的NullReferenceException在HtmlAgilityPack

Question

我試圖用xpath從下文提到的網址

string url = "http://www.album-cover-art.org/search.php?q=Ruin+-+Live+Album+Version+Lamb+of+God" 

我的代碼提取link：

HtmlAgilityPack.HtmlWeb web = new HtmlAgilityPack.HtmlWeb(); 
HtmlAgilityPack.HtmlDocument htmlDoc = new HtmlAgilityPack.HtmlDocument(); 
htmlDoc = web.Load(url); //Exception generated here Line 23 

if (htmlDoc.DocumentNode != null) 
{ 
    HtmlNode linkNode = htmlDoc.DocumentNode.SelectSingleNode(".//*[@id='related_search_row']/img/@src"); 
    if (linkNode != null) 
     Console.WriteLine(linkNode.InnerText); 
} 

上面的代碼編譯罰款，但是當我嘗試運行它產生例外

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object. 

Complete stacktrace

System.NullReferenceException: Object reference not set to an instance of an object. 
    at HtmlAgilityPack.HtmlDocument.ReadDocumentEncoding(HtmlNode node) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlDocument.cs:line 1916 
    at HtmlAgilityPack.HtmlDocument.PushNodeEnd(Int32 index, Boolean close) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlDocument.cs:line 1805 
    at HtmlAgilityPack.HtmlDocument.Parse() in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlDocument.cs:line 1468 
    at HtmlAgilityPack.HtmlDocument.Load(TextReader reader) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlDocument.cs:line 769 
    at HtmlAgilityPack.HtmlWeb.Get(Uri uri, String method, String path, HtmlDocument doc, IWebProxy proxy, ICredentials creds) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlWeb.cs:line 1515 
    at HtmlAgilityPack.HtmlWeb.LoadUrl(Uri uri, String method, WebProxy proxy, NetworkCredential creds) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlWeb.cs:line 1563 
    at HtmlAgilityPack.HtmlWeb.Load(String url, String method) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlWeb.cs:line 1149 
    at HtmlAgilityPack.HtmlWeb.Load(String url) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlWeb.cs:line 1107 
    at ScreenScrapping.Program.Main(String[] args) in c:\Users\ranveer\csharp\ScreenScrapping\ScreenScrapping\Program.cs:line 23 

所以，我的問題是爲什麼我得到這個異常。

Answer 1

這是HtmlAgilityPack中的一個錯誤。您嘗試解析的文檔有<meta http-equiv="Content-Type" content="text/html; charset=iso-utf-8">，其中charset值（iso-utf-8）無法通過AgilityPack解析爲有效的編碼名稱。正如Simon Mourier said,這是在1.4.0.0中引入的一個錯誤。

爲了避免這種情況，手動從流裝入原稿並手動設置的編碼這樣的：

var htmlDoc = new HtmlDocument(); 
htmlDoc.OptionReadEncoding = false; 
var request = (HttpWebRequest)WebRequest.Create(url); 
request.Method = "GET"; 
using (var response = (HttpWebResponse)request.GetResponse()) 
{ 
    using (var stream = response.GetResponseStream()) 
    { 
     htmlDoc.Load(stream, Encoding.UTF8); 
    } 
}