如何更正此示例说明WPF DependencyProperty用法？

Question

我试图运行下面附加的文章“Dependency Properties in WPF”中的代码。在线SetValue(MyDependencyProperty, value);

但应用程式的空档，出现异常：

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

内部异常：

{“_3DP_CallBack_DefaultValue.MainWindow '符合指定 '构造对 类型调用'绑定约束抛出了一个异常。'行号 '3' 和线 位置 '9'。“}

我应该怎么才能运行这个程序改变？

WPF应用程序的代码：

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：

上面是与2以前的WPF应用三维（增量）版本的WPF应用程序中，我运行没有任何错误

第二个版本有：

• 完全相同XAML代码
• 完全相同的C＃MainWindow()的身体构造/方法;
• 在class DependencyPropertySample : DependencyObject{}
  • OnMyPropertyChanged( DependencyObject dObject, DependencyPropertyChangedEventArgs e)
  • OnValidateMyProperty(object myObj)
  • OnCoerceValue(DependencyObject dObject, object val)

和public static readonly DependencyProperty MyDependencyProperty = DependencyProperty.Register()缺席方法不同：

这里是与c从工作的第二个版本的DependencyPropertySample类颂歌：

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); 
    } 
} 

这里是DependencyPropertySample类的从失败的应用的3D版本的代码：

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;** 
    } 

该函数返回的String.Empty比较后

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

然后，此验证返回false。由于验证失败，您会收到错误“''不是属性'MyProperty'的有效值。”对这些功能进行适当的更改。