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c# - HtmlAgilityPack NullReferenceException 错误

问题描述

我正在尝试获取文本，但总是收到 NullReferenceException 错误。我快疯了。它工作了一会儿，但我又开始遇到这个错误。我只会得到帖子的标题。

我试图改变我的 XPath。我尝试了不同的 XPath。

    private void button1_Click(object sender, EventArgs e)
    {
        listView1.Items.Clear();

        for (int i = 4; i < 9; i++)
        {
            VeriAl(Url: "https://cracked.to/Forum-Combolists?sortby=started&order=desc&datecut=9999&prefix=0", XPath: "//table[@class='tborder clear']//tr[" + i + "]//td[2]//div[1]//span[1]//span[1]//a", tag: "title",CikanSonuc: listView1);
        }
    }

    public void VeriAl (String Url, String XPath, String tag,ListView CikanSonuc)
    {
        try
        {
            url = new Uri(Url);
        }
        catch (UriFormatException)
        {
            if (MessageBox.Show(text: "UriFormatException", caption: "Hata", buttons: MessageBoxButtons.OK, icon: MessageBoxIcon.Error) == DialogResult.OK)
            {

            }
        }
        catch (ArgumentNullException)
        {
            if (MessageBox.Show(text: "ArgumentNullException", caption: "Hata", buttons: MessageBoxButtons.OK, icon: MessageBoxIcon.Error) == DialogResult.OK)
            {

            }
        }

        WebClient client = new WebClient();
        try
        {
            html = client.DownloadString(url);
        }
        catch (WebException)
        {
            if (MessageBox.Show(text: "WebException", caption: "Hata", buttons: MessageBoxButtons.OK, icon: MessageBoxIcon.Error) == DialogResult.OK)
            {

            }
        }

        HtmlAgilityPack.HtmlDocument doc = new HtmlAgilityPack.HtmlDocument();
        doc.LoadHtml(html);
        try
        {
            CikanSonuc.Items.Add(doc.DocumentNode.SelectSingleNode(XPath).Attributes[tag].Value);
        }
        catch (NullReferenceException)
        {
            if (MessageBox.Show(text: "NullReferenceException", caption: "Hata", buttons: MessageBoxButtons.OK, icon: MessageBoxIcon.Error) == DialogResult.OK)
            {

            }
        }

标签： c#html-agility-pack

解决方案

问题是在您尝试获取 HTML 时内容不存在。如果您调用WebClient.DownloadString(). 获得完全加载的网页的唯一方法是使用 WebBrowser 控件加载页面并在加载完成后获取所有内容：

public static class WebViewExtension
{
    public static HtmlAgilityPack.HtmlDocument GetHtmlDocument(this WebBrowser wView)
    {
        HtmlAgilityPack.HtmlDocument doc = new HtmlAgilityPack.HtmlDocument();
        doc.LoadHtml(wView.Document.Body.OuterHtml);
        return doc;
    }

    public static async Task<HtmlAgilityPack.HtmlDocument> LoadSiteAndGetHtml(this WebBrowser wView, string siteurl)
    {
        await wView.NavigateAndWait(siteurl);
        HtmlAgilityPack.HtmlDocument doc = wView.GetHtmlDocument();
        return doc;
    }

    public static async Task NavigateAndWait(this WebBrowser wView, string siteurl)
    {
        TaskCompletionSource<bool> loaded = new TaskCompletionSource<bool>();
        wView.Navigate(new Uri(siteurl));
        wView.DocumentCompleted += delegate (object sender, WebBrowserDocumentCompletedEventArgs args)
        {
            loaded?.TrySetResult(true);
        };

        //wait until the website is loaded
        await loaded.Task;
    }
}

您可以像这样使用这些方法：

    WebBrowser client = new WebBrowser();
    try
    {
        HtmlDocument doc = await client.LoadSiteAndGetHtml(url);
    }
    catch (WebException)
    {
        if (MessageBox.Show(text: "WebException", caption: "Hata", buttons: MessageBoxButtons.OK, icon: MessageBoxIcon.Error) == DialogResult.OK)
        {

        }
    }

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