Showing how to do as-you-type search in WP7

KeydownSearch.cs

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Windows;
using Posten.Pakkesporing.Lib.Model;
using Posten.Pakkesporing.Lib.Services;

namespace Posten.Pakkesporing.Lib.ViewModels
{
    public class PostUnitsViewModel : BaseViewModel
    {
        public ObservableCollection<PostUnitAnnotation> PostUnits { get; private set; }
        public List<PostUnitAnnotation> AllPostUnits { get; private set; }
        private readonly PostUnitsService _unitsService;
        private readonly Timer _timer;
        private string _searchTerm;
        private bool _isSearching;        

        public PostUnitsViewModel()
        {
            _unitsService = new PostUnitsService();

            PostUnits = new ObservableCollection<PostUnitAnnotation>();
            AllPostUnits = new List<PostUnitAnnotation>();
            _timer = new Timer(ExecuteSearch, null, int.MaxValue, int.MaxValue);
        }

        public void Search(string term)
        {
            _searchTerm = term.ToLower();
            
            if(string.IsNullOrEmpty(_searchTerm))
            {
                _timer.Change(int.MaxValue, int.MaxValue);
                SearchByLocation();
            }
            else
            {
                _timer.Change(300, int.MaxValue);
            }
        }

        private void ExecuteSearch(object state)
        {
            if (_isSearching) _timer.Change(300, int.MaxValue);

            ThreadPool.QueueUserWorkItem(o => {
                _searchEvent.WaitOne();
                _searchEvent.Reset();

                _isSearching = true;
                LoadUnitsIfEmpty();

                Deployment.Current.Dispatcher.BeginInvoke(() => PostUnits.Clear());

                foreach (var unit in AllPostUnits)
                {
                    if (unit.Name.ToLower().Contains(_searchTerm) ||
                        unit.Address.ToLower().Contains(_searchTerm))
                    {
                        unit.DisplayDistance = false;
                        PostUnitAnnotation dispatchUnit = unit;
                        Deployment.Current.Dispatcher.BeginInvoke(() => PostUnits.Add(dispatchUnit));
                    }
                }
                _isSearching = false;
                _searchEvent.Set();
            });
        }      

        public void LoadUnitsIfEmpty()
        {
            lock (AllPostUnits)
            {
                if (AllPostUnits.Count == 0)
                {
                    AllPostUnits.AddRange(_unitsService.GetAnnotationsForAllUnits());
                }
            }
        }

        private readonly ManualResetEvent _searchEvent = new ManualResetEvent(true);
    }
}