创建异步服务的正确模式是什么?

我要做的是:

public class EmployeeService
{
    public void GetEmployeesAsyc(Action<IEnumerable<Employees>> callback)
    {
        ThreadPool.QueueUserWorkItem(x => {
             var employees = //...fetch employees ...//
             callback(employees);
        });
    }
}

public class ViewModel
{
    private EmployeeService _employeeService;
    private bool _isLoaded;

    public ViewModel() 
    { 
         _employeeService = new EmployeeService();   
         EmployeeList = new ObservableCollection<Employee>();
    }

    public ObservableCollection<Employee> EmployeeList { get; set; }

    public void LoadData()
    {
         if(_isLoaded) return;

         _employeeService.GetEmployeesAsync(employees => 
         {
             EmployeeList.Clear();
             employees.ForEach(employee => EmployeeList.Add(employee));          
         });

         _isLoaded = true;
    }
}

public partial class View : PhoneApplicationPage
{
     private ViewModel _vm;

     public View()
     {
         InitializeComponent();
         _vm = new ViewModel();
         this.Loaded += (sender, e) => _vm.LoadData();
     }
}

这里的问题是,这会引发一个跨线程异常:

         _employeeService.GetEmployeesAsync(employees => 
         {
             EmployeeList.Clear();
             employees.ForEach(employee => EmployeeList.Add(employee));          
         });

         _employeeService.GetEmployeesAsync(employees => 
         {
             Dispatcher.BeginInvoke(() =>
             {
                 EmployeeList.Clear();
                 employees.ForEach(employee => EmployeeList.Add(employee));          
             });
         });

但我不确定这样做是否正确。有什么建议吗?