C#HTML Agility Pack单一选择节点返回null [英] C# HTML Agility Pack Single Select Node returning null

问题描述

我有一个使用C#，Windows窗体和HTML Agility Pack开发的Web抓取工具.

I have a web scraper developed using C#, windows forms and the HTML Agility Pack.

当网站更改其代码并将其破解时，我的一切工作都很好.我知道它经常发生在刮板机上，但是现在我在弄清楚如何解决该问题上遇到了麻烦.

I had it all working great when the site changed it's code and broke it. I know it happens often with web scrapers but now I am having trouble figuring out how to correct the issue.

这时，我的抓取工具会循环访问多个URL，并从每个页面抓取数据.

At this time my scraper loops thru multiple URL's and scrapes data from each page.

我遇到的问题是，它循环通过的网站模板将随机显示较新的模板，该模板不具有我在程序中定义的相同的HTML类和ID.我想做的是运行一个简单的if，它检查单个节点是否为null以及是否为新模板运行单独的代码集.

The problem I am running into is that the template of the site it loops thru will randomly show the newer template which does not have the same HTML classes and ID's that I have defined in the program. What I am trying to do is run a simple if that checks if a single node if null and if it is runs a separate set of code for the new template.

我遇到的问题是我的程序在if语句上抛出了NullReferenceException.

The problem I am having is that my program throws a NullReferenceException on my if statement.

这是我用来检查其是否为空的语句:

Here is the statement I am using to check if it is null:

var varitem = doc.DocumentNode.SelectSingleNode("//h1[@class='producttitle']").InnerText;

 if (varitem == null) MessageBox.Show("no titles");

它在定义变量的第一行引发异常，甚至没有在if语句中出现.

It throws the exception at the first line defining the varitem and doesn't even make it to the if statement.

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推荐答案

首先，您应该检查

 doc.DocumentNode.SelectSingleNode("//h1[@class='producttitle']")

返回空值.

如果为空，您将从null.InnerText

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