using System;

using System.Collections.Concurrent;

using System.Threading.Tasks.Dataflow;

using System.Threading.Tasks;

using System.Threading;

using System.Collections.Generic;

​

public static class Program

{

    public static async Task Main()

    {

        var completeFiles = new ConcurrentQueue<string>();

        var filesToProcess = new Queue<string>(CreateListOfFakeFilePaths());

        // - =======================================

        var executionOptions = new ExecutionDataflowBlockOptions

        {

            MaxDegreeOfParallelism = 3,

            EnsureOrdered = true

        };

        // Create a background work pool with an ActionBlock<T> with a lambda

        var backgroundDispatcher = new ActionBlock<string>(filePath =>

        {

            Console.WriteLine($"Background Thread [{{Environment.CurrentManagedThreadId}}] | Processing [{filePath}]");

            Thread.Sleep(500); // Simulate work

            completeFiles.Enqueue(filePath);

        }, executionOptions);

        // - =======================================

        // Queue work in the background delegated off this main thread.

        for (var count = 0; filesToProcess.Count > 0; count++)

        {

            backgroundDispatcher.Post(filesToProcess.Dequeue());

            // Interrupt background by not feeding it anymore (pausing)

            if (count > 7)

                break;

        }

​

        Console.WriteLine("Parent Thread Sleeping");

        Thread.Sleep(500); // Pretend parent thread is doing other work

        Console.WriteLine("Parent Thread Resuming");

        // Resume and finish the work.

        while (filesToProcess.Count > 0)

        {

            backgroundDispatcher.Post(filesToProcess.Dequeue());

        }

​

        // Allow graceful spindown of all background threads the ActionBlock used.

        backgroundDispatcher.Complete();

        await backgroundDispatcher.Completion;

        Console.WriteLine(string.Join("\n", completeFiles.ToArray()));

    }

    private static string[] CreateListOfFakeFilePaths(int howMany = 25)

    {

        var filePaths = new List<string>();

        for (var i = 0; i < howMany; i++)

        {

            filePaths.Add($"/Path/To/Temp/{i}.png");

        }

​

        return filePaths.ToArray();

    }

}

​