WPF无法从代码访问DependencyProperty [重复]

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
    }

此代码最初由 Luke Malpass (AngelSix) 编写

我用过的财产是这样的

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

在 Xaml 中：

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

SomeType 是一个普通的类，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，targetType 总是 null。

我做错了什么？

提前感谢

Answer 1

您应该在UserControl 上调用GetValueValue 附加属性已设置：

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。