选择索引更改了两次触发

我想知道为什么我的选择索引更改在单击列表中的项目时触发两次。

这是我在选择索引中使用的代码更改

 private void listBoxFolders_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        // Taking the name of the folder to pass in the parameters
        if ((Folder)listBoxFolders.SelectedItem != null)
        {
            folderTmp = (Folder)listBoxFolders.SelectedItem;
        }
        // Connection to the webservice to get the subfolders and also the files
        WebClient wc = new WebClient();
        wc.DownloadStringCompleted += new DownloadStringCompletedEventHandler(wc_DownloadStringCompleted2);
        wc.DownloadStringAsync(new Uri("http://clients.uicentric.net/IISHostedCalcService/FilesService.svc/GetFoldersAndFiles?selectedFolder=" + folderTmp.Name));
    }

这是在其中发射两次的方法：

 public void wc_DownloadStringCompleted2(object sender, DownloadStringCompletedEventArgs e)
    {
        if (e.Error == null)
        {
            XDocument xdoc = XDocument.Parse(e.Result, LoadOptions.None);
            XNamespace aNamespace = XNamespace.Get("http://schemas.datacontract.org/2004/07/System.IO");
            try
            {
                // Retrieving the subfolders
                var folders = from query in xdoc.Descendants(aNamespace.GetName("DirectoryInfo"))
                              select new Folder
                              {
                                  Name = (string)query.Element("OriginalPath"),
                              };
                _lFolders = new ObservableCollection<Folder>();
                foreach (Folder f in folders)
                {
                    LFolders.Add(f);
                }
                listBoxFolders.ItemsSource = LFolders;
                listBoxFolders.DisplayMemberPath = "Name";

                // Retrieving the files
                var files = from query in xdoc.Descendants(aNamespace.GetName("FileInfo"))
                            select new File
                         {
                             Name = (string)query.Element("OriginalPath"),
                         };

                _lFiles = new ObservableCollection<File>();
                foreach (File f in files)
                {
                    LFiles.Add(f);
                }
                listBoxFiles.ItemsSource = LFiles;
                listBoxFiles.DisplayMemberPath = "Name";
                listBoxFiles.SelectionChanged += new SelectionChangedEventHandler(listBoxFiles_SelectionChanged);
            }
            catch { }
        }
    }