AppDomain.FirstChanceException Event

public:
 event EventHandler<System::Runtime::ExceptionServices::FirstChanceExceptionEventArgs ^> ^ FirstChanceException;

public event EventHandler<System.Runtime.ExceptionServices.FirstChanceExceptionEventArgs>? FirstChanceException;

public event EventHandler<System.Runtime.ExceptionServices.FirstChanceExceptionEventArgs> FirstChanceException;

[add: System.Security.SecurityCritical]
[remove: System.Security.SecurityCritical]
public event EventHandler<System.Runtime.ExceptionServices.FirstChanceExceptionEventArgs> FirstChanceException;

member this.FirstChanceException : EventHandler<System.Runtime.ExceptionServices.FirstChanceExceptionEventArgs> 

[<add: System.Security.SecurityCritical>]
[<remove: System.Security.SecurityCritical>]
member this.FirstChanceException : EventHandler<System.Runtime.ExceptionServices.FirstChanceExceptionEventArgs> 

Public Custom Event FirstChanceException As EventHandler(Of FirstChanceExceptionEventArgs) 

Event Type

Attributes

SecurityCriticalAttribute

Examples

The following example creates a series of application domains named AD0 through AD3, with a Worker object in each application domain. Each Worker object has a reference to the Worker object in the next application domain, except for the Worker in the last application domain. The FirstChanceException event is handled in all application domains except AD1.

When the application domains have been created, the default application domain calls the TestException method for the first application domain. Each Worker object calls the TestException method for the next application domain, until the last Worker throws an exception that is either handled or unhandled. Thus, the current thread passes through all the application domains, and TestException is added to the stack in each application domain.

When the last Worker object handles the exception, the FirstChanceException event is raised only in the last application domain. The other application domains never get a chance to handle the exception, so the event is not raised.

When the last Worker object does not handle the exception, the FirstChanceException event is raised in each application domain that has an event handler. After each event handler has finished, the stack continues to unwind until the exception is caught by the default application domain.

using System;
using System.Reflection;
using System.Runtime.ExceptionServices;

class FirstChanceExceptionSnippet
{
 static void Main()
 {
 AppDomain.CurrentDomain.FirstChanceException += FirstChanceHandler;

 // Create a set of application domains, with a Worker object in each one.
 // Each Worker object creates the next application domain.
 AppDomain ad = AppDomain.CreateDomain("AD0");
 Worker w = (Worker) ad.CreateInstanceAndUnwrap(
 typeof(Worker).Assembly.FullName, "Worker");
 w.Initialize(0, 3);

 Console.WriteLine("\r\nThe last application domain throws an exception and catches it:");
 Console.WriteLine();
 w.TestException(true);

 try
 {
 Console.WriteLine(
 "\r\nThe last application domain throws an exception and does not catch it:");
 Console.WriteLine();
 w.TestException(false);
 }
 catch (ArgumentException ex)
 {
 Console.WriteLine("ArgumentException caught in {0}: {1}",
 AppDomain.CurrentDomain.FriendlyName, ex.Message);
 }
 }

 static void FirstChanceHandler(object source, FirstChanceExceptionEventArgs e)
 {
 Console.WriteLine("FirstChanceException event raised in {0}: {1}",
 AppDomain.CurrentDomain.FriendlyName, e.Exception.Message);
 }
}

public class Worker : MarshalByRefObject
{
 private AppDomain ad = null;
 private Worker w = null;

 public void Initialize(int count, int max)
 {
 // Handle the FirstChanceException event in all application domains except
 // AD1.
 if (count != 1)
 {
 AppDomain.CurrentDomain.FirstChanceException += FirstChanceHandler;
 }

 // Create another application domain, until the maximum is reached.
 // Field w remains null in the last application domain, as a signal
 // to TestException().
 if (count < max)
 {
 int next = count + 1;
 ad = AppDomain.CreateDomain("AD" + next);
 w = (Worker) ad.CreateInstanceAndUnwrap(
 typeof(Worker).Assembly.FullName, "Worker");
 w.Initialize(next, max);
 }
 }

 public void TestException(bool handled)
 {
 // As long as there is another application domain, call TestException() on
 // its Worker object. When the last application domain is reached, throw a
 // handled or unhandled exception.
 if (w != null)
 {
 w.TestException(handled);
 }
 else if (handled)
 {
 try
 {
 throw new ArgumentException("Thrown in " + AppDomain.CurrentDomain.FriendlyName);
 }
 catch (ArgumentException ex)
 {
 Console.WriteLine("ArgumentException caught in {0}: {1}",
 AppDomain.CurrentDomain.FriendlyName, ex.Message);
 }
 }
 else
 {
 throw new ArgumentException("Thrown in " + AppDomain.CurrentDomain.FriendlyName);
 }
 }

 static void FirstChanceHandler(object source, FirstChanceExceptionEventArgs e)
 {
 Console.WriteLine("FirstChanceException event raised in {0}: {1}",
 AppDomain.CurrentDomain.FriendlyName, e.Exception.Message);
 }
}

