Implementing the Producer/Consumer Pattern in C#

Question

How can I implement the Producer/Consumer patterns in C# using Events and Delegates? What do I need to keep an eye out for when it comes to resources when using these design patterns? Are there any edge cases I need to be aware of?

Answer 1

I know this thread is quite a bit old, but since I came across it sometimes in my searches, I decided to share this producer-consumer code for people wondering how to implement a simple generic producer-consumer job queue.

The Job class is used to 'store' an object's method call in the form of a delegate. The delegate is then called when the job is processed. Any relevant arguments are also stored in this Job class.

With this simple pattern it's possible to achieve multi-threading in the enqueue AND dequeue processes. Actually this is just the easiest part: multi-threading brings new challenges to your code, you'll notice them later ;-)

I've originally posted this code in this thread.

using System;
using System.Collections.Concurrent;
using System.Diagnostics;
using System.Threading;

// Compiled and tested in: Visual Studio 2017, DotNET 4.6.1

namespace MyNamespace
{
    public class Program
    {
        public static void Main(string[] args)
        {
            MyApplication app = new MyApplication();
            app.Run();
        }
    }

    public class MyApplication
    {
        private BlockingCollection<Job> JobQueue = new BlockingCollection<Job>();
        private CancellationTokenSource JobCancellationTokenSource = new CancellationTokenSource();
        private CancellationToken JobCancellationToken;
        private Timer Timer;
        private Thread UserInputThread;



        public void Run()
        {
            // Give a name to the main thread:
            Thread.CurrentThread.Name = "Main";

            // Fires a Timer thread:
            Timer = new Timer(new TimerCallback(TimerCallback), null, 1000, 2000);

            // Fires a thread to read user inputs:
            UserInputThread = new Thread(new ThreadStart(ReadUserInputs))
            {
                Name = "UserInputs",
                IsBackground = true
            };
            UserInputThread.Start();

            // Prepares a token to cancel the job queue:
            JobCancellationToken = JobCancellationTokenSource.Token;

            // Start processing jobs:
            ProcessJobs();

            // Clean up:
            JobQueue.Dispose();
            Timer.Dispose();
            UserInputThread.Abort();

            Console.WriteLine("Done.");
        }



        private void ProcessJobs()
        {
            try
            {
                // Checks if the blocking collection is still up for dequeueing:
                while (!JobQueue.IsCompleted)
                {
                    // The following line blocks the thread until a job is available or throws an exception in case the token is cancelled:
                    JobQueue.Take(JobCancellationToken).Run();
                }
            }
            catch { }
        }



        private void ReadUserInputs()
        {
            // User input thread is running here.
            ConsoleKey key = ConsoleKey.Enter;

            // Reads user inputs and queue them for processing until the escape key is pressed:
            while ((key = Console.ReadKey(true).Key) != ConsoleKey.Escape)
            {
                Job userInputJob = new Job("UserInput", this, new Action<ConsoleKey>(ProcessUserInputs), key);
                JobQueue.Add(userInputJob);
            }
            // Stops processing the JobQueue:
            JobCancellationTokenSource.Cancel();
        }

        private void ProcessUserInputs(ConsoleKey key)
        {
            // Main thread is running here.
            Console.WriteLine($"You just typed '{key}'. (Thread: {Thread.CurrentThread.Name})");
        }



        private void TimerCallback(object param)
        {
            // Timer thread is running here.
            Job job = new Job("TimerJob", this, new Action<string>(ProcessTimer), "A job from timer callback was processed.");
            JobQueue.TryAdd(job); // Just enqueues the job for later processing
        }

        private void ProcessTimer(string message)
        {
            // Main thread is running here.
            Console.WriteLine($"{message} (Thread: {Thread.CurrentThread.Name})");
        }
    }



    /// <summary>
    /// The Job class wraps an object's method call, with or without arguments. This method is called later, during the Job execution.
    /// </summary>
    public class Job
    {
        public string Name { get; }
        private object TargetObject;
        private Delegate TargetMethod;
        private object[] Arguments;

        public Job(string name, object obj, Delegate method, params object[] args)
        {
            Name = name;
            TargetObject = obj;
            TargetMethod = method;
            Arguments = args;
        }

        public void Run()
        {
            try
            {
                TargetMethod.Method.Invoke(TargetObject, Arguments);
            }
            catch(Exception ex)
            {
                Debug.WriteLine($"Unexpected error running job '{Name}': {ex}");
            }
        }

    }
}