ShutDownListener.cs source code in C# .NET

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                         / 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / wpf / src / Base / MS / Internal / ShutDownListener.cs / 1305600 / ShutDownListener.cs
                        
                        
                            //---------------------------------------------------------------------------- 
//
// 
//    Copyright (C) Microsoft Corporation.  All rights reserved.
//  
//
// Description: Listen for shut down events on behalf of a target, in a way that 
//          does not leak the target.  ShutDown events include: 
//                  AppDomain.DomainUnload
//                  AppDomain.ProcessExit 
//                  Dispatcher.ShutdownFinished
//          Listening to these events directly can cause leaks, since the AppDomain
//          lives longer than the target.
// 
//          The WeakEvent pattern has similar goals, but can't be used here because
//          the WeakEvent table itself needs to listen for shutdown events. 
// 
//          "Target" refers to the actual consumer of the event(s).  Each class
//          XYZ that wants to consume these events should define a class 
//          XYZShutDownListener deriving from ShutDownListener that overrides the
//          OnShutDown method.  The target's constructor typically creates an
//          instance of the XYZShutDownListener, which holds a weak reference to
//          the target, and listens for the desired events.  When an event occurs, 
//          the OnShutDown override is passed a (normal) reference to the target
//          object, and typically calls an appropriate target method that reacts 
//          to the event.  (See examples in WeakEventTable, DataBindEngine, etc.) 
//
//          Shutdown is a "one-time" process.  When the ShutDownListener receives 
//          any one of the desired events, it stops listening to all events.
//---------------------------------------------------------------------------

using System; 
using System.Security;              // [SecurityCritical]
using System.Threading;             // Interlocked 
using System.Windows.Threading;     // Dispatcher 
using MS.Internal.WindowsBase;      // [FriendAccessAllowed]
 
namespace MS.Internal
{
    [FriendAccessAllowed]   // defined in Base, also used in Framework
    [Flags] 
    internal enum ShutDownEvents : ushort
    { 
        DomainUnload        = 0x0001, 
        ProcessExit         = 0x0002,
        DispatcherShutdown  = 0x0004, 

        AppDomain           = DomainUnload | ProcessExit,
        All                 = AppDomain | DispatcherShutdown,
    } 

    [FriendAccessAllowed]   // defined in Base, also used in Framework 
    internal abstract class ShutDownListener : WeakReference 
    {
        ///  
        /// Critical - accesses AppDomain.DomainUnload event.
        /// not SecurityTreatAsSafe.
        /// 
        [SecurityCritical] 
        internal ShutDownListener(object target)
            : this(target, ShutDownEvents.All) 
        { 
        }
 
        /// 
        /// Critical - accesses AppDomain.DomainUnload and AppDomain.ProcessExit events (which have link demands).
        /// not SecurityTreatAsSafe.
        ///  
        [SecurityCritical]
        internal ShutDownListener(object target, ShutDownEvents events) 
            : base(target) 
        {
            _flags = ((PrivateFlags)events) | PrivateFlags.Listening; 

            if (target == null)
            {
                _flags |= PrivateFlags.Static; 
            }
 
            if ((_flags & PrivateFlags.DomainUnload) != 0) 
            {
                AppDomain.CurrentDomain.DomainUnload += new EventHandler(HandleShutDown); 
            }

            if ((_flags & PrivateFlags.ProcessExit) != 0)
            { 
                AppDomain.CurrentDomain.ProcessExit += new EventHandler(HandleShutDown);
            } 
 
            if ((_flags & PrivateFlags.DispatcherShutdown) != 0)
            { 
                Dispatcher dispatcher = Dispatcher.CurrentDispatcher;
                dispatcher.ShutdownFinished += new EventHandler(HandleShutDown);
                _dispatcherWR = new WeakReference(dispatcher);
            } 
        }
 
        // derived class should override this method to inform the target that a shutdown 
        // event has occurred.  This method might be called on any thread (e.g.
        // AppDomain.DomainUnload events are typically raised on worker threads). 
        abstract internal void OnShutDown(object target, object sender, EventArgs e);

        // stop listening for shutdown events
        ///  
        ///     Critical: accesses AppDomain.DomainUnload event
        ///     TreatAsSafe: This code does not take any parameter or return state. 
        ///                  It simply unattaches private callbacks. 
        /// 
        [SecurityCritical,SecurityTreatAsSafe] 
        internal void StopListening()
        {
            if ((_flags & PrivateFlags.Listening) == 0)
                return; 

            _flags = _flags & ~PrivateFlags.Listening; 
 
            if ((_flags & PrivateFlags.DomainUnload) != 0)
            { 
                AppDomain.CurrentDomain.DomainUnload -= new EventHandler(HandleShutDown);
            }

            if ((_flags & PrivateFlags.ProcessExit) != 0) 
            {
                AppDomain.CurrentDomain.ProcessExit -= new EventHandler(HandleShutDown); 
            } 

            if ((_flags & PrivateFlags.DispatcherShutdown) != 0) 
            {
                Dispatcher dispatcher = (Dispatcher)_dispatcherWR.Target;
                if (dispatcher != null)
                { 
                    dispatcher.ShutdownFinished -= new EventHandler(HandleShutDown);
                } 
                _dispatcherWR = null; 
            }
        } 

        // handle a shutdown event
        private void HandleShutDown(object sender, EventArgs e)
        { 
            // The dispatcher and AppDomain events might arrive on separate threads
            // at the same time.  The interlock assures that we only do the work 
            // once. 
            if (Interlocked.Exchange(ref _inShutDown, 1) == 0)
            { 
                // ShutDown is a one-time event.  Stop listening (thus releasing
                // references to the ShutDownListener).
                StopListening();
 
                // do the shutdown work, unless the target has been GC'd already.
                object target = Target; 
                if (target != null || (_flags & PrivateFlags.Static) != 0) 
                {
                    OnShutDown(target, sender, e); 
                }
            }
        }
 
        [Flags]
        enum PrivateFlags : ushort 
        { 
            DomainUnload        = ShutDownEvents.DomainUnload,
            ProcessExit         = ShutDownEvents.ProcessExit, 
            DispatcherShutdown  = ShutDownEvents.DispatcherShutdown,

            Static              = 0x4000,
            Listening           = 0x8000, 
        }
 
        PrivateFlags _flags; 
        WeakReference _dispatcherWR;
        int _inShutDown; 
    }
}

// File provided for Reference Use Only by Microsoft Corporation (c) 2007.
// Copyright (c) Microsoft Corporation. All rights reserved.
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ShutDownListener.cs source code in C# .NET

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