Pausing a new BackGroundWorker until previous completes

I am struggling with threading. The problem is when I am iterating trough foreach loop.

When setting this.Document, the application performs login, that is triggered with an event and takes few seconds to complete. In the worker_RunWorkerCompleted method I need to perform some actions that depend on current login information.

The problem is that before I can perform this action for the first file, the this.Document already changes making the application perform another login. This way I can never actually perform my actions.

My question is: How can I pause the next thread until previous thread has completed. Is there any other solution to my problem?

I tried with AutoResetEvent but I got no luck. I set waitOne() just after the RunWorkerAsync call and .Set() in the RunWorkerCompleted. The code never gets to RunWorkerCompleted...

Here is the code:

    public void Start(object obj)
    {
       try
       {
          foreach (KeyValuePair<string, Stream> pair in this.CollectionOfFiles)
          {
              Worker = new BackgroundWorker();
              Worker.DoWork += new DoWorkEventHandler(worker_DoWork);
              Worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(worker_RunWorkerCompleted);

          using (Stream stream = pair.Value)
              {
                primaryDocument = new Document(stream);

                DataHolderClass dataHolder = new DataHolderClass();
                dataHolder.FileName = pair.Key;
                dataHolder.Doc = secondaryDocument;

               //background thread call
                Worker.RunWorkerAsync(dataHolder);
              }
            }
          }

       catch (Exception ex)
       {
          // exception logic
}
       finally
       {
          // complete logic
       }
    }


    private void worker_DoWork(object sender, DoWorkEventArgs e)
    {
       DataHolderClass dataHolder = ((DataHolderClass)e.Argument);
       // setting this attribute triggers execution of login event
       this.Document = dataHolder.Doc;
       e.Result = (dataHolder);
    }


    private void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
       // here I need to perform some actions that are depending on the current login
       DataHolderClass dataHolder = ((DataHolderClass)e.Result);
       this.eventAggregator.GetEvent<ActionEvent>().Publish(new Message(EMessageType.Info) { Title = dataHolder.FileName });
    }