HTML Agility Pack错误

Question

我是第一次尝试HTML Agility Pack，我正在使用一段示例代码来解析HTML中的URL。但是我得到了一个错误，我不确定为什么我会得到它。有人能指出我哪里做错了吗？

下面是源代码(html是传入的HTML字符串)：

 StringBuilder sb = new StringBuilder();

 HtmlDocument htmldoc = new HtmlDocument();
 htmldoc.LoadHtml(html);

 foreach (HtmlNode link in htmldoc.DocumentNode.SelectNodes("//a[@HREF]"))
     {
     HtmlAttribute att = link.Attributes["HREF"];
     sb.AppendLine(att.Value + "|");
     }
 return sb.ToString();

当我调试我的应用程序时，我收到以下错误(调试器将它放在“foreach”之后)：

System.NullReferenceException was unhandled
  Message=Object reference not set to an instance of an object.
  Source=ScreenScraper
  StackTrace:
       at ScreenScraper.its.GetITSLoadID(String html) in C:\Web_Projects\ScreenScaper\ScreenScraper\its.cs:line 22
       at ScreenScraper.frm1.btnStartScraping_Click(Object sender, EventArgs e) in C:\Web_Projects\ScreenScaper\ScreenScraper\frm1.cs:line 43
       at System.Windows.Forms.Control.OnClick(EventArgs e)
       at System.Windows.Forms.Button.OnClick(EventArgs e)
       at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
       at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.ButtonBase.WndProc(Message& m)
       at System.Windows.Forms.Button.WndProc(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.Run(Form mainForm)
       at ScreenScraper.Program.Main() in C:\Web_Projects\ScreenScaper\ScreenScraper\Program.cs:line 18
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.Runtime.Hosting.ManifestRunner.Run(Boolean checkAptModel)
       at System.Runtime.Hosting.ManifestRunner.ExecuteAsAssembly()
       at System.Runtime.Hosting.ApplicationActivator.CreateInstance(ActivationContext activationContext, String[] activationCustomData)
       at System.Runtime.Hosting.ApplicationActivator.CreateInstance(ActivationContext activationContext)
       at System.Activator.CreateInstance(ActivationContext activationContext)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssemblyDebugInZone()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

Answer 1

Html Agility Pack有一个“设计错误”，它为空集合返回一个null。因此，您需要这样做：

HtmlNodeList list = htmldoc.DocumentNode.SelectNodes("//a[@HREF]");
if (list != null)
{
  foreach (HtmlNode link in list)
  ...
}

顺便说一句，XPATH表达式中指定的所有标记都必须是小写的，即使它们在HTML文本中以不同的方式声明(因为HTML不区分大小写，所以默认的Html Agility Pack XPATH约定是使用小写标记)。因此，您应该改为这样写：

HtmlNodeList list = htmldoc.DocumentNode.SelectNodes("//a[@href]");