如何更正此示例說明WPF DependencyProperty用法？

Question

我試圖運行下面附加的文章「Dependency Properties in WPF」中的代碼。在線SetValue(MyDependencyProperty, value);

但應用程式的空檔，出現異常：

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

內部異常：

{「_3DP_CallBack_DefaultValue.MainWindow '符合指定 '構造對 類型調用'綁定約束拋出了一個異常。'行號 '3' 和線 位置 '9'。「}

我應該怎麼才能運行這個程序改變？

WPF應用程序的代碼：

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：

上面是與2以前的WPF應用三維（增量）版本的WPF應用程序中，我運行沒有任何錯誤

第二個版本有：

• 完全相同XAML代碼
• 完全相同的C＃MainWindow()的身體構造/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：

這裏是與c從工作的第二個版本的DependencyPropertySample類頌歌：

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); 
    } 
} 

這裏是DependencyPropertySample類的從失敗的應用的3D版本的代碼：

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;** 
    } 

該函數返回的String.Empty比較後

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

然後，此驗證返回false。由於驗證失敗，您會收到錯誤「''不是屬性'MyProperty'的有效值。」對這些功能進行適當的更改。