问从XAML创建的GUI获取上一个文本值

EN

<Label x:Name="Label_F" Content="F" Margin="5" VerticalAlignment="Center" Width="20"/>
<TextBox x:Name="Box_F" Text="{Binding F, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" IsEnabled="{Binding IntegralCageCheck, Converter={StaticResource booleaninverter}}" Margin="5" Width="50" VerticalAlignment="Center" HorizontalContentAlignment="Center" VerticalContentAlignment="Center"/>

public string F
{
    get { return _F; }
    set
    {
        _F = value;
        double temp;

        bool result = double.TryParse(value, out temp);

        if (!char.IsDigit(Convert.ToChar(_F)) && _F != ".")
            {
            _F = _F.previousvalue; // Using as example
            }
        else { FrontPanelVariables.F = temp * 25.4; }

        RaisePropertyChanged("F");