VB.NET下载器下载后切换单选按钮触发空引用异常问题

错误原因 #

你遇到的空引用错误，根源在下载完成后的清理代码中：
在httpClient_DownloadProgressChanged方法里，你错误地执行了txtLocation = Nothing和txtFilename = Nothing——这是把控件对象本身设置为null，而不是清空控件的文本内容。当后续切换单选按钮触发CheckedChanged事件时，代码尝试访问txtFilename.Text，但此时txtFilename已经是不存在的对象，自然抛出空引用异常。

另外还有一个潜在问题：用ProgressBar1.Value = 100来判断下载完成并不严谨，因为DownloadProgressChanged事件触发到100%时，异步下载操作可能还未完全结束，正确的做法是通过DownloadFileCompleted事件处理下载完成后的逻辑。

修复步骤 #

1. 修正控件清空逻辑：将赋值控件对象为Nothing的代码，改为清空控件的Text属性。
2. 改用DownloadFileCompleted事件处理完成逻辑：把下载完成后的清理、提示逻辑移到这个事件里，确保下载操作真正完成后再执行清理。
3. 移除ProgressChanged中的完成判断代码：避免提前执行清理操作导致的问题。

修改后的完整代码 #

Option Explicit On
Option Strict On

Imports System.Net

Public Class Form1

    Private WithEvents httpClient As WebClient

    Dim UserName As String
    Dim Password As String
    Dim USER As String = Environment.UserName
    Dim SAVE As String
    Dim Filename As String
    Dim Destination As String
    Dim URL As String

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        UserName = txtUserName.Text
        Password = txtPassword.Text
        URL = txtURL.Text
        Destination = txtLocation.Text
        Filename = txtFilename.Text

        SAVE = "C:\Users\" & USER & "\Desktop\" & txtName.Text & "\" & Destination & "\" & Filename

        httpClient = New WebClient

        Try
            httpClient.Credentials = New NetworkCredential(UserName, Password)
            httpClient.DownloadFileAsync(New Uri(URL), SAVE)
        Catch ex As Exception
            MessageBox.Show(ex.Message, "Critical")
        End Try
    End Sub

    Private Sub httpClient_DownloadProgressChanged(sender As Object, e As DownloadProgressChangedEventArgs) Handles httpClient.DownloadProgressChanged
        ProgressBar1.Value = e.ProgressPercentage
        Label6.Text = CInt(e.BytesReceived / 1048576) & " MB"
    End Sub

    Private Sub httpClient_DownloadFileCompleted(sender As Object, e As System.ComponentModel.AsyncCompletedEventArgs) Handles httpClient.DownloadFileCompleted
        ' 处理下载完成（包括成功或失败）
        If e.Error Is Nothing Then
            MessageBox.Show(Filename & " has been successfully downloaded!", "Success!", MessageBoxButtons.OK, MessageBoxIcon.Information)
            ' 清空控件文本，而非控件对象
            Label6.Text = Nothing
            txtURL.Text = Nothing
            txtLocation.Text = Nothing
            txtFilename.Text = Nothing
            ProgressBar1.Value = 0
            ' 取消单选按钮选中状态
            rdoAVI.Checked = False
            rdoMP4.Checked = False
            rdoMKV.Checked = False
        Else
            MessageBox.Show("Download failed: " & e.Error.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End If
        ' 释放WebClient资源
        httpClient.Dispose()
        httpClient = Nothing ' 可选，避免后续误访问
    End Sub

    Private Sub rdoAVI_CheckedChanged(sender As Object, e As EventArgs) Handles rdoAVI.CheckedChanged
        If rdoAVI.Checked = True Then
            txtFilename.Text = txtName.Text & " S" & txtSeason.Text & " E" & txtEpisode.Text & ".avi"
        End If
    End Sub

    Private Sub rdoMP4_CheckedChanged(sender As Object, e As EventArgs) Handles rdoMP4.CheckedChanged
        If rdoMP4.Checked = True Then
            txtFilename.Text = txtName.Text & " S" & txtSeason.Text & " E" & txtEpisode.Text & ".mp4"
        End If
    End Sub

    Private Sub rdoMKV_CheckedChanged(sender As Object, e As EventArgs) Handles rdoMKV.CheckedChanged
        If rdoMKV.Checked = True Then
            txtFilename.Text = txtName.Text & " S" & txtSeason.Text & " E" & txtEpisode.Text & ".mkv"
        End If
    End Sub
End Class

额外说明 #

• 改用DownloadFileCompleted事件可以准确捕获下载完成的时机，同时还能处理下载过程中出现的异常（比如网络中断、文件无法写入等）。
• 清空控件时始终操作Text属性，而不是控件对象本身，这是WinForms开发中的基础规范，避免控件对象被意外销毁导致后续操作报错。

内容的提问来源于stack exchange，提问作者Wilko84