Pushpin.DataContext等于null

Question

我今天正在努力解决一个问题。我有来自webservice的图钉的地图，我有这样的约束力：

string mynearjesonstring = e.Result.ToString();
                var mynearitems = JsonConvert.DeserializeObject<RootObject>(mynearjesonstring);
                NearSightingsItems.ItemsSource = mynearitems.sightings;

然后在XAML中：

<maps:MapItemsControl x:Name="NearSightingsItems" Width="450" Tap="NearSightingsItems_Tap" >
                    <maps:MapItemsControl.ItemTemplate>
                        <DataTemplate>
                            <maps:Pushpin x:Name="NearSightingPushpin" Location="{Binding location}" Template="{StaticResource PushpinControlTemplate1}"/>
                        </DataTemplate>
                    </maps:MapItemsControl.ItemTemplate>
                </maps:MapItemsControl>

运作良好，我可以看到所有的图钉。然后在pushpin上点击事件我想要更改一些TextBox.Text所以我尝试获取数据上下文...并且它等于null：

private void NearSightingsItems_Tap(object sender, System.Windows.Input.GestureEventArgs e)
        {
            var PushpinModel = (sender as Pushpin).DataContext as Sighting;
            NameTextBlock.Text = PushpinModel.name;

        }

也许问题是我的mynearitems.sightings在列表中？