private static ManualResetEvent mre = new ManualResetEvent(false);
private Result result;

{
 BackgroundWorker bw = new BackgroundWorker();
 bw.DoWork += new DoWorkEventHandler(bw_DoWork);
 bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);
 bw.RunWorkerAsync(new BackgroundWorkerArgument(bitmap, image));

Rectangle rectangle = DetectRectangle(bitmapClone));
mre.WaitOne();

<код дальше>
}

void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            result= (Result)e.Result;
        }

        void bw_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorkerArgument argument = (BackgroundWorkerArgument)e.Argument;
            e.Result = CreateResult(argument.Bitmap, argument.Image);
            mre.Set();