如何在文本框和双精度值之间实现双向绑定

这是我窗口的代码：

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="leartWPF.ControlTestWindow"
    x:Name="Window"
    Title="ControlTestWindow"
    Width="640" Height="480">
    <Grid x:Name="LayoutRoot">
        <TextBlock Height="26" Margin="45,26,241,0" TextWrapping="Wrap" Text="Please, enter an ariphmetical expression to calculate:" VerticalAlignment="Top"/>
        <TextBox Margin="48,72,63,201" TextWrapping="Wrap" Text="{Binding Input, ElementName=Window, FallbackValue=1+1, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" TextChanged="TextBox_TextChanged" >
        </TextBox>
        <!--<TextBlock Margin="282,208,266,167" TextWrapping="Wrap" Text="=" FontSize="64"/>-->
        <TextBlock Height="90" Margin="83,0,77,60" TextWrapping="Wrap" VerticalAlignment="Bottom" FontSize="48" Text="{Binding Result, ElementName=Window, Mode=TwoWay}"/>
        <Button Content="=" Height="27" Margin="233,0,263,166" VerticalAlignment="Bottom" FontSize="16"/>
    </Grid>
</Window>

和类：

public partial class ControlTestWindow : Window
{
    private string _input;
    public double Result { get; set; }
    private static VsaEngine _engine = VsaEngine.CreateEngine();
    public string Input
    {
        get { return _input; }
        set
        {
            Result = double.Parse(Eval.JScriptEvaluate(value, _engine).ToString());
            _input = value;
        }
    }

    public ControlTestWindow()
    {
        this.InitializeComponent();
        // Insert code required on object creation below this point.
    }
    private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
    {
    }
}

输入将更新，结果值将更改，但它永远不会显示在相应的文本块上。

我应该更改什么才能使其正常工作？