c#之如何调用 WebBrowser Navigate 遍历多个 url

要收集网页上的信息，我可以使用 WebBrowser.Navigated 事件。

首先，导航到 url:

WebBrowser wbCourseOverview = new WebBrowser(); 
wbCourseOverview.ScriptErrorsSuppressed = true; 
wbCourseOverview.Navigate(url); 
wbCourseOverview.Navigated += wbCourseOverview_Navigated; 

然后在调用Navigated时处理网页:

void wbCourseOverview_Navigated(object sender, WebBrowserNavigatedEventArgs e) 
    { 
        //Find the control and invoke "Click" event... 
    } 

当我尝试遍历 url 字符串数组时，困难的部分就来了。

foreach (var u in courseUrls) 
        { 
            WebBrowser wbCourseOverview = new WebBrowser(); 
            wbCourseOverview.ScriptErrorsSuppressed = true; 
            wbCourseOverview.Navigate(u); 
 
            wbCourseOverview.Navigated += wbCourseOverview_Navigated; 
        } 

此处，由于页面加载需要时间，因此永远不会到达 wbCourseOverview_Navigated。

我尝试在 C#5 中使用 async await。任务和基于事件的异步模式 (EAP) 可在 here 中找到.另一个示例可以在 The Task-based Asynchronous Pattern 中找到.

问题是 WebClient 有像 DownloadDataAsync 和 DownloadStringAsync 这样的异步方法。但是 WebBrowser 中没有 NavigateAsync。

有哪位高手可以给我一些建议吗？谢谢。

---

StackOverflow 中有一篇文章 ( here )。但是，有谁知道如何在其答案中实现该 strut 吗？

---

再次更新。

在 another post here in StackOverflow 中建议,

public static Task WhenDocumentCompleted(this WebBrowser browser) 
{ 
    var tcs = new TaskCompletionSource<bool>(); 
    browser.DocumentCompleted += (s, args) => tcs.SetResult(true); 
    return tcs.Task; 
} 

所以我有:

foreach (var c in courseBriefs) 
    { 
        wbCourseOverview.Navigate(c.Url); 
        await wbCourseOverview.WhenDocumentCompleted(); 
    } 

在我的网络浏览器访问第二个 url 之前，它看起来不错。

An attempt was made to transition a task to a final state when it had already completed.

我知道我一定是在 foreach 循环中犯了一个错误。因为循环到第二轮的时候还没有引发DocumentCompleted事件。在 foreach 循环中编写此 await 的正确方法是什么？

请您参考如下方法:

There is a post in StackOverflow (here). But, does anyone know how to implement that strut in its answer?

好的，所以你需要一些带有 awaiter 的代码。我做了两段代码。 第一个使用 TPL 的内置等待器:

 public partial class Form1 : Form 
    { 
        public Form1() 
        { 
            InitializeComponent(); 
        } 
 
        private void button1_Click(object sender, EventArgs e) 
        { 
            ProcessUrlsAsync(new[] { "http://google.com", "http://microsoft.com", "http://yahoo.com" }) 
                .Start(); 
        } 
 
        private Task ProcessUrlsAsync(string[] urls) 
        { 
            return new Task(() => 
            { 
                foreach (string url in urls) 
                { 
                    TaskAwaiter<string> awaiter = ProcessUrlAsync(url); 
                    // or the next line, in case we use method * 
                    // TaskAwaiter<string> awaiter = ProcessUrlAsync(url).GetAwaiter();                      
                    string result = awaiter.GetResult(); 
 
                    MessageBox.Show(result); 
                } 
            }); 
        }         
 
        // Awaiter inside 
        private TaskAwaiter<string> ProcessUrlAsync(string url) 
        { 
            TaskCompletionSource<string> taskCompletionSource = new TaskCompletionSource<string>(); 
            var handler = new WebBrowserDocumentCompletedEventHandler((s, e) => 
            { 
                // TODO: put custom processing of document right here 
                taskCompletionSource.SetResult(e.Url + ": " + webBrowser1.Document.Title); 
            }); 
            webBrowser1.DocumentCompleted += handler; 
            taskCompletionSource.Task.ContinueWith(s => { webBrowser1.DocumentCompleted -= handler; }); 
 
            webBrowser1.Navigate(url); 
            return taskCompletionSource.Task.GetAwaiter(); 
        } 
 
        // (*) Task<string> instead of Awaiter 
        //private Task<string> ProcessUrlAsync(string url) 
        //{ 
        //    TaskCompletionSource<string> taskCompletionSource = new TaskCompletionSource<string>(); 
        //    var handler = new WebBrowserDocumentCompletedEventHandler((s, e) => 
        //    { 
        //        taskCompletionSource.SetResult(e.Url + ": " + webBrowser1.Document.Title); 
        //    }); 
        //    webBrowser1.DocumentCompleted += handler; 
        //    taskCompletionSource.Task.ContinueWith(s => { webBrowser1.DocumentCompleted -= handler; }); 
 
        //    webBrowser1.Navigate(url); 
        //    return taskCompletionSource.Task; 
        //} 

下一个示例包含 Eric Lippert 正在谈论的 awaiter struct 的示例实现 here .

public partial class Form1 : Form 
    { 
        public struct WebBrowserAwaiter 
        { 
            private readonly WebBrowser _webBrowser; 
            private readonly string _url; 
 
            private readonly TaskAwaiter<string> _innerAwaiter; 
 
            public bool IsCompleted 
            { 
                get 
                { 
                    return _innerAwaiter.IsCompleted; 
                } 
            } 
 
            public WebBrowserAwaiter(WebBrowser webBrowser, string url) 
            { 
                _url = url; 
                _webBrowser = webBrowser; 
                _innerAwaiter = ProcessUrlAwaitable(_webBrowser, url); 
            } 
 
            public string GetResult() 
            { 
                return _innerAwaiter.GetResult(); 
 
            } 
 
            public void OnCompleted(Action continuation) 
            { 
                _innerAwaiter.OnCompleted(continuation); 
            } 
 
            private TaskAwaiter<string> ProcessUrlAwaitable(WebBrowser webBrowser, string url) 
            { 
                TaskCompletionSource<string> taskCompletionSource = new TaskCompletionSource<string>(); 
                var handler = new WebBrowserDocumentCompletedEventHandler((s, e) => 
                { 
                    // TODO: put custom processing of document here 
                    taskCompletionSource.SetResult(e.Url + ": " + webBrowser.Document.Title); 
                }); 
                webBrowser.DocumentCompleted += handler; 
                taskCompletionSource.Task.ContinueWith(s => { webBrowser.DocumentCompleted -= handler; }); 
 
                webBrowser.Navigate(url); 
                return taskCompletionSource.Task.GetAwaiter(); 
            } 
        } 
 
        public Form1() 
        { 
            InitializeComponent(); 
        } 
 
        private void button1_Click(object sender, EventArgs e) 
        { 
            ProcessUrlsAsync(new[] { "http://google.com", "http://microsoft.com", "http://yahoo.com" }) 
                .Start(); 
        } 
 
        private Task ProcessUrlsAsync(string[] urls) 
        { 
            return new Task(() => 
            { 
                foreach (string url in urls) 
                { 
                    var awaiter = new WebBrowserAwaiter(webBrowser1, url); 
                    string result = awaiter.GetResult(); 
 
                    MessageBox.Show(result); 
                } 
            }); 
        } 
    }    
        } 

希望这对您有所帮助。