using System;

using System.Threading;

using System.Collections.Concurrent;

using System.Linq;

​

public class Proxy

{

    static ImportFileInfo _inProgressRequest = null;

    static readonly ConcurrentQueue<ImportFileInfo> WaitingRequests = new ConcurrentQueue<ImportFileInfo>();

    //

    // Expose a one-way, asynchronous operation to do the work

    //

    public void UploadAndImport(ImportFileInfo request)

    {       

        //

        // Perform initial work in parallel

        //      

        UploadFile(request);

        //

        // Synchronize subsequent work

        //

        WaitingRequests.Enqueue(request);

        while (true)

        {

            WaitingRequests.TryPeek(out var next);

            if (next == request &&

                null != Interlocked.CompareExchange(ref _inProgressRequest, request, null))

            {

                break;

            }

            Thread.Sleep(500);

        }

        //

        // Run import file on the same thread that performed the initial work

        //

        ImportFile(request);

        Interlocked.Exchange(ref _inProgressRequest, null);

        WaitingRequests.TryDequeue(out _);

    }

​

    //

    // Expose a two way operation to query on the status of the work

    //

    public bool UploadAndImportIsComplete(Guid operationId) => 

        WaitingRequests.All(x => x.OperationId != operationId);

    private void UploadFile(ImportFileInfo request) 

    {

        // can run in parallel

    }

    private void ImportFile(ImportFileInfo request)

    {

        // must run one at a time

    }

}

​

public class ImportFileInfo

{

    public Guid OperationId

    {

        get;

        set;

    }

}