XAML 双向绑定到可为空的类型

环境： WinRt/XAML/C#

我正在尝试为浮点类型属性进行双向绑定。但是，如果这是可为 null 的类型（浮点型？或可为 Null），则绑定不起作用。我为此应用了事件转换器，但它仍然没有显示出任何可绑定性的迹象。

C#：视图模型

public class MyViewModel : INotifyPropertyChanged
{
    private float _quantity1;
    public float Quantity1
    {
        get
        {
            return this._quantity1;
        }
        set
        {
            this._quantity1 = value;
            RaisePropertyChanged("Quantity1");
        }
    }
    private float? _quantity2;
    public float? Quantity2
    {
        get
        {
            return this._quantity2;
        }
        set
        {
            this._quantity2 = value;
            RaisePropertyChanged("Quantity2");
        }
    }
    private Nullable<float> _quantity3;
    public Nullable<float> Quantity3
    {
        get
        {
            return this._quantity3;
        }
        set
        {
            this._quantity3 = value;
            RaisePropertyChanged("Quantity3");
        }
    }
    public MyViewModel()
    {
        this.Quantity1 = 100.01F;
        this.Quantity2 = 200.02F;
        this.Quantity3 = 300.03F;
    }
    public event PropertyChangedEventHandler PropertyChanged;
    protected void RaisePropertyChanged(string propertyName)
    {
        System.ComponentModel.PropertyChangedEventHandler propertyChanged = this.PropertyChanged;
        if ((propertyChanged != null))
        {
            propertyChanged(this, new System.ComponentModel.PropertyChangedEventArgs(propertyName));
        }
    }
}

C# ： 转换器：

public sealed class NullableFloatConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, string language)
    {
        if (value == null)
            return 0F;
        else
            return value;
    }
    public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        if (value != null)
            return value;
        else
            return 0;
    }
}

XAML：

<Page
x:Class="Test_Binding.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Test_Binding"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Page.Resources>
    <local:NullableFloatConverter x:Key="nullConverter" />
</Page.Resources>
    <Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
    <StackPanel>
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Quantity1: " Width="150" />
            <TextBox Text="{Binding Quantity1, Mode=TwoWay}" />
            <TextBlock Text="{Binding Quantity1}" />
        </StackPanel>
        <!-- the second text block doesnt get an updated value -->
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Quantity2: " Width="150"/>
            <TextBox Text="{Binding Quantity2, Mode=TwoWay, Converter={StaticResource nullConverter}}" />
            <TextBlock Text="{Binding Quantity2, Converter={StaticResource nullConverter}}" />
        </StackPanel>
        <!-- the second text block doesnt get an updated value -->
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Quantity2 (No Converter): " Width="150"/>
            <TextBox Text="{Binding Quantity2, Mode=TwoWay}" />
            <TextBlock Text="{Binding Quantity2}" />
        </StackPanel>
        <!-- the second text block doesnt get an updated value -->
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Quantity3: " Width="150"/>
            <TextBox Text="{Binding Quantity3, Mode=TwoWay}" />
            <TextBlock Text="{Binding Quantity3}" />
        </StackPanel>
    </StackPanel>
</Grid>

只有第一个文本块会更新（即对于数量 1）。我无法让其他人（Quantity2和Quantity3）获得更新。有什么建议吗？

public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        string s = value as string;
        float result;
        if (!string.IsNullOrWhiteSpace(s) && float.TryParse(s, out result))
        {
            return result;
        }
        return null;
    }