throw an exception. 
private void button2_Click(object sender, System.EventArgs e)
{
    ThreadStart newThreadStart = new ThreadStart(newThread_Execute);
    newThread = new Thread(newThreadStart);
    newThread.Start();
}

// The thread we start up to demonstrate non-UI exception handling.  
void newThread_Execute()
{
    throw new Exception("The method or operation is not implemented.");
}

// Handle the UI exceptions by showing a dialog box, and asking the user whether 
// or not they wish to abort execution. 
private static void Form1_UIThreadException(object sender, ThreadExceptionEventArgs t)
{
    DialogResult result = 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();
        }
    }

    // Exits the program when the user clicks Abort. 
    if (result == DialogResult.Abort)
        Application.Exit();
}

// 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 static void CurrentDomain_UnhandledException(object sender, UnhandledExceptionEventArgs e)
{
    try
    {
        Exception ex = (Exception)e.ExceptionObject;
        string errorMsg = "An application error occurred. Please contact the adminstrator " +
            "with the following information:\n\n";

        // Since we can't prevent the app from terminating, log this to the event log. 
        if (!EventLog.SourceExists("ThreadException"))
        {
            EventLog.CreateEventSource("ThreadException", "Application");
        }

        // Create an EventLog instance and assign its source.
        EventLog myLog = new EventLog();
        myLog.Source = "ThreadException";
        myLog.WriteEntry(errorMsg + ex.Message + "\n\nStack Trace:\n" + ex.StackTrace);
    }
    catch (Exception exc)
    {
        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();
        }
    }
}

// Creates the error message and displays it. 
private static DialogResult ShowThreadExceptionDialog(string title, Exception e)
{
    string errorMsg = "An application error occurred. Please contact the adminstrator " +
        "with the following information:\n\n";
    errorMsg = errorMsg + e.Message + "\n\nStack Trace:\n" + e.StackTrace;
    return MessageBox.Show(errorMsg, title, MessageBoxButtons.AbortRetryIgnore,
        MessageBoxIcon.Stop);
}

.NET Framework

Supported in: 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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