依赖项属性，无法创建新的类实例

本文关键字：实例 创建 属性 依赖 | 更新日期: 2023-09-27 18:35:30

我正在设计一个可以有两个主要状态的控件。 状态将通过依赖项属性设置。 我想通过 XAML 设置属性。 我知道如果通过 XAML 设置了 setValue 代码，则资源库只会运行该代码，如下所述：WPF：XAML 属性声明不是通过 Sseters 设置的？

所以我尝试设置一个 DependencyPropertyChangedEvent 事件，但是当我运行代码时，我收到此错误： 

XamlParseException Occurred
'The invocation of the constructor on type 'RouteOneClient.VehicleInfoControl'
that matches the specified binding constraints threw an exception.' Line number 
'134' and line position '34'.

当我查看它时，它告诉我无法在事件中创建控件的新实例。 C# 逻辑如下所示： 

    public enum VehicleType
    {
        Sale = 1,
        Trade = 2,
    }
    public static readonly DependencyProperty VehicleTypeProperty =  DependencyProperty.Register(
        "VehicleTYPE", 
        typeof(VehicleType), 
        typeof(VehicleInfoControl), 
        new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender, new PropertyChangedCallback(OnVehTypeChanged)));  // optionally metadata for defaults etc.
    public VehicleInfoControl()
    {
        InitializeComponent();
    }
    public VehicleType VehicleTYPE
    {
        get { return (VehicleType)GetValue(VehicleTypeProperty); }
        set
        {
            SetValue(VehicleTypeProperty, value);
        }          
    }
    private static void OnVehTypeChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        VehicleInfoControl instance = d as VehicleInfoControl;
        if (instance.VehicleTYPE == VehicleType.Sale)
        {
            instance.TitleLabel.Content = "Sale Vehicle";
            instance.IntendedUse.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleNewUsed.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleVin.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleInceptionMiles.Visibility = System.Windows.Visibility.Visible;
            instance.IntendedUseLabel.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleNewUsedLabel.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleVinLabel.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleInceptionMilesLabel.Visibility = System.Windows.Visibility.Visible;
        }
        else
        {
            instance.TitleLabel.Content = "Trade Vehicle";
            instance.IntendedUse.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleNewUsed.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleVin.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleInceptionMiles.Visibility = System.Windows.Visibility.Hidden;
            instance.IntendedUseLabel.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleNewUsedLabel.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleVinLabel.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleInceptionMilesLabel.Visibility = System.Windows.Visibility.Hidden;
        }
    }

和 XAML：

<Expander Header="Vehicle Information" Width="Auto" Height="Auto" Grid.Row="2">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width=".3*" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <custom:VehicleInfoControl x:Name="SaleVeh" Width="Auto" Height="Auto" Grid.Column="0" VehicleTYPE="Sale"/>
        <custom:VehicleInfoControl x:Name="TradeVeh" Width="Auto" Height="Auto" Grid.Column="2" VehicleTYPE="Trade"/>
    </Grid>
</Expander>

上面给出的错误指向该行。

我觉得我正确地遵循了表单，但它没有运行。 有什么想法吗？

依赖项属性，无法创建新的类实例

依赖项

属性的类型是枚举，枚举是 ValueType，但您可以在属性元数据中指定null作为默认属性值。这是行不通的，因为null不是 ValueType 的有效值。

在注册依赖项属性时，将"null"传递到"FrameworkPropertyMetadata"构造函数中。

要传递的第一个参数将是依赖项属性的默认值。在您的情况下，您将 Null 作为 ENUM 类型的依赖项属性的默认值传递，这是不允许的。

传递有效的默认值可能会解决您的问题。

像这样的事情可能会做

new FrameworkPropertyMetadata(VehicleType.Sale,  FrameworkPropertyMetadataOptions.AffectsRender,  new PropertyChangedCallback(OnVehTypeChanged)));
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