当ItemsSource绑定到从服务获取的数据时，ListPicker不会'tick'选择项目

我按照http://mobileworld.appamundi.com/blogs/andywigley/archive/2012/02/02/how-to-databind-selecteditems-of-the-listpicker-and-recurringdayspicker.aspx的说明在多选ListPicker中绑定SelectedItems .

当我将ItemsSource绑定到ViewModel中创建的对象的ObservableCollection时，ListPicker可以正常工作 . 但是，当我将ItemsSource绑定到从WCF服务获取的ObservableCollection时，问题就开始了 . ListPicker正确显示所有项目，也可以在主屏幕上选择并显示为所选项目 . 但是，当我单击选择器再次选择时，ListPicker无法“勾选”模板上的选定项目 .

我可以重写从服务中获取的所有元素的列表，但我想知道是否有一种解决该问题的简洁方法？

项目信息：WP 7.0，WCF服务，EntityFramework，大量咖啡

实体类：

DataContract(IsReference=true)]
public class TypeOfDish
{
    [DataMember]
    public int TypeOfDishID { get; set; }
    [DataMember]
    public string NameToDisplay { get; set; }
}

WCF服务：

[ServiceContract]
public interface IMyService
{
    [OperationContract]
    [ApplyDataContractResolver]
    IEnumerable<TypeOfDish> GetDishTypes();
}

XAML：

<StackPanel x:Name="DishTypeGroup" Orientation="Vertical" Width="400">
    <helpers:ListPickerExtended x:Name="TypeOfDishPicker" >
         ItemsSource="{Binding DishTypeList}" 
         SelectionMode="Multiple"
         SelectedItems="{Binding SelectedDishTypes, Mode=TwoWay}"
         Margin="8,0,0,0" HorizontalAlignment="Stretch" VerticalAlignment="Center">

        <toolkit:ListPicker.FullModeItemTemplate>
            <DataTemplate>
                <TextBlock Text="{Binding Path=NameToDisplay}" />
            </DataTemplate>
        </toolkit:ListPicker.FullModeItemTemplate>
    </helpers:ListPickerExtended>
</StackPanel>

ListPickerExtended.cs

public class ListPickerExtended : ListPicker
{
    public new IList SelectedItems
    {
        get
        {
            return (IList)GetValue(SelectedItemsProperty);
        }
        set
        {
            base.SetValue(SelectedItemsProperty, value);
        }
    }
}

视图模型

public ObservableCollection<TypeOfDish> DishTypeList
{
    get
    {
        //myModel.DichTypes is loaded with data from calling 
        //GetDishTypesAsync() on my service client
        return myModel.DishTypes;
    }
}

ObservableCollection<object> _selectedDishes = new ObservableCollection<object>();

public ObservableCollection<object> SelectedDishTypes
{
    get { return _selectedDishes; }
    set
    {
       if (_selectedDishes == value) { return; }
       _selectedDishes = value;
       RaisePropertyChanged("SelectedDishTypes");
    }
}