How to programmatically exit from a second message loop?

Question

I'm trying to create a second message loop to process/filter low level messages asynchronously in C#. It works by creating a hidden form, exposing it's Handle property to be hooked, and run a second message loop in a separate thread. At the moment I'm quite happy with the results but I'm unable to exit from the second loop properly. The only workaround was setting the IsBackground property to true, so the second thread will be simply terminated (without processing all the pending messages) at main application exit.

The question is: how to proper quit that message loop so the second Application.Run() returns? I tried differents approaches creating a separate ApplicationContext and controlling various events (Application.ApplicationExit, Application.ThreadExit, ApplicationContext.ThreadExit) but they all failed with race conditions I'm unable to debug.

Any hint? Thanks

This is the code:

public class MessagePump
{
    public delegate void HandleHelper(IntPtr handle);

    public MessagePump(HandleHelper handleHelper, Filter filter)
    {
        Thread thread = new Thread(delegate()
        {
            ApplicationContext applicationContext = new ApplicationContext();
            Form form = new Form();
            handleHelper(form.Handle);
            Application.AddMessageFilter(new MessageFilter(filter));
            Application.Run(applicationContext);
        });
        thread.SetApartmentState(ApartmentState.STA);
        thread.IsBackground = true; // <-- The workaround
        thread.Start();
    }
}

public delegate bool Filter(ref Message m);

internal class MessageFilter : IMessageFilter
{
    private Filter _Filter;

    public MessageFilter(Filter filter)
    {
        _Filter = filter;
    }

    #region IMessageFilter Members

    public bool PreFilterMessage(ref Message m)
    {
        return _Filter(ref m);
    }

    #endregion // IMessageFilter Members
}

I use it in the main Form constructor in this way:

_Completion = new ManualResetEvent(false);

MessagePump pump = new MessagePump(
delegate(IntPtr handle)
{
    // Sample code, I did this form twain drivers low level wrapping
    _Scanner = new TwainSM(handle);
    _Scanner.LoadDs("EPSON Perfection V30/V300");
},
delegate(ref Message m)
{
    // Asyncrhronous processing of the messages
    // When the correct message is found -->
    _Completion.Set();
}

EDIT: Full solution in my answer.

Answer 1

You should pass the Form instance to the ApplicationContext ctor as a parameter:

applicationContext = new ApplicationContext(form); 

Right now, you are basically creating a no-context instance, which doesn't care about your form being closed.

Also, it is a good practice to do some cleanup, like removing the filter when you don't need it anymore:

Form form = new Form();
ApplicationContext applicationContext = new ApplicationContext(form);
handleHelper(form.Handle);

MessageFilter filter = new MessageFilter(filter);
Application.AddMessageFilter(filter);
Application.Run(applicationContext);
Application.RemoveMessageFilter(filter);

[Edit]

If you don't want to show the form, then you can use the paramaterless ctor, but you will have to close the context manually by calling the ApplicationContext.ExitThread method. This method actually gets called when your form fires the FormClosed event, if you pass the form in the constructor.

Since hidden form is not related to the context, you need to exit them both at some time.

Answer 2

I eventually realized that the thread.IsBackground = true; was not bad, because it was the only way to determine "hey, I'm the last thread running, I'm supposed to quit". Correct resource cleaning is still needed, tough. For this, a third delegate for resource cleaning is needed and I just registered it to the AppDomain.CurrentDomain.ProcessExit event. I even provided a ExitLoop() method to the MessageLoop class (was MessagePump in the question). In this way, I can terminate the message loop anytime. Critical sections of ExitLoop() and ProcessExit handler are mutexed.

