Question

I am working on an application that uses a Mutex to ensure that it is the only instance of the application running on the system.

When another instance of the application attempts to start, I want a method to run in the original instance.

Can I invoke a specific method in my application from another instance of the application?

I’ve found some examples using RegisterWindowMessage/PostMessage Win32 APIs by sending the message to HWND_BROADCAST, but I couldn’t get them to work, and I’ve read elsewhere that using HWND_BROADCAST can be dangerous.

Is there a better way to do this that doesn’t involve the app needing to be run in privileged mode?

Answer 1

Here’s a little helper I wrote.

To use it:

var pipeListener = new NamedPipeListener<String>(); // instantiate an instance
pipeListener.MessageReceived += (sender, e) => MessageBox.Show(e.Message); // when a message is received, show a messagebox with the message
pipeListener.Error += (sender, e) => MessageBox.Show("Error ({0}): {1}", e.ErrorType, e.Exception.Message); // oh noes!
pipeListener.Start(); // when you're ready, start listening

From another process:

NamedPipeListener<String>.SendMessage("Howdy howdy howdy");

Note that it uses the full name of the PipeListener as the default name of the pipe. If you need to get more discrete than that, use the constructor overload that takes a pipe name.

Here’s the class:

using System;
using System.IO.Pipes;
using System.Runtime.Serialization.Formatters.Binary;

namespace FunWithNamedPipes
{
    /// <summary>Contains event data for <see cref="NamedPipeMessageReceiveHandler{TMessage}" /> events.</summary>
    /// <typeparam name="TMessage"></typeparam>
    public class NamedPipeListenerMessageReceivedEventArgs<TMessage> : EventArgs
    {
        /// <summary>Initializes an instance of <see cref="NamedPipeListenerMessageReceivedEventArgs{TMessage}" /> with the specified <paramref name="message" />.</summary>
        /// <param name="message">The message passed by the event.</param>
        public NamedPipeListenerMessageReceivedEventArgs(TMessage message)
        {
            this.Message = message;
        }

        /// <summary>Gets the message passed by the event.</summary>
        public TMessage Message { get; private set; }
    }

    /// <summary>Contains event data for <see cref="NamedPipeListenerErrorEventHandler" /> events.</summary>
    public class NamedPipeListenerErrorEventArgs : EventArgs
    {
        /// <summary>Initializes an instance of <see cref="NamedPipeListenerErrorEventArgs" /> with the specified <paramref name="errorType" /> and <paramref name="exception" />.</summary>
        /// <param name="errorType">A <see cref="NamedPipeListenerErrorType" /> describing the part of the listener process where the error was caught.</param>
        /// <param name="ex">The <see cref="Exception" /> that was thrown.</param>
        public NamedPipeListenerErrorEventArgs(NamedPipeListenerErrorType errorType, Exception ex)
        {
            this.ErrorType = errorType;
            this.Exception = ex;
        }

        /// <summary>Gets a <see cref="NamedPipeListenerErrorType" /> describing the part of the listener process where the error was caught.</summary>
        public NamedPipeListenerErrorType ErrorType { get; private set; }

        /// <summary>Gets the <see cref="Exception" /> that was caught.</summary>
        public Exception Exception { get; private set; }
    }

    /// <summary>Represents a method that will handle an event where a message is received via named pipes.</summary>
    /// <typeparam name="TMessage">The type of message that will be received.</typeparam>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">The event data passed by the event, which includes the message that was received.</param>
    public delegate void NamedPipeMessageReceivedHandler<TMessage>(Object sender, NamedPipeListenerMessageReceivedEventArgs<TMessage> e);

    /// <summary>Represents a method that will handle an event that is fired when an exception is caught.</summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">The event data passed by the event, included the error type and exception that was caught.</param>
    public delegate void NamedPipeMessageErrorHandler(Object sender, NamedPipeListenerErrorEventArgs e);

    /// <summary>Includes different types of errors that describe where in the listening process an exception was caught.</summary>
    public enum NamedPipeListenerErrorType : byte
    {
        /// <summary>Indicates that an exception was caught while calling <see cref="NamedPipeServerStream.BeginWaitForConnection" />.</summary>
        BeginWaitForConnection = 1,

        /// <summary>Indicates that an exception was caught while calling <see cref="NamedPipeServerStream.EndWaitForConnection" />.</summary>
        EndWaitForConnection = 2,

        /// <summary>Indicates that an exception was caught while deserializing a message received from the named pipe.</summary>
        DeserializeMessage = 3,

        /// <summary>Indicates that an exception was caught while closing or disposing a used named pipe.</summary>
        CloseAndDisposePipe = 4,

        /// <summary>Indicates that an exception was caught while invoking the <see cref="NamedPipeListener{TMessage}.MessageReceived"/> event.</summary>
        NotifyMessageReceived = 5
    }

