xamarin picker SelectedItem在可观察集合上返回null

在我的xamarin项目picker binding中,SelectedItem不工作。当我将ItemSource设置为列表时,SelectedItem工作,但是当我将ItemSource更改为observatecollection时,SelectedItem始终返回null。有人能看出我做错了什么吗?

xaml公司

        <Picker x:Name="PickerMarket2" Title="Market2" ClassId="PickerMarket2"
                ItemsSource="{Binding TestList2}"
                ItemDisplayBinding="{Binding ShortDesc}"
                SelectedItem="{Binding SelectedMarket}"
                Grid.Row="0" Grid.Column="1" >
        </Picker>

视图模型

    class VamiMarketViewModel: INotifyPropertyChanged
{
    private List<string> _testList;
    public List<string> TestList
    {
        get { return _testList; }
        set
        {
            {
                _testList = value;
                NotifyPropertyChanged();
            }
        }
    }

    private ObservableCollection<Performance> _testList2;
    public ObservableCollection<Performance> TestList2
    {
        get { return _testList2; }
        set
        {
            {
                _testList2 = value;
                NotifyPropertyChanged();
            }
        }
    }

    private string _selectedMarket;
    public string SelectedMarket
    {
        get { return _selectedMarket; }
        set
        {
            {
                _selectedMarket = value;
                NotifyPropertyChanged();
            }
        }
    }