多线程可观察收集

Question

我有ObservableCollection这样：

 private string Username;
 private string Password;

 ObservableCollection<Employee> employees = new ObservableCollection<Employee>();
 employees.Add(new Employee() { Username = "Bill54", Password = "sundayfunday"});
 employees.Add(new Employee() { Username = "Sarah66", Password = "doglover"});
 employees.Add(new Employee() { Username = "Nick42", Password = "gopatriots"});
 return employees;

我希望每个项目都通过Parallel.ForEach使用其字符串值执行此操作：

using (var client = new CookieAwareWebclient)
{             
    var values = new NameValueCollection();
    values.Add("username", Username);
    values.Add("login_password", Password);
    var responseString = client.UploadValues(url, values);
}

我该怎么做？