The code:

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Threading;

namespace System
{
    public class MessageLoop
    {
        #region Fields

        private Object _Lock;
        private ApplicationContext _ApplicationContext;
        private CustomMessageFilter _MessageFilter;
        private HandleProvider _ResourceCleaner;
        private ManualResetEvent _Completion;
        private bool _Disposed;

        #endregion // Fields

        #region Constructors

        /// <summary>
        /// Run a second message pump that will filter messages asyncronously
        /// </summary>
        /// <param name="provideHandle">A delegate that provide a window handle for
        ///     resource initializing</param>
        /// <param name="messageFilter">A delegate for message filtering</param>
        /// <param name="cleanResources">A delegate for proper resource cleaning
        ///     before quitting the loop</param>
        /// <param name="background">State if the loop should be run on a background
        ///     thread or not. If background = false, please be aware of the
        ///     possible race conditions on application shut-down.</param>
        public MessageLoop(HandleProvider initializeResources, MessageFilter messageFilter,
            HandleProvider cleanResources, bool background)
        {
            _Lock = new Object();
            _ResourceCleaner = cleanResources;
            _Completion = new ManualResetEvent(false);
            _Disposed = false;

            Thread thread = new Thread(delegate()
            {
                _ApplicationContext = new ApplicationContext();
                WindowHandle window = new WindowHandle();
                initializeResources(window.Handle);
                _MessageFilter = new CustomMessageFilter(messageFilter);
                Application.AddMessageFilter(_MessageFilter);

                // Signal resources initalizated
                _Completion.Set();

                // If background = true, do resource cleaning on ProcessExit event
                if (background)
                {
                    AppDomain.CurrentDomain.ProcessExit +=
                        new EventHandler(CurrentDomain_ProcessExit);
                }

                // Run the message loop
                Application.Run(_ApplicationContext);

                // Clean resource before leaving the thread
                cleanResources(window.Handle);

                // Signal resources cleaned
                _Completion.Set();
            });
            thread.SetApartmentState(ApartmentState.STA);
            thread.IsBackground = background;
            thread.Start();

            // Before returning the instace, wait for thread resources initialization
            _Completion.WaitOne();
        }

        #endregion // Constructors

        #region Inquiry

        /// <summary>
        /// Early exit the message loop
        /// </summary>
        public void ExitLoop()
        {
            lock (_Lock)
            {
                if (_Disposed)
                    return;

                // Completion was already signaled in the constructor 
                _Completion.Reset();

                // Tell the message loop thread to quit
                _ApplicationContext.ExitThread();

                // Wait for thread resources cleaning
                _Completion.WaitOne();

                _Disposed = true;
            }
        }

        #endregion // Inquiry

        #region Event handlers

        void CurrentDomain_ProcessExit(object sender, EventArgs e)
        {
            lock (_Lock)
            {
                if (_Disposed)
                    return;

                // Completion was already signaled in the constructor 
                _Completion.Reset();

                // Tell the message loop thread to quit
                _ApplicationContext.ExitThread();

                // Wait for thread resources cleaning
                _Completion.WaitOne();

                _Disposed = true;
            }
        }

        #endregion // Event handlers

        #region Support

        public delegate void HandleProvider(IntPtr handle);
        public delegate bool MessageFilter(ref Message m);

        internal class CustomMessageFilter : IMessageFilter
        {
            private MessageFilter _Filter;

            public CustomMessageFilter(MessageFilter filter)
            {
                _Filter = filter;
            }

            #region IMessageFilter Members

            public bool PreFilterMessage(ref Message m)
            {
                return _Filter(ref m);
            }

            #endregion // IMessageFilter Members
        }

        #endregion // Support
    }

    public class WindowHandle : NativeWindow
    {
        public WindowHandle()
        {
            CreateParams parms = new CreateParams();
            CreateHandle(parms);
        }
        ~WindowHandle()
        {
            DestroyHandle();
        }
    }
}

Can be used this way:

_Completion = new ManualResetEvent(false);

MessageLoop messageLoop = new MessageLoop(
delegate(IntPtr handle) // Resource initializing
{
    // Sample code, I did this form twain drivers low level wrapping
    _Scanner = new TwainSM(handle);
    _Scanner.LoadDs("EPSON Perfection V30/V300");
},
delegate(ref Message m) // Message filtering
{
    // Asyncrhronous processing of the messages
    // When the correct message is found -->
    _Completion.Set();
},
delegate(IntPtr handle) // Resource cleaning
{
    // Resource cleaning/disposing. In my case, it's the following...
    _Scanner.Dispose();
}, true); // Automatically quit on main application shut-down

// Anytime you can exit the loop
messageLoop.ExitLoop();