public ObservableCollection<T> Items => INotifyPropertyChanged

public void Foo()
{
    var svc = ServiceUtility.GetClient();
    svc.GetCategoriesCompleted += delegate(object sender, GetCategoriesCompletedEventArgs e)
    {
        if (e.Error != null)
        {
            throw new System.Exception(e.Error.Message);
        }
        this.Items = e.Result;
     };
     svc.GetCategoriesAsync();
     svc.CloseAsync();
}

public void GetCategories()
{
    this.Items = e.Result;
}

public void GetCategoriesCount()
{
    /// ...
    this.Count = e.Result;
}

private void OnCompleted(object sender, OnCompletedEventArgs e ) 
{     
    Dispatcher.BeginInvoke(() => // доступ к UI // ); 
}

void SomeMethod()
{
   if(serviceProxy.GetItemsCount()==0)
  { 
      viewModel.Items = serviceProxy.GetOtherItems();
      if(!serviceProxy.TryFillDefaultItems())
      {
          ....
      }
  }
}

var svc = ServiceUtility.GetClient();
svc.GetItemsCountCompleted += delegate(object service1, GetItemsCountCompletedEventArgs args1)
{
    if (args1.Result == 0)
    {
        var client = ServiceUtility.GetClient();
        client.GetItemsCountCompleted += delegate(object service2, GetItemsCountCompletedEventArgs args2)
        {
            // телодвижения
        };
        client.GetItemsCountAsync();
        client.CloseAsync();
    }
};
svc.GetItemsCountAsync();
svc.CloseAsync();