Silverlight组合框SelectedValue双向绑定不工作

我在应用程序中使用了许多组合框，所有组合框都能正常工作。但是，我现在找不到问题。我已将SelectedValuePath设置为"Tag"属性。但是属性在更改ComboBox所选项目后没有更新。我读过StackOverflow的其他问题，但没有烦恼有帮助。

它是xaml:

xmlns:vms="clr namespace:SilverlightApplication1"

<UserControl.DataContext>
    <vms:MainViewModel />
</UserControl.DataContext>
<Grid x:Name="LayoutRoot" Background="White">
    <ComboBox Width="100" VerticalAlignment="Center" FontFamily="Segoe UI"
         Height="30" Margin="0,5,0,0"  HorizontalAlignment="Left" 
         SelectedValue="{Binding SelectedDifStatusComparer, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
         SelectedValuePath="Tag">
        <ComboBox.Items>
            <ComboBoxItem Tag="H" >High</ComboBoxItem>
            <ComboBoxItem Tag="L" >Low</ComboBoxItem>
            <ComboBoxItem Tag="E" >Equal</ComboBoxItem>
        </ComboBox.Items>
    </ComboBox>
</Grid>

这是ViewModel：

public class MainViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
        private string _selectedDifStatusComparer = "";
        private string SelectedDifStatusComparer
        {
            get { return _selectedDifStatusComparer; }
            set
            {
                _selectedDifStatusComparer = value;
                MessageBox.Show(_selectedDifStatusComparer);
                OnPropertyChanged("SelectedDifStatusComparer");
            }
        }
        public MainViewModel()
        {
            SelectedDifStatusComparer = "E"; // It is working, the MessageBox is apperaing
        }
    }

 public class MainViewModel : INotifyPropertyChanged
        {
            public event PropertyChangedEventHandler PropertyChanged;
            private void OnPropertyChanged(string propertyName)
            {
                if (PropertyChanged != null)
                {
                    PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
                }
            }
            private string _selectedDifStatusComparer = "";
            public string SelectedDifStatusComparer
            {
                get { return _selectedDifStatusComparer; }
                set
                {
                    _selectedDifStatusComparer = value;
                    MessageBox.Show(_selectedDifStatusComparer);
                    OnPropertyChanged("SelectedDifStatusComparer");
                }
            }
            public MainViewModel()
            {
                SelectedDifStatusComparer = "E"; // It is working, the MessageBox is apperaing
            }
        }