AppDomain.UnhandledException Event

Definition

Namespace:

System

Assembly:

System.Runtime.dll

Occurs when an exception is not caught.

public:
 event UnhandledExceptionEventHandler ^ UnhandledException;

public event UnhandledExceptionEventHandler? UnhandledException;

member this.UnhandledException : UnhandledExceptionEventHandler 

Public Custom Event UnhandledException As UnhandledExceptionEventHandler 

Event Type

UnhandledExceptionEventHandler

Examples

The following example demonstrates the UnhandledException event. It defines an event handler, MyHandler, that is invoked whenever an unhandled exception is thrown in the default application domain. It then throws two exceptions. The first is handled by a try/catch block. The second is unhandled and invokes the MyHandle routine before the application terminates.

// The example should be compiled with the /clr:pure compiler option.
using namespace System;
using namespace System::Security::Permissions;

public ref class Example
{


private:
   static void MyHandler(Object^ sender, UnhandledExceptionEventArgs^ args)
   {
      Exception^ e = dynamic_cast<Exception^>(args->ExceptionObject);
      Console::WriteLine( "MyHandler caught : {0}", e->Message );
      Console::WriteLine("Runtime terminating: {0}", args->IsTerminating);
   }
   
public: 
   [SecurityPermissionAttribute( SecurityAction::Demand, ControlAppDomain = true )]
   static void Main()
   {
      AppDomain^ currentDomain = AppDomain::CurrentDomain;
      currentDomain->UnhandledException += gcnew UnhandledExceptionEventHandler(Example::MyHandler);
      try
      {
         throw gcnew Exception("1");
      }
      catch (Exception^ e) 
      {
         Console::WriteLine( "Catch clause caught : {0}\n", e->Message );
      }

      throw gcnew Exception("2");
   }
};

void main()
{
   Example::Main();
}   
// The example displays the following output:
//       Catch clause caught : 1
//       
//       MyHandler caught : 2
//       Runtime terminating: True
//       
//       Unhandled Exception: System.Exception: 2
//          at Example.Main()
//          at mainCRTStartup(String[] arguments)

using System;

public class Example
{
   public static void Main()
   {
      AppDomain currentDomain = AppDomain.CurrentDomain;
      currentDomain.UnhandledException += new UnhandledExceptionEventHandler(MyHandler);

      try {
         throw new Exception("1");
      } catch (Exception e) {
         Console.WriteLine("Catch clause caught : {0} \n", e.Message);
      }

      throw new Exception("2");
   }

   static void MyHandler(object sender, UnhandledExceptionEventArgs args)
   {
      Exception e = (Exception) args.ExceptionObject;
      Console.WriteLine("MyHandler caught : " + e.Message);
      Console.WriteLine("Runtime terminating: {0}", args.IsTerminating);
   }
}
// The example displays the following output:
//       Catch clause caught : 1
//
//       MyHandler caught : 2
//       Runtime terminating: True
//
//       Unhandled Exception: System.Exception: 2
//          at Example.Main()

open System
open System.Security.Permissions

let myHandler _ (args: UnhandledExceptionEventArgs) =
    let e = args.ExceptionObject :?> Exception
    printfn $"MyHandler caught : {e.Message}"
    printfn $"Runtime terminating: {args.IsTerminating}"

[<EntryPoint>]
let main _ =
    let currentDomain = AppDomain.CurrentDomain
    currentDomain.UnhandledException.AddHandler(UnhandledExceptionEventHandler myHandler)

    try
        failwith "1"
    with e ->
        printfn $"Catch clause caught : {e.Message} \n"

    failwith "2"

// The example displays the following output:
//       Catch clause caught : 1
//
//       MyHandler caught : 2
//       Runtime terminating: True
//
//       Unhandled Exception: System.Exception: 2
//          at Example.main()

Module Example
   Sub Main()
      Dim currentDomain As AppDomain = AppDomain.CurrentDomain
      AddHandler currentDomain.UnhandledException, AddressOf MyHandler
      
      Try
         Throw New Exception("1")
      Catch e As Exception
         Console.WriteLine("Catch clause caught : " + e.Message)
         Console.WriteLine()
      End Try
      
