using System;

using System.Threading;

using System.Threading.Tasks;

using System.Linq;

using System.Collections.Concurrent;

​

public class Program

{

    public static void Main()

    {

        var proxy = new Proxy();

        var tasks = Enumerable

            .Range(0, 25)

            .Select(i => Task.Factory.StartNew(() => proxy.UploadAndImport(new Request(i))))

            .ToList();

        Task.WhenAll(tasks).GetAwaiter().GetResult();

    }

}

​

public class Proxy

{

    static Request _importInProgressItem = null;

    static readonly ConcurrentQueue<Request> ImportQueue = new ConcurrentQueue<Request>();

    public void UploadAndImport(Request request)

    {

        UploadFile(request);

        ImportFileSynchronized(request);

    }

​

    // Synchronize the file import, 

    // because the database allows a user to perform only one write at a time.

    private void ImportFileSynchronized(Request request)

    {

        ImportQueue.Enqueue(request);

        do

        {

            ImportQueue.TryPeek(out var next);

            if (null != Interlocked.CompareExchange(ref _importInProgressItem, next, null))

            {

                // Queue processing is already under way in another thread.

                return;

            }

​

            ImportFile(next);

            ImportQueue.TryDequeue(out _);

            Interlocked.Exchange(ref _importInProgressItem, null);

        }

        while (ImportQueue.Any());

    }

​

    public bool UploadAndImportIsComplete(int operationId) => ImportQueue.All(waiting => waiting.OperationId != operationId);

    void UploadFile(Request request) => Console.WriteLine($"Uploaded {request.OperationId} on {Thread.CurrentThread.ManagedThreadId}");

    void ImportFile(Request request) => Console.WriteLine($"Imported {request.OperationId} on {Thread.CurrentThread.ManagedThreadId}");

}

​

public class Request

{

    // Use Guids in production.

    public int OperationId

    {

        get;

        set;

    }

​

    public Request(int id)

    {

        OperationId = id;

    }

}