/* This example produces output similar to the following:

The last application domain throws an exception and catches it:

FirstChanceException event raised in AD3: Thrown in AD3
ArgumentException caught in AD3: Thrown in AD3

The last application domain throws an exception and does not catch it:

FirstChanceException event raised in AD3: Thrown in AD3
FirstChanceException event raised in AD2: Thrown in AD3
FirstChanceException event raised in AD0: Thrown in AD3
FirstChanceException event raised in Example.exe: Thrown in AD3
ArgumentException caught in Example.exe: Thrown in AD3
 */

open System
open System.Runtime.ExceptionServices

let firstChanceHandler _ (e: FirstChanceExceptionEventArgs) =
 printfn $"FirstChanceException event raised in {AppDomain.CurrentDomain.FriendlyName}: {e.Exception.Message}"

type Worker() =
 inherit MarshalByRefObject()

 let mutable w = Unchecked.defaultof<Worker>

 member _.Initialize(count, max) =
 // Handle the FirstChanceException event in all application domains except
 // AD1.
 if count <> 1 then
 AppDomain.CurrentDomain.FirstChanceException.AddHandler firstChanceHandler

 // Create another application domain, until the maximum is reached.
 // Field w remains null in the last application domain, as a signal
 // to TestException().
 if count < max then
 let next = count + 1
 let ad = AppDomain.CreateDomain("AD" + string next)
 w <-
 ad.CreateInstanceAndUnwrap(typeof<Worker>.Assembly.FullName, "Worker") :?> Worker
 w.Initialize(next, max)

 member _.TestException(handled) =
 // As long as there is another application domain, call TestException() on
 // its Worker object. When the last application domain is reached, throw a
 // handled or unhandled exception.
 if isNull (box w) then
 w.TestException handled
 elif handled then
 try
 raise (ArgumentException $"Thrown in {AppDomain.CurrentDomain.FriendlyName}")
 with :? ArgumentException as ex ->
 printfn $"ArgumentException caught in {AppDomain.CurrentDomain.FriendlyName}: {ex.Message}"
 else
 raise (ArgumentException $"Thrown in {AppDomain.CurrentDomain.FriendlyName}")

AppDomain.CurrentDomain.FirstChanceException.AddHandler firstChanceHandler

// Create a set of application domains, with a Worker object in each one.
// Each Worker object creates the next application domain.
let ad = AppDomain.CreateDomain "AD0"
let w = ad.CreateInstanceAndUnwrap(typeof<Worker>.Assembly.FullName, "Worker") :?> Worker
w.Initialize(0, 3)

printfn "\nThe last application domain throws an exception and catches it:\n"
w.TestException true

try
 printfn "\nThe last application domain throws an exception and does not catch it:\n"
 w.TestException false
with :? ArgumentException as ex ->
 printfn"ArgumentException caught in {AppDomain.CurrentDomain.FriendlyName}: {ex.Message}"

(* This example produces output similar to the following:

The last application domain throws an exception and catches it:

FirstChanceException event raised in AD3: Thrown in AD3
ArgumentException caught in AD3: Thrown in AD3

The last application domain throws an exception and does not catch it:

FirstChanceException event raised in AD3: Thrown in AD3
FirstChanceException event raised in AD2: Thrown in AD3
FirstChanceException event raised in AD0: Thrown in AD3
FirstChanceException event raised in Example.exe: Thrown in AD3
ArgumentException caught in Example.exe: Thrown in AD3
 *)

Imports System.Reflection
Imports System.Runtime.ExceptionServices

Class Example

 Shared Sub Main()
 
 AddHandler AppDomain.CurrentDomain.FirstChanceException, AddressOf FirstChanceHandler

 ' Create a set of application domains, with a Worker object in each one.
 ' Each Worker object creates the next application domain.
 Dim ad As AppDomain = AppDomain.CreateDomain("AD0")
 Dim w As Worker = CType(ad.CreateInstanceAndUnwrap(
 GetType(Worker).Assembly.FullName, "Worker"),
 Worker)
 w.Initialize(0, 3)

 Console.WriteLine(vbCrLf & "The last application domain throws an exception and catches it:")
 Console.WriteLine()
 w.TestException(true)

