使用.NET WebClient调用DownloadFileAsync与DownloadFile无响应不抛异常求助

核心问题点 #

• 回调事件绑定错误：你调用的是DownloadFileAsync方法，对应的完成回调事件是DownloadFileCompleted，而非你当前代码绑定的DownloadDataCompleted，这是你看不到任何下载完成/报错提示的核心原因。
• User-Agent被站点拦截：sec.gov有严格的反爬校验规则，你使用的User-Agent: Other属于无效标识，会被服务器直接拦截请求，不会返回文件内容。同步下载生成空文件就是因为拦截后无有效响应内容写入。
• 额外注意：如果是控制台场景运行代码，调用异步下载后不要让主线程直接退出，否则进程终止会直接中断下载且无任何提示。

修正后代码示例 #

Public DownloadClient As WebClient
' 控制台场景可加信号量等待下载完成
Private ReadOnly downloadResetEvent As New Threading.ManualResetEvent(False)

Public Sub DownloadAsync()
    Dim ArchiveUrl As String = "https://www.sec.gov/Archives/edgar/daily-index/xbrl/companyfacts.zip"
    Dim UserFolder As String = System.IO.Path.GetTempPath()
    Dim ArchiveTemp As String = Now().ToString("yy-MM-dd_HH-mm") & ".zip"
    Dim savePath As String = System.IO.Path.Combine(UserFolder, ArchiveTemp)

    If DownloadClient IsNot Nothing AndAlso DownloadClient.IsBusy Then
        Debug.WriteLine("File download in progress")
        Return
    End If

    Debug.Print("Downloading to: " & savePath)

    ' 提前配置安全协议，建议放在程序启动入口执行一次即可
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 Or SecurityProtocolType.Tls13
    ServicePointManager.DefaultConnectionLimit = 10

    DownloadClient = New WebClient()
    ' 替换为正常浏览器UA
    DownloadClient.Headers.Add("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36")
    ' 补充必要请求头避免拦截
    DownloadClient.Headers.Add("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8")
    DownloadClient.Headers.Add("Accept-Language", "zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2")
    DownloadClient.Headers.Add("Accept-Encoding", "gzip, deflate, br")

    ' 修正事件绑定：对应DownloadFileAsync的回调事件
    AddHandler DownloadClient.DownloadFileCompleted, AddressOf DownloadComplete
    AddHandler DownloadClient.DownloadProgressChanged, AddressOf DownloadProgress

    Try
        downloadResetEvent.Reset()
        DownloadClient.DownloadFileAsync(New Uri(ArchiveUrl), savePath)
        ' 控制台场景打开下面这句等待下载完成，WinForm/WPF场景不需要
        ' downloadResetEvent.WaitOne()
    Catch ex As WebException
        If ex.Response IsNot Nothing Then
            Dim dataStream As IO.Stream = ex.Response.GetResponseStream()
            Dim reader As New IO.StreamReader(dataStream)
            Debug.WriteLine(reader.ReadToEnd())
        Else
            Debug.WriteLine("No Response Stream")
        End If
    End Try

End Sub


Public Sub DownloadProgress(sender As Object, e As DownloadProgressChangedEventArgs)
    Debug.WriteLine($"Downloading: {e.ProgressPercentage}%，已下载{e.BytesReceived}/{e.TotalBytesToReceive}字节")
End Sub

Public Sub DownloadComplete(sender As Object, e As System.ComponentModel.AsyncCompletedEventArgs)
    If e.Error IsNot Nothing Then
        Debug.WriteLine($"下载出错：{e.Error.ToString()}")
        downloadResetEvent.Set()
        Return
    End If
    If e.Cancelled Then
        Debug.WriteLine("下载被取消")
        downloadResetEvent.Set()
        Return
    End If

    Threading.Thread.Sleep(3000)
    Debug.WriteLine("Download Complete!")
    downloadResetEvent.Set()
End Sub

额外优化建议 #

如果经常遇到下载超时问题，可以自定义继承WebClient的子类，设置更长的超时时间：

Public Class CustomWebClient
    Inherits WebClient
    Public Property Timeout As Integer = 600000 ' 单位毫秒，这里设为10分钟
    Protected Overrides Function GetWebRequest(address As Uri) As WebRequest
        Dim request = MyBase.GetWebRequest(address)
        request.Timeout = Timeout
        CType(request, HttpWebRequest).ReadWriteTimeout = Timeout
        Return request
    End Function
End Class

使用时替换原来的WebClient为CustomWebClient即可。

内容的提问来源于stack exchange，提问作者Berries