      Throw New Exception("2")
   End Sub
   
   Sub MyHandler(sender As Object, args As UnhandledExceptionEventArgs)
      Dim e As Exception = DirectCast(args.ExceptionObject, Exception)
      Console.WriteLine("MyHandler caught : " + e.Message)
      Console.WriteLine("Runtime terminating: {0}", args.IsTerminating)
   End Sub
End Module
' The example displays the following output:
'       Catch clause caught : 1
'       
'       MyHandler caught : 2
'       Runtime terminating: True
'       
'       Unhandled Exception: System.Exception: 2
'          at Example.Main()

Remarks

This event provides notification of uncaught exceptions. It allows the application to log information about the exception before the system default handler reports the exception to the user and terminates the application. If sufficient information about the state of the application is available, other actions may be undertaken - such as saving program data for later recovery. Caution is advised, because program data can become corrupted when exceptions are not handled.

Note

In the .NET Framework versions 1.0 and 1.1, application termination and debugging options are reported to the user before this event is raised, rather than after.

This event can be handled in any application domain. However, the event is not necessarily raised in the application domain where the exception occurred. An exception is unhandled only if the entire stack for the thread has been unwound without finding an applicable exception handler, so the first place the event can be raised is in the application domain where the thread originated.

Note

In the .NET Framework versions 1.0 and 1.1, this event occurs only for the default application domain that is created by the system when an application is started. If an application creates additional application domains, specifying a delegate for this event in those applications domains has no effect.

If the UnhandledException event is handled in the default application domain, it is raised there for any unhandled exception in any thread, no matter what application domain the thread started in. If the thread started in an application domain that has an event handler for UnhandledException, the event is raised in that application domain. If that application domain is not the default application domain, and there is also an event handler in the default application domain, the event is raised in both application domains.

For example, suppose a thread starts in application domain "AD1", calls a method in application domain "AD2", and from there calls a method in application domain "AD3", where it throws an exception. The first application domain in which the UnhandledException event can be raised is "AD1". If that application domain is not the default application domain, the event can also be raised in the default application domain.

Note

The common language runtime suspends thread aborts while event handlers for the UnhandledException event are executing.

If the event handler has a ReliabilityContractAttribute attribute with the appropriate flags, the event handler is treated as a constrained execution region.

Starting with the .NET Framework 4, this event is not raised for exceptions that corrupt the state of the process, such as stack overflows or access violations, unless the event handler is security-critical and has the HandleProcessCorruptedStateExceptionsAttribute attribute.

In the .NET Framework versions 1.0 and 1.1, an unhandled exception that occurs in a thread other than the main application thread is caught by the runtime and therefore does not cause the application to terminate. Thus, it is possible for the UnhandledException event to be raised without the application terminating. Starting with the .NET Framework version 2.0, this backstop for unhandled exceptions in child threads was removed, because the cumulative effect of such silent failures included performance degradation, corrupted data, and lockups, all of which were difficult to debug. For more information, including a list of cases in which the runtime does not terminate, see Exceptions in Managed Threads.

To register an event handler for this event, you must have the required permissions, or a SecurityException is thrown.

For more information about handling events, see Handling and Raising Events.

Other Events for Unhandled Exceptions

For certain application models, the UnhandledException event can be preempted by other events if the unhandled exception occurs in the main application thread.

In applications that use Windows Forms, unhandled exceptions in the main application thread cause the Application.ThreadException event to be raised. If this event is handled, the default behavior is that the unhandled exception does not terminate the application, although the application is left in an unknown state. In that case, the UnhandledException event is not raised. This behavior can be changed by using the application configuration file, or by using the Application.SetUnhandledExceptionMode method to change the mode to UnhandledExceptionMode.ThrowException before the ThreadException event handler is hooked up. This applies only to the main application thread. The UnhandledException event is raised for unhandled exceptions thrown in other threads.

Starting with Microsoft Visual Studio 2005, the Visual Basic application framework provides another event for unhandled exceptions in the main application thread. See the WindowsFormsApplicationBase.UnhandledException event. This event has an event arguments object with the same name as the event arguments object used by AppDomain.UnhandledException, but with different properties. In particular, this event arguments object has an ExitApplication property that allows the application to continue running, ignoring the unhandled exception (and leaving the application in an unknown state). In that case, the AppDomain.UnhandledException event is not raised.