从服务器到客户端机器ASP.NET下载文件组

Question

我无法弄清楚如何让用户浏览本地计算机上的目录，并从我的ASP.NET Web应用程序中的服务器下载一些文件。这是我的代码，目前硬编码到文件夹。我希望用户能够选择一个文件夹并下载整个文件。为什么我不能在VB.NET中使用OpenFileDialog来让他们选择一个文件夹？

Protected Sub materialsClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim materialsButton As Button = sender
    Dim rowIndex As Integer = Convert.ToInt32(materialsButton.Attributes("RowIndex"))
    Dim presentationDate As Date = Convert.ToDateTime(gvPresentations.