在我的vb.net项目中需要帮助，阅读http响应标头时出现问题 [英] Need Help in my vb.net project ,problem in reading http response headers

问题描述

大家好，我即将在vb.net 2008中使用webclient创建一个下载程序。我想要获取的是http响应标头并在列表框中显示它们...但是我收到此消息{对象参考不是在Foreach行中设置为对象的实例。}这是我的代码

  Imports  System.Net 
 公开 类 Form1 
  Dim   WithEvents  wc  As  新 WebClient 
 私有  Sub  Button1_Click（ ByVal  sender < span class =code-keyword> As  System。 Object ， ByVal  e  As  System.EventArgs） Handles  Butto n1.Click 
  Dim  s  As   String  
 
 wc.DownloadFileAsync（ New  Uri（TextBox1.Text），  c：\ downloads \ a.mp4）
 
 对于 每个名称作为 字符串 在 wc.ResponseHeaders.Keys 
 ListBox1.Items.Add（Name +  ： + wc.ResponseHeaders（Name））
 下一步 
 
 
 < span class =code-keyword> End   Sub  
 
  Private   Sub  wc_DownloadProgressChanged（ ByVal 发​​件人作为 对象， ByVal  e  As  System.Net.DownloadProgressChangedEventArgs）句柄 wc.DownloadProgressChanged 
 ProgressBar1.Value = e.ProgressPercentage 
 结束  Sub  
 结束 类 

希望你能帮助我..提前感谢

解决方案

DownloadFileAsync 方法返回之前下载已完成。在引发 DownloadFileCompleted 事件之前，您无法读取响应标头。

  Imports  System.Net 
 
 公共 类 Form1 
  Dim   WithEvents  wc  As  新 WebClient 
 
 私有  Sub  Button1_Click（ ByVal  sender  As  System。对象， ByVal  e  As  EventArgs）句柄 Button1.Click 
 Button1.Enabled =  False  
 wc.DownloadFileAsync（ New  Uri（T \\ textBox1.Text），  c：\ downloads \ a.mp4）
 结束  Sub  
 
 私有  Sub  wc_DownloadProgressChanged（ ByVal  sender  As  对象， ByVal  e  As  DownloadProgressChangedEventArgs） 句柄 wc.DownloadProgressChanged 
 ProgressBar1.Value = e.ProgressPercentage 
 结束  Sub  
 
 私有  Sub  wc_DownloadFileCompleted（ ByVal  sender  As  对象， ByVal  e  As  AsyncCompletedEventArgs）句柄 wc.DownloadFileCompleted 
 对于 每个名称作为 字符串 在 wc.ResponseHeaders.Keys 
 ListBox1 .Items.Add（Name +  ： + wc.ResponseHeaders（Name））
 下一步 
 Button1.Enabled =  True  
 结束  Sub  
 结束 类 

Hi all , im about to create a downloader using a webclient in vb.net 2008 .. what im trying to get is the http response headers and show them in a listbox ...but i get this message {Object reference not set to an instance of an object.} in the Foreach line ... Here is my code

Imports System.Net
Public Class Form1
    Dim WithEvents wc As New WebClient
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim s As String

        wc.DownloadFileAsync(New Uri(TextBox1.Text), "c:\downloads\a.mp4")

        For Each Name As String In wc.ResponseHeaders.Keys
            ListBox1.Items.Add(Name + ":" + wc.ResponseHeaders(Name))
        Next


    End Sub

    Private Sub wc_DownloadProgressChanged(ByVal sender As Object, ByVal e As System.Net.DownloadProgressChangedEventArgs) Handles wc.DownloadProgressChanged
        ProgressBar1.Value = e.ProgressPercentage
    End Sub
End Class

Hope you will help me .. thanks in advance

解决方案

The DownloadFileAsync method returns before the download has completed. You cannot read the response headers until the DownloadFileCompleted event is raised.

Imports System.Net

Public Class Form1
    Dim WithEvents wc As New WebClient
    
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As EventArgs) Handles Button1.Click
        Button1.Enabled = False
        wc.DownloadFileAsync(New Uri(TextBox1.Text), "c:\downloads\a.mp4")
    End Sub
 
    Private Sub wc_DownloadProgressChanged(ByVal sender As Object, ByVal e As DownloadProgressChangedEventArgs) Handles wc.DownloadProgressChanged
        ProgressBar1.Value = e.ProgressPercentage
    End Sub
    
    Private Sub wc_DownloadFileCompleted(ByVal sender As Object, ByVal e As AsyncCompletedEventArgs) Handles wc.DownloadFileCompleted
        For Each Name As String In wc.ResponseHeaders.Keys
            ListBox1.Items.Add(Name + ":" + wc.ResponseHeaders(Name))
        Next
        Button1.Enabled = True
    End Sub
End Class

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