如何更正说明 WPF DependencyProperty 用法的示例？

Question

我正在尝试运行我附在下面的文章“Dependency Properties in WPF”中的代码。

但应用程序在SetValue(MyDependencyProperty, value); 线上中断，但有例外：

System.Windows.Markup.XamlParseException   
"'' is not a valid value for property 'MyProperty'."

内部异常：

{"'对类型的构造函数的调用 '_3DP_CallBack_DefaultValue.MainWindow' 匹配指定的 绑定约束引发了异常。行号“3”和行 位置‘9’。”}

为了运行这个应用，我应该改变什么？

WPF应用代码：

namespace _3DP_CallBack_DefaultValue
{
  public partial class MainWindow : Window
  {
    public MainWindow()
    {
      InitializeComponent();
      DependencyPropertySample dpSample = new DependencyPropertySample();
      dpSample.MyProperty = "Dependency Property Test";//???

      Binding mybinding = new Binding("MyProperty");
      mybinding.Mode = BindingMode.OneWay;
      mybinding.Source = dpSample;
      BindingOperations.SetBinding(MyTextblock, TextBox.TextProperty, mybinding);
    }
  }
  public class DependencyPropertySample : DependencyObject
  {
    //Register Dependency Property
    public static readonly DependencyProperty MyDependencyProperty 
      = DependencyProperty.Register
           (
               "MyProperty", typeof(string), typeof(DependencyPropertySample), 
               new PropertyMetadata
                 (
                    "Test", 
                    new PropertyChangedCallback(OnMyPropertyChanged), 
                    new CoerceValueCallback(OnCoerceValue)
                 ), 
                 new ValidateValueCallback(OnValidateMyProperty) 
           );

    public string MyProperty
    {
      get
      {
        return (string)GetValue(MyDependencyProperty);
      }
      set
      {
//***************************
//breaking on the following line trying to set any string value   
// in this case "Dependency Property Test"
        SetValue(MyDependencyProperty, value);
      }
    }

    public static void OnMyPropertyChanged(DependencyObject dObject,
          DependencyPropertyChangedEventArgs e)
    {
      MessageBox.Show(e.NewValue.ToString());
    }
    public static string OnCoerceValue(DependencyObject dObject, object val)
    {
      if (val.ToString().CompareTo("Test") == 1)
      {
        return val.ToString();
      }
      return string.Empty;
    }
    public static bool OnValidateMyProperty(object myObj)
    {
      if (myObj.ToString() == string.Empty)
        return false;
      return true;
    }
  }
}

XAML：

<Window x:Class="_3DP_CallBack_DefaultValue.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
        </Grid.RowDefinitions>
        <Label Content="Enter String:" Grid.Row="0"
            VerticalAlignment="Center" />
        <TextBox Text="" Name="MyTextblock" Height="25"
             Width="150" HorizontalAlignment="Right" />
    </Grid>
</Window>

更新：

上面是一个 3d（增量）版本的 WPF 应用程序，带有 2 个以前的 WPF 应用程序，我已经运行没有任何错误

第二个版本有：

• 完全相同的 XAML 代码
• 完全相同的C#MainWindow()的构造函数/方法体；
• class DependencyPropertySample : DependencyObject{} 中缺少的方法
  • OnMyPropertyChanged( DependencyObject dObject, DependencyPropertyChangedEventArgs e)
  • OnValidateMyProperty(object myObj)
  • OnCoerceValue(DependencyObject dObject, object val)

而public static readonly DependencyProperty MyDependencyProperty = DependencyProperty.Register() 不同：

这是DependencyPropertySample类的代码来自工作的第二版：

public class DependencyPropertySample : DependencyObject
{
  //Register Dependency Property
  public static readonly DependencyProperty MyDependencyProperty = 
           DependencyProperty.Register
              ("MyProperty", typeof(string), typeof(DependencyPropertySample));
public string MyProperty
{
  get
  {
    return (string)GetValue(MyDependencyProperty);
  }
  set
  {
    SetValue(MyDependencyProperty, value);
  }
}

这是 DependencyPropertySample 类的代码来自失败的 3d 版应用程序：

  public class DependencyPropertySample : DependencyObject
  {
    //Register Dependency Property
    public static readonly DependencyProperty MyDependencyProperty 
      = DependencyProperty.Register
           (
               "MyProperty", typeof(string), typeof(DependencyPropertySample), 
               new PropertyMetadata
                 (
                    "Test", 
                    new PropertyChangedCallback(OnMyPropertyChanged), 
                    new CoerceValueCallback(OnCoerceValue)
                 ), 
                 new ValidateValueCallback(OnValidateMyProperty) 
           );

    public string MyProperty
    {
      get
      {
        return (string)GetValue(MyDependencyProperty);
      }
      set
      {
        SetValue(MyDependencyProperty, value);
      }
    }
    public static void OnMyPropertyChanged(DependencyObject dObject,
          DependencyPropertyChangedEventArgs e)
    {
      MessageBox.Show(e.NewValue.ToString());
    }
    public static string OnCoerceValue(DependencyObject dObject, object val)
    {
      if (val.ToString().CompareTo("Test") == 1)
      {
        return val.ToString();
      }
      return string.Empty;
    }
    public static bool OnValidateMyProperty(object myObj)
    {
      if (myObj.ToString() == string.Empty)
        return false;
      return true;
    }
  }

Answer 1

public static string OnCoerceValue(DependencyObject dObject, object val)
    {
      if (val.ToString().CompareTo("Test") == 1)
      {
        return val.ToString();
      }
      **return string.Empty;**
    }

这个函数在比较后返回string.Empty

public static bool OnValidateMyProperty(object myObj)
        {
            if (myObj.ToString() == string.Empty)
                **return false;**
            return true;
        }

然后这个验证返回 false。由于验证失败，您会收到错误消息“'' is not a valid value for property 'MyProperty'。”对这些函数进行适当的更改。