 Try
 Console.WriteLine(vbCrLf & 
 "The last application domain throws an exception and does not catch it:")
 Console.WriteLine()
 w.TestException(false) 

 Catch ex As ArgumentException
 
 Console.WriteLine("ArgumentException caught in {0}: {1}", 
 AppDomain.CurrentDomain.FriendlyName, ex.Message)
 End Try
 End Sub

 Shared Sub FirstChanceHandler(ByVal source As Object, 
 ByVal e As FirstChanceExceptionEventArgs)
 
 Console.WriteLine("FirstChanceException event raised in {0}: {1}",
 AppDomain.CurrentDomain.FriendlyName, e.Exception.Message)
 End Sub
End Class

Public Class Worker
 Inherits MarshalByRefObject

 Private ad As AppDomain = Nothing
 Private w As Worker = Nothing

 Public Sub Initialize(ByVal count As Integer, ByVal max As Integer)
 
 ' Handle the FirstChanceException event in all application domains except
 ' AD1.
 If count <> 1
 
 AddHandler AppDomain.CurrentDomain.FirstChanceException, AddressOf FirstChanceHandler

 End If

 ' Create another application domain, until the maximum is reached.
 ' Field w remains Nothing in the last application domain, as a signal 
 ' to TestException(). 
 If count < max
 Dim nextAD As Integer = count + 1
 ad = AppDomain.CreateDomain("AD" & nextAD)
 w = CType(ad.CreateInstanceAndUnwrap(
 GetType(Worker).Assembly.FullName, "Worker"),
 Worker)
 w.Initialize(nextAD, max)
 End If
 End Sub

 Public Sub TestException(ByVal handled As Boolean)
 
 ' As long as there is another application domain, call TestException() on
 ' its Worker object. When the last application domain is reached, throw a
 ' handled or unhandled exception.
 If w IsNot Nothing
 
 w.TestException(handled)

 Else If handled
 
 Try
 Throw New ArgumentException("Thrown in " & AppDomain.CurrentDomain.FriendlyName)

 Catch ex As ArgumentException
 
 Console.WriteLine("ArgumentException caught in {0}: {1}", 
 AppDomain.CurrentDomain.FriendlyName, ex.Message)
 End Try
 Else
 
 Throw New ArgumentException("Thrown in " & AppDomain.CurrentDomain.FriendlyName)
 End If
 End Sub

 Shared Sub FirstChanceHandler(ByVal source As Object, 
 ByVal e As FirstChanceExceptionEventArgs)
 
 Console.WriteLine("FirstChanceException event raised in {0}: {1}",
 AppDomain.CurrentDomain.FriendlyName, e.Exception.Message)
 End Sub
End Class

' This example produces output similar to the following:
'
'The last application domain throws an exception and catches it:
'
'FirstChanceException event raised in AD3: Thrown in AD3
'ArgumentException caught in AD3: Thrown in AD3
'
'The last application domain throws an exception and does not catch it:
'
'FirstChanceException event raised in AD3: Thrown in AD3
'FirstChanceException event raised in AD2: Thrown in AD3
'FirstChanceException event raised in AD0: Thrown in AD3
'FirstChanceException event raised in Example.exe: Thrown in AD3
'ArgumentException caught in Example.exe: Thrown in AD3

Remarks

This event is only a notification. Handling this event does not handle the exception or affect subsequent exception handling in any way. After the event has been raised and event handlers have been invoked, the common language runtime (CLR) begins to search for a handler for the exception. FirstChanceException provides the application domain with a first chance to examine any managed exception.

The event can be handled per application domain. If a thread passes through multiple application domains while executing a call, the event is raised in each application domain that has registered an event handler, before the CLR begins searching for a matching exception handler in that application domain. After the event has been handled, a search is made for a matching exception handler in that application domain. If none is found, the event is raised in the next application domain.

You must handle all exceptions that occur in the event handler for the FirstChanceException event. Otherwise, FirstChanceException is raised recursively. This could result in a stack overflow and termination of the application. We recommend that you implement event handlers for this event as constrained execution regions (CERs), to keep infrastructure-related exceptions such as out-of-memory or stack overflow from affecting the virtual machine while the exception notification is being processed.

This event is not raised for exceptions that indicate corruption of process state, such as access violations, unless the event handler is security-critical and has the HandleProcessCorruptedStateExceptionsAttribute attribute.

The common language runtime suspends thread aborts while this notification event is being handled.

See also

• FirstChanceExceptionEventArgs
• UnhandledException
• How to: Receive First-Chance Exception Notifications