public async Task SaveTodoItemAsync (TodoItem item, bool isNewItem = false)
{
  // RestUrl = http://developer.xamarin.com:8081/api/todoitems/
  var uri = new Uri (string.Format (Constants.RestUrl, string.Empty));

  ...
  var json = JsonConvert.SerializeObject (item);
  var content = new StringContent (json, Encoding.UTF8, "application/json");

  HttpResponseMessage response = null;
  if (isNewItem) {
    response = await client.PostAsync (uri, content);
  }
  ...

  if (response.IsSuccessStatusCode) {
    Debug.WriteLine (@"                TodoItem successfully saved.");

  }
  ...
}

توضیحات بیشتر