c# - UWP - 将 TextBox.Text 绑定(bind)到 Nullable

Question

当前无法绑定(bind)到任何 Nullable<T> 是否正确？在通用 XAML 应用程序中？

我在 2013 年找到了这个链接:

https://social.msdn.microsoft.com/Forums/en-US/befb9603-b8d6-468d-ad36-ef82a9e29749/textbox-text-binding-on-nullable-types?forum=winappswithcsharp

说明:

Windows 8 应用商店应用不支持绑定(bind)到可为 null 的值。它只是没有进入这个版本。我们已经为 v.Next 找到了有关此行为的错误。

但难道真的还没有修复好吗？

我的绑定(bind):

<TextBox Text="{Binding Serves, Mode=TwoWay}" Header="Serves"/>

我的属性(property):

public int? Serves
{
    get { return _serves; ; }
    set
    {
        _serves = value;
        OnPropertyChanged();
    }
}

我在输出中得到的错误:

Error: Cannot save value from target back to source. 
BindingExpression: 
    Path='Serves' 
    DataItem='MyAssembly.MyNamespace.RecipeViewModel, MyAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'; target element is 'Windows.UI.Xaml.Controls.TextBox' (Name='null'); target property is 'Text' (type 'String').

Answer 1

好像不是固定的。由于 XAML 使用内置转换器，在这种情况下，您可能可以将其与您自己的转换器交换，处理可空值:

XAML:

<StackPanel Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <StackPanel.Resources>
        <local:NullConverter x:Key="NullableIntConverter"/>
    </StackPanel.Resources>
    <TextBox Text="{Binding Serves, Mode=TwoWay, Converter={StaticResource NullableIntConverter}}" Header="Serves"/>
</StackPanel>

代码隐藏:

public class NullConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, string language)
    { return value; }

    public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        int temp;
        if (string.IsNullOrEmpty((string)value) || !int.TryParse((string)value, out temp)) return null;
        else return temp;
    }
}

public sealed partial class MainPage : Page, INotifyPropertyChanged
{
    private int? _serves;

    public event PropertyChangedEventHandler PropertyChanged;
    public void RaiseProperty(string name) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));

    public int? Serves
    {
        get { return _serves; }
        set { _serves = value; RaiseProperty("Serves"); }
    }

    public MainPage()
    {
        this.InitializeComponent();
        DataContext = this;
    }
}