依赖属性PropertyChangedCallback异常 [英] Dependency Property PropertyChangedCallback exception

问题描述

  public   bool  HasConnection 
 {
  get  { return （ bool ）GetValue （HasConnectionProperty）; } 
  set  {SetValue（HasConnectionProperty， value ）; } 
} 
 
  public   static   readonly  DependencyProperty HasConnectionProperty = 
 DependencyProperty.Register（
   HasConnection ，
  typeof （ bool ），
  typeof （NetworkNode），
  new  FrameworkPropertyMetadata（
  false ，
 FrameworkPropertyMetadataOptions.BindsTwoWayByDefault，
  new  PropertyChangedCallback（HasConnectionChangedCallBack）））; 
 
 
  private   static   void  HasConnectionChangedCallBack（DependencyObject d，DependencyPropertyChangedEventArgs e）
 {
 NetworkNode nn =（NetworkNode）d; 
 Ellipse el = nn.GetTemplateChild（  PART_inner） as  Ellipse; 
 如果（el.PART_inner.Visibility == ...）<<  -  exception el  is   null  
  //  。 .code ..  
} 

此代码工作正常，除非我在视觉中更改HasConnection属性工作室属性面板，或在xaml中我设置HasConnection =True，并且我运行程序抛出一个例外：对象引用未设置为对象的实例。

解决方案

< blockquote>出于某种原因， el 或 el.PART_inner 在某些条件下变为null。你将不得不调试它。



不用担心。这是检测和修复的最简单的案例之一。它只是意味着某些引用类型的某个成员/变量通过使用和它的实例（非静态）成员解除引用，这要求此成员/变量为非null，但实际上它似乎为null。只需在调试器下执行它，它将停止抛出异常的执行。在该行上设置一个断点，重新启动应用程序并再次到达这一点。评估下一行中涉及的所有引用，并查看哪一个为null，而不需要为null。解决这个问题之后，修复代码：要么确保将成员/变量正确初始化为非空引用，要么将其检查为null，如果为null，则执行其他操作。



另请参阅：想要在按钮点击时显示下一条记录。但是如果下一条记录功能的条件对象引用没有设置为对象的实例 [ ^ ]。



有时候，你不能在调试器下，由于一个或另一个原因做到这一点。一个非常讨厌的情况是，只有在调试信息不​​可用时构建软件时才会出现问题。在这种情况下，你必须使用更难的方式。首先，你需要确保你永远不会通过静默处理异常来阻止异常的传播（这是开发人员对自己的犯罪，但很常见）。您需要在每个线程的最顶层堆栈帧上捕获绝对所有异常。如果处理类型 System.Exception 的异常，则可以执行此操作。在处理程序中，您需要记录所有异常信息，尤其是 System.Exception.StackTrace ：

http://msdn.microsoft.com/en-us/library/system.exception.aspx [ ^ ]，

http://msdn.microsoft.com/en-us/library/system.exception。 stacktrace.aspx [ ^ ]。



堆栈跟踪只是一个字符串，显示从throw语句到处理程序的异常传播的完整路径。通过阅读，您总能找到目的。对于日志记录，使用类 System.Diagnostics.EventLog 是最好的（在大多数情况下）：

http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx [ ^ ] 。



祝你好运，

-SA

这是具有依赖属性的自定义控件

 [TemplatePart（Name =   PART_outter，Type =  typeof （Ellipse））] 
 [TemplatePart（Name =   PART_inner，Type =  typeof （Ellipse） ）] 
 [TemplatePart（Name =   PART_ canvas，Type =  typeof （Canvas））] 
  public   class  NetworkNode：Control 
 {
  static  NetworkNode（）
 {
 DefaultStyleKeyProperty.OverrideMetadata（ typeof （NetworkNode）， new  FrameworkPropertyMetadata（ typeof运算（NetworkNode）））; 
} 
 
 私人 Canvas PART_canvas; 
  private  Ellipse PART_outter; 
  private  Ellipse PART_inner; 
 
  public  覆盖  void  OnApplyTemplate（）
 {
  base  .OnApplyTemplate（）; 
 
 PART_canvas =  this  .GetTemplateChild（  PART_canvas） as  Canvas; 
 PART_outter = 此 .GetTemplateChild（  PART_outter ） as  Ellipse; 
 PART_inner =  this  .GetTemplateChild（  PART_inner ） as  Ellipse; 
} 
 
  public   bool  HasConnection 
 {
  get  { return （ bool  ）的GetValue（HasConnectionProperty）; } 
  set  {SetValue（HasConnectionProperty， value ）; } 
} 
 
  public   static   readonly  DependencyProperty HasConnectionProperty = 
 DependencyProperty.Register（
   HasConnection ，
  typeof （ bool ），
  typeof （NetworkNode），
  new  FrameworkPropertyMetadata（ false ，
  new  PropertyChangedCallback（callBack）））; 
 
  private   static   void  callBack（DependencyObject d，DependencyPropertyChangedEventArgs e）
 {
 NetworkNode nn =（NetworkNode）d; 
  if （nn.Template ==  null ）返回; 
 
