WPF 无法从代码访问依赖项属性

Question

我正在使用WPF 4.5.2，.Net 4.7.2，C# 7

这是我的附加属性的基类的代码

public abstract class BaseAP<Parent, Property> where Parent : BaseAP<Parent , Property>, new()
{
#region Public Events

/// <summary>
/// Fire when the value changes
/// </summary>
public event Action<DependencyObject , DependencyPropertyChangedEventArgs> ValueChanged = ( sender , e ) => { };

#endregion

#region Properties
/// <summary>
/// A singleton instance of the parent class
/// </summary>
public static Parent Instance { get; private set; } = new Parent();

#endregion

#region Attached Properties Definitions
/// <summary>
/// The Attached Property for this class
/// </summary>
public static readonly DependencyProperty ValueProperty = DependencyProperty.RegisterAttached( "Value" , typeof( Property ) , typeof( BaseAP<Parent , Property> ) , new PropertyMetadata( new PropertyChangedCallback( OnValuePropertyChanged ) ) );

/// <summary>
/// The callback event when the <see cref="ValueProperty"/> is changed
/// </summary>
/// <param name="d">The UI-Element that had it's property changed</param>
/// <param name="e">The arguments for the event</param>
private static void OnValuePropertyChanged( DependencyObject d , DependencyPropertyChangedEventArgs e )
{
// --- Call the parent function
Instance.OnValueChanged( d , e );

// --- Call the event listeners
Instance.ValueChanged( d , e );            
}


/// <summary>
/// Gets the attached property
/// </summary>
/// <param name="d">The element to get the property from</param>
/// <returns></returns>
public static Property GetValue( DependencyObject d )
{
return ( (Property) d.GetValue( ValueProperty ) );
}
/// <summary>
/// Sets the attached property
/// </summary>
/// <param name="d">The element to set the property to</param>
/// <param name="value">The value to set to the element</param>
public static void SetValue( DependencyObject d , Property value )
{
d.SetValue( ValueProperty , value );
}
#endregion

#region Event Methods

/// <summary>
/// The method is called when any attached property of this type is changed
/// </summary>
/// <param name="d">The ui element that this property was changed for</param>
/// <param name="e">The arguments for this event</param>
public virtual void OnValueChanged( DependencyObject d , DependencyPropertyChangedEventArgs e )
{
SetValue( d , (Property) e.NewValue );
}

#endregion
}

这段代码最初由Luke Malpass(AngelSix(编写。

我的二手房产看起来像这样

public class APType : BaseAP<APType , Type> { }

在 Xaml 中：

<UserControl local:APType.Value={x:Type local:SomeType} />

SomeType是普通课程，没什么特别的

在后面的代码中，我正在尝试这个：

Type targetType = GetValue( APType.ValueProperty ) as Type;

不幸的是，目标类型始终为空。

我做错了什么？

谢谢

Answer 1

设置Value附加属性后，您应该在UserControl上调用GetValue：

Type type = uc.GetValue(APType.ValueProperty) as Type;

XAML：

<UserControl x:Name="uc" local:APType.Value="{x:Type local:SomeType}">

在创建UserControl之前无法设置该属性。