how to async callback a recursive function in WCF?

Introduction: I have a Server [WCF service library] and Client [Winform], client connects to server using netTcpBinding.

the server job is to share Computer Files to the client with a file manager features { New Folder,Move,Copy,Delete,Properties, Attributes and SEARCH }.

The Problem : the search function is a recursive function, it adds item to client ListView instantly (using client CALLBACK) when it finds a (folder/file) name that contains the search key.
so it was all working perfect, until I added a stop search button, which suppose to allow the user to stop the recursive function _Search(), what happens when I try to stop searching is freezing the GUI and never get back from freezing mode till I "STOP Debugging".
In fact when I set points to see what's wrong with the search function in debugging mode, it works and the search stops.

This's the Code I use for searching:

WCF lib Side:

     [ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple, InstanceContextMode = InstanceContextMode.PerSession)]
  public class MainService : IFileManager,ITaskManager
  {

    IFileManagerCallback callback = OperationContext.Current.GetCallbackChannel<IFileManagerCallback>();

 bool stopSearch = false;

    public void StopSearch()    //client call this function to stop SEARCHING.
    {
        stopSearch = true;
    }

    public void Search(string path, string name)     //client call this function to start searching
    {
        _Search(path, name);
        callback.SearchEnd();
        if (stopSearch)
        {
            callback.InfoLabel("Search Cancelled", InfoState.Info);
            stopSearch = false;
            return;
        }
        callback.InfoLabel("Search Done.", InfoState.Done);
    }

    private void _Search(string path, string name)    //the evil recursive function
    {
        if (stopSearch) return;
        DirectoryInfo Roots = new DirectoryInfo(path);

        foreach (FileInfo file in Roots.GetFiles())
        {
            if (stopSearch) return;
            if (file.Name.IndexOf(name, StringComparison.InvariantCultureIgnoreCase) > -1)
            {
                _File item = new _File();
                item.Name = file.Name;
                item.Size = file.Length;
                item.Path = file.FullName;
                callback.File(item);
            }
        }
        foreach (DirectoryInfo folder in Roots.GetDirectories())
        {
            if (stopSearch) return;
            if (folder.Name.IndexOf(name, StringComparison.InvariantCultureIgnoreCase) > -1)
            {
                _Folder item = new _Folder();
                item.Name = folder.Name;
                item.Path = folder.FullName;
                callback.Folder(item);
            }
            _Search(folder.FullName, name);
        }
    } 
 }

WCF Interface:

  [ServiceContract(CallbackContract = typeof(IFileManagerCallback))]
public interface IFileManager
{
    [OperationContract]
    void StopSearch();
     [OperationContract(IsOneWay = true)]
    void Search(string path, string name);
}

     public interface IFileManagerCallback
{
    [OperationContract]
    void File(_File file);

    [OperationContract]
    void Folder(_Folder folder);

    [OperationContract]
    void InfoLabel(string value, InfoState state);

    [OperationContract]
    void SearchEnd();

}

Client side :

 class Callback : IFileManagerCallback
{
    public delegate void OnFileReceived(object sender, _File item);
    private OnFileReceived _fileReceivedHandler = null;
    public event OnFileReceived OnFileReceivedEvent
    {
        add { _fileReceivedHandler += value; }
        remove { _fileReceivedHandler -= value; }
    }
    private void RaiseFileEvents(_File file)
    {
        if (_fileReceivedHandler != null)
        {
            _fileReceivedHandler(this, file);
        }
    }
    public void File(_File file)
    {
        RaiseFileEvents(file);
    }
    // **I WILL AVOID POSTING Folder event and handler it's the same of the file.**
    public void Folder(_Folder folder)
    {
        RaiseFolderEvents(folder);
    }

Client Form1.cs :

  public partial class Form1 : Form
{

       private void callback_FileReceivedEvent(object sender, _File file)
    {
        ListViewItem item = new ListViewItem();
        item.Text = file.Name;
        item.ToolTipText = file.Path;
        item.Tag = item.ImageIndex;
        item.Name = item.Text;
        item.SubItems.Add(CnvrtUnit(file.Size));
        item.Group = listView1.Groups[0];
        item.ImageIndex = _iconListManager.AddFileIcon(file.Path);
        listView1.Items.Add(item);
    }
    bool IsSearch = false;
    private void btnSearch_Click(object sender, EventArgs e)
    {
        if (!IsSearch)
        {
           IsSearch = true;
           listView1.Items.Clear();
           client.Search(currAddress, txtAddress.Text);
           return;
        }
        client.StopSearch();
    }
    public void StopSearching()
    {
        UpdateLabel();    //updating GUI label "Selected 0:,Items: 0"
        IsSearch = false;
    }
}

I'm really confused about fixing it, I'm not sure if I did choose the right title for my question ,so if that's happening because I need an Async callback, how would I convert my search function and Async Callback with WCF?