    /// <summary>A helper class for sending and receiving messages using named pipes.</summary>
    /// <typeparam name="TMessage">The type of message that will be sent or received.</typeparam>
    public class NamedPipeListener<TMessage> : IDisposable
    {
        /// <summary>Occurs when a message is received.</summary>
        public event NamedPipeMessageReceivedHandler<TMessage> MessageReceived;

        /// <summary>Occurs when an exception is caught.</summary>
        public event NamedPipeMessageErrorHandler Error;

        static readonly String DEFAULT_PIPENAME = typeof(NamedPipeListener<TMessage>).FullName;
        static readonly BinaryFormatter formatter = new BinaryFormatter();

        NamedPipeServerStream pipeServer;

        /// <summary>Initializes a new instance of <see cref="NamedPipeListener{TMessage}" /> using the specified <paramref name="pipeName" />.</summary>
        /// <param name="pipeName">The name of the named pipe that will be used to listen on.</param>
        public NamedPipeListener(String pipeName)
        {
            this.PipeName = pipeName;
        }

        /// <summary>Initializes a new instance of <see cref="NamedPipeListener{TMessage}" /> using the default pipe name.</summary>
        /// <remarks>The default pipe name is the full name of the type of the instance.</remarks>
        public NamedPipeListener()
            : this(DEFAULT_PIPENAME) { }

        /// <summary>The name of the named pipe that will be used to listen on.</summary>
        public String PipeName { get; private set; }

        /// <summary>Starts listening on the named pipe specified for the instance.</summary>
        internal void Start()
        {
            if (pipeServer == null) pipeServer = new NamedPipeServerStream(DEFAULT_PIPENAME, PipeDirection.In, 1, PipeTransmissionMode.Message, PipeOptions.Asynchronous);

            try { pipeServer.BeginWaitForConnection(new AsyncCallback(PipeConnectionCallback), null); }
            catch (Exception ex) { this.OnError(NamedPipeListenerErrorType.BeginWaitForConnection, ex); }
        }

        private void PipeConnectionCallback(IAsyncResult result)
        {
            try
            {
                pipeServer.EndWaitForConnection(result);
            }
            catch (Exception ex)
            {
                this.OnError(NamedPipeListenerErrorType.EndWaitForConnection, ex);
                return;
            }

            TMessage message;
            try
            {
                message = (TMessage)formatter.Deserialize(pipeServer);
            }
            catch (Exception ex)
            {
                this.OnError(NamedPipeListenerErrorType.DeserializeMessage, ex);
                return;
            }

            try
            {
                this.OnMessageReceived(new NamedPipeListenerMessageReceivedEventArgs<TMessage>(message));
            }
            catch (Exception ex)
            {
                this.OnError(NamedPipeListenerErrorType.NotifyMessageReceived, ex);
                return;
            }

            if (this.End())
            {
                this.Start();
            }
        }

        internal Boolean End()
        {
            try
            {
                pipeServer.Close();
                pipeServer.Dispose();
                pipeServer = null;

                return true;
            }
            catch (Exception ex)
            {
                this.OnError(NamedPipeListenerErrorType.CloseAndDisposePipe, ex);
                return false;
            }
        }

        private void OnMessageReceived(TMessage message)
        {
            this.OnMessageReceived(new NamedPipeListenerMessageReceivedEventArgs<TMessage>(message));
        }

        protected virtual void OnMessageReceived(NamedPipeListenerMessageReceivedEventArgs<TMessage> e)
        {
            if (this.MessageReceived != null)
            {
                this.MessageReceived(this, e);
            }
        }

        private void OnError(NamedPipeListenerErrorType errorType, Exception ex)
        {
            this.OnError(new NamedPipeListenerErrorEventArgs(errorType, ex));
        }

        protected virtual void OnError(NamedPipeListenerErrorEventArgs e)
        {
            if (this.Error != null)
            {
                this.Error(this, e);
            }
        }

        void IDisposable.Dispose()
        {
            if(pipeServer != null)
            {
                try { pipeServer.Disconnect(); }
                catch { }

                try { pipeServer.Close(); }
                catch { }

                try { pipeServer.Dispose(); }
                catch { }
            }
        }

        /// <summary>Sends the specified <paramref name="message" /> to the default named pipe for the message.</summary>        
        /// <param name="message">The message to send.</param>
        public static void SendMessage(TMessage message)
        {
            NamedPipeListener<TMessage>.SendMessage(DEFAULT_PIPENAME, message);
        }

        /// <summary>Sends the specified <paramref name="message" /> to the specified named pipe.</summary>
        /// <param name="pipeName">The name of the named pipe the message will be sent to.</param>
        /// <param name="message">The message to send.</param>
        public static void SendMessage(String pipeName, TMessage message)
        {
            using (var pipeClient = new NamedPipeClientStream(".", DEFAULT_PIPENAME, PipeDirection.Out, PipeOptions.None))
            {
                pipeClient.Connect();

                formatter.Serialize(pipeClient, message);
                pipeClient.Flush();

                pipeClient.WaitForPipeDrain();
                pipeClient.Close();
            }
        }
    }
}