在Visual Basic中实现文件上传可以使用WebClient类来实现。首先需要创建一个WebClient对象，并设置其上传文件的URL地址和上传方式。然后通过调用UploadFileAsync方法来实现文件上传，并使用UploadProgressChanged和UploadFileCompleted事件来处理上传进度和完成后的操作。处理上传过程中的错误可以在UploadFileCompleted事件中捕获异常并处理。以下是一个示例代码：
Imports System.IO
Imports System.Net
Public Class Uploader
Private Sub btnUpload_Click(sender As Object, e As EventArgs) Handles btnUpload.Click
Dim client As WebClient = New WebClient()
AddHandler client.UploadProgressChanged, AddressOf UploadProgressChanged
AddHandler client.UploadFileCompleted, AddressOf UploadFileCompleted
client.UploadFileAsync(New Uri("http://example.com/upload.php"), "POST", "C:\myfile.txt")
End Sub
Private Sub UploadProgressChanged(sender As Object, e As UploadProgressChangedEventArgs)
' 更新上传进度
progressBar.Value = e.ProgressPercentage
End Sub
Private Sub UploadFileCompleted(sender As Object, e As UploadFileCompletedEventArgs)
If e.[Error] IsNot Nothing Then
' 处理上传错误
MessageBox.Show("Error: " + e.[Error].Message)
Else
' 处理上传成功
MessageBox.Show("Upload completed!")
End If
End Sub
End Class