 Ellipse el = nn.GetTemplateChild（  PART_inner） as  Ellipse; 
  var  test = el.Visibility; 
  switch （test）
 {
  case  Visibility.Collapsed： 
  break ; 
 
  case  Visibility.Hidden：
 el.Visibility = Visibility.Visible; 
  break ; 
 
  case  Visibility.Visible：
 el.Visibility = Visibility.Hidden; 
  break ; 
 
 默认：
  break ; 
} 
} 
} 
 

如果我执行HasConnection =False没问题，编译运行正常。

问题是，我认为，当我在xaml中执行HasConnection =True时，依赖项prop检测到值的变化并调用方法callBack，但自定义控件尚未实例化，因此为null异常。随着绑定问题的解决。但我想在代码背后做。

 <  < span class =code-leadattribute> ioncontrolslibrary：networknode     x：name   =  uiNode    horizo​​ntalalignment   = 左    margin   =  59,427,0,0     verticalalignment   =  Top    hasconnection   = 真    xmlns：x   = ＃unknown    xmlns：ioncontrolslibrary   = ＃unknown    /  >  

您的媒体资源的默认值为假。因此，如果你将它设置为false，则永远不会调用回调，所以你不会看到错误。

如果在回调中元素为null，那么在之类的内容中执行相同的操作OnApplyTemplate （应用控制模板后）

public bool HasConnection
{
    get { return (bool)GetValue(HasConnectionProperty); }
    set { SetValue(HasConnectionProperty, value); }
}
 
public static readonly DependencyProperty HasConnectionProperty =
 DependencyProperty.Register(
  "HasConnection",
        typeof(bool),
        typeof(NetworkNode),
        new FrameworkPropertyMetadata(
            false,
            FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
            new PropertyChangedCallback(HasConnectionChangedCallBack)));
 

private static void HasConnectionChangedCallBack(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
 NetworkNode nn = (NetworkNode)d;
 Ellipse el = nn.GetTemplateChild("PART_inner") as Ellipse;
    if (el.PART_inner.Visibility == ...) <<-- exception el is null
            //..code..
}

This code works fine, except if I change the HasConnection property in visual studio properties panel, or in xaml I set HasConnection = "True", and I run the program throws an excetion: Object reference not set to an instance of an object.

解决方案

By some reason, el or el.PART_inner becomes null under certain conditions. You will have to debug it.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"[^].

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx[^],
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx[^].

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it''s the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx[^].

Good luck,

—SA

This is the Custom Control With Dependency Property

[TemplatePart(Name = "PART_outter", Type = typeof(Ellipse))]
[TemplatePart(Name = "PART_inner", Type = typeof(Ellipse))]
[TemplatePart(Name = "PART_canvas", Type = typeof(Canvas))]
public class NetworkNode : Control
{
    static NetworkNode()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(NetworkNode), new FrameworkPropertyMetadata(typeof(NetworkNode)));
    }

    private Canvas PART_canvas;
    private Ellipse PART_outter;
    private Ellipse PART_inner;

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();

        PART_canvas = this.GetTemplateChild("PART_canvas") as Canvas;
        PART_outter = this.GetTemplateChild("PART_outter") as Ellipse;
        PART_inner = this.GetTemplateChild("PART_inner") as Ellipse;
    }

    public bool HasConnection
    {
        get { return (bool)GetValue(HasConnectionProperty); }
        set { SetValue(HasConnectionProperty, value); }
    }

    public static readonly DependencyProperty HasConnectionProperty =
        DependencyProperty.Register(
        "HasConnection",
        typeof(bool),
        typeof(NetworkNode),
        new FrameworkPropertyMetadata(false,
            new PropertyChangedCallback(callBack)));

    private static void callBack(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        NetworkNode nn = (NetworkNode)d;
        if (nn.Template == null) return;

        Ellipse el = nn.GetTemplateChild("PART_inner") as Ellipse;
        var test = el.Visibility;
        switch (test)
        {
            case Visibility.Collapsed:
                break;

            case Visibility.Hidden:
                el.Visibility = Visibility.Visible;
                break;

            case Visibility.Visible:
                el.Visibility = Visibility.Hidden;
                break;

            default:
                break;
        }
    }
}

If i do HasConnection="False" no problem at all, it compile and runs fine.
The problem is, "i think", when I do HasConnection="True" in xaml, the dependency prop detects the value change and call the method callBack, but the custom control isnt yet instantiated and therefore the null exception. With binding problem solved. but i wanna do it in code behind.

<ioncontrolslibrary:networknode x:name="uiNode" horizontalalignment="Left" margin="59,427,0,0" verticalalignment="Top" hasconnection="True" xmlns:x="#unknown" xmlns:ioncontrolslibrary="#unknown" />

The default value for your property is False. so if you set it to false, callback is never called, so you don''t see errors.
If element is null in callback, do the same handling in something like OnApplyTemplate (after control template is applied)

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