NullReferenceException异常在C＃中的HTMLDocument参考 [英] NullReferenceException with HtmlDocument reference in C#

问题描述

我使用 HtmlAgilityPack 为了凑信息关闭谷歌的翻译进行翻译计划。我已经下载了 HtmlAgilityPack 的dll，并成功地在我的程序中引用它。我正在使用统一大会。下面是我对两个程序代码：

I am using HtmlAgilityPack in order to scrape information off of Google Translate for a translation program. I have downloaded the HtmlAgilityPack dll, and successfully referenced it in my program. I am using Assembly in Unity. Below is my code for the two programs:

using UnityEngine;
using System.Collections;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net;
using HtmlAgilityPack;

public class GUIScript : MonoBehaviour {
    private string textField = "";
    private string input;
    public Texture2D icon;
    Dictionary search;
    Encoding code;
    // Use this for initialization
    void Start () { 
        search = new Dictionary();
        input = " ";
        code = Encoding.UTF8;
        //This is what is run to translate
        print (search.Translate("Hola","es|en",code));
    }

    // Update is called once per frame
    void Update () {

    }
    void OnGUI(){
        textField = GUI.TextField(new Rect(0, Screen.height -50, Screen.width-80, 40), textField);
        if(GUI.Button(new Rect(Screen.width-80, Screen.height -50, 80,40), icon)){
            input = textField;
            textField = "";

        }
        //GUI.Label(new Rect(0,Screen.height -70, Screen.width-80,20), search.Translate("Hola","es|en",code));
        //print (search.Translate("Hola","es|en",code));
    }
}

这是引用我的<$ C $代码如下图所示C>字典类：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using UnityEngine;
using System.Collections;
using System.Net;
using HtmlAgilityPack;


public class Dictionary{
    string[] formatParams;
    HtmlDocument doc;
    public Dictionary(){
        formatParams = new string[2];
        doc = new HtmlDocument();
    }
    public string Translate(String input, String languagePair, Encoding encoding)
     {
        formatParams[0]= input;
        formatParams[1]= languagePair;
        string url = String.Format("http://www.google.com/translate_t?hl=en&ie=UTF8&text={0}&langpair={1}", formatParams);

        string result = String.Empty;

        using (WebClient webClient = new WebClient())
        {
            webClient.Encoding = encoding;
            result = webClient.DownloadString(url);
        }       
        doc.LoadHtml(result);
        return doc.DocumentNode.SelectSingleNode("//span[@title=input]").InnerText;
    }
    // Use this for initialization
    void Start () {

    }
}

在运行此，我收到错误消息：

When running this, I receive the error:

NullReferenceException: Object reference not set to an instance of an object
Dictionary.Translate (System.String input, System.String languagePair,System.Text.Encoding encoding) (at Assets/Dictionary.cs:32)
GUIScript.Start () (at Assets/GUIScript.cs:22)

我试图改变代码，仰视的解决方案，为的HTMLDocument 的API，以及如何解决 NullReferenceException异常，但由于某些原因，我想不出为什么我收到一个的NullReferenceException 。这个问题已经拖我的一两个星期，现在我需要用我的项目继续前进。 ！任何帮助将不胜感激。

I have tried changing code, looking up solutions, the API for HtmlDocument, and how to fix NullReferenceExceptions, but for some reason I cannot figure out why I am getting a NullReferenceException. This problem has been holding me back for a week or two now and I need to move on with my project. Any help would be greatly appreciated!

推荐答案

如果我计算正确，这是第32行：

If I've counted correctly, this is line 32:

return doc.DocumentNode.SelectSingleNode("//span[@title=input]").InnerText

这意味着要么 doc.DocumentNode 为空或 DocumentNode.SelectSingleNode（ //跨度[@标题=输入]）终止返回null。

That means either doc.DocumentNode is null or DocumentNode.SelectSingleNode("//span[@title=input]") is returning null.

如果是前者，请检查您收到了实际文档的反面。您的网址可能不正确编码。另请参见为什么HTML敏捷性包HtmlDocument.DocumentNode为空？

If it is the former, check you are receiving an actual document back. Your URL may not be encoded correctly. See also why HTML Agility Pack HtmlDocument.DocumentNode is null?

如果是后者，它可能是一些奇怪的使用XPath发生。我不知道如何相关，这是因为 DocumentNode 应该是文档的根在的 http://htmlagilitypack.codeplex.com/discussions/249129 可以申请。根据这一点，'//'是从搜索的文件的，你可能必须尝试 doc.DocumentNode.SelectSingleNode（.//跨度[@title的根=输入]）代替（添加。字符串的开头）。

If it is the latter, it could be something odd happening with XPath. I don't know how relevant this is as DocumentNode should be the root of the document, the discussion at http://htmlagilitypack.codeplex.com/discussions/249129 could apply. According to this, '//' is searches from the root of the document, and you may have to try doc.DocumentNode.SelectSingleNode(".//span[@title=input]") instead (adding a . to the beginning of the string).

调试方法，看到这些调用的确切值将完成这项工作。

Debugging the method and seeing exactly the values of these calls will finish the job.

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