Thread hangs when accessing mainthread variables

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Automation;
using System.Diagnostics;
using System.Threading;
using Microsoft.Win32;
using System.IO;
using System.Windows;
using System.Windows.Forms;
using System.Net.Mail;
using System.Net;
using System.Net.Mime;
using System.Runtime.InteropServices;
using System.Media;

namespace TestEHWin
{
    internal static class Worker
    {
        internal static AutomationElement eventSenderElement = null;
        internal static List<AutomationElement> dialogs = new List<AutomationElement>();

        internal static Thread mainThread;
        internal static string debugFile = @"C:\Test_logging.txt";
        internal static string debugFileEvent = @"C:\Test_logging_Event.txt";

        internal static void Start()
        {
            StreamWriter sw = new StreamWriter(Worker.debugFileEvent);
            sw.Close();

            int _counter = 0;
            mainThread = Thread.CurrentThread;
            DebugWrite("M - Mainthread id: " + mainThread.ManagedThreadId);

            dialogs = GetChildDialogs(AutomationElement.RootElement);

            foreach (AutomationElement _item in dialogs)
            {
                DebugWrite("M - Dialog: " + _item.Current.Name);
            }

            Automation.AddStructureChangedEventHandler(AutomationElement.RootElement, TreeScope.Children, new StructureChangedEventHandler(strucChangedHandler));

            DateTime stop = DateTime.Now.AddSeconds(20);
            do
            {
                _counter++;
                DebugWrite("M - Iteration number #" + _counter.ToString());
                Sleep(10);
            } while (DateTime.Now < stop);

            Automation.RemoveStructureChangedEventHandler(AutomationElement.RootElement, strucChangedHandler);

            MessageBox.Show("Finished....");
        }

        private static void strucChangedHandler(object _sender, StructureChangedEventArgs _e)
        {
            AutomationElement _senderElement = _sender as AutomationElement;
            Object _windowPattern;

            if (_e.StructureChangeType == StructureChangeType.ChildAdded)
            {
                if (false == _senderElement.TryGetCurrentPattern(WindowPattern.Pattern, out _windowPattern))
                {
                    return;
                }

                eventSenderElement = _senderElement;
                System.Threading.Thread _thread = new Thread(new ThreadStart(testHandler));
                _thread.Priority = ThreadPriority.Highest;
                _thread.Start();
            }
        }

        private static void testHandler()
        {
            mainThread.Suspend();
            DebugWriteEvent("EHT - Thread #" + mainThread.ManagedThreadId.ToString() + " will be suspended.");
            DebugWriteEvent("EHT - ThreadID: " + Thread.CurrentThread.ManagedThreadId);

            DebugWriteEvent("EHT - New dialog name: " + eventSenderElement.Current.Name);

            dialogs = GetChildDialogs(AutomationElement.RootElement);
            DebugWriteEvent("EHT - Dialogs number: #" + dialogs.Count.ToString());

            for (int i = 0; i < dialogs.Count; i++)
            {
                DebugWriteEvent("EHT - Dialog#" + (i + 1).ToString() + " in root: " + dialogs[i].Current.Name);
                Thread.Sleep(100);
            }

            DebugWriteEvent("EHT - Thread #" + mainThread.ManagedThreadId.ToString() + " will be alive again.");
            mainThread.Resume();
        }

        internal static void Sleep(int timeoutInMs)
        {
            TimeSpan wait = DateTime.Now.TimeOfDay;
            wait = wait.Add(new TimeSpan(0, 0, 0, 0, timeoutInMs));

            while (DateTime.Now.TimeOfDay < wait)
            {
                Application.DoEvents();
            }
        }

        internal static List<AutomationElement> GetChildDialogs(AutomationElement _basedialog)
        {
            List<AutomationElement> _dialoglist = new List<AutomationElement>();
            TreeWalker _walker = new TreeWalker(new PropertyCondition(AutomationElement.ControlTypeProperty, ControlType.Window));
            AutomationElement _node = _walker.GetFirstChild(_basedialog);
            while (_node != null)
            {
                _dialoglist.Add(_node);
                _node = _walker.GetNextSibling(_node);
            }
            return _dialoglist;
        }

        internal static void DebugWrite(string _text)
        {
            StreamWriter sw = File.AppendText(Worker.debugFile);
            sw.WriteLine(DateTime.Now.ToString("HHmmss_fffffff: ") + _text + " Thread ID: " + Thread.CurrentThread.ManagedThreadId.ToString());
            sw.Flush();
            sw.Close();
            return;
        }

        internal static void DebugWriteEvent(string _text)
        {

            StreamWriter sw = File.AppendText(Worker.debugFileEvent);
            sw.WriteLine(DateTime.Now.ToString("HHmmss_fffffff: ") + _text + " Thread ID: " + Thread.CurrentThread.ManagedThreadId.ToString());
            sw.Flush();
            sw.Close();
            return;
        }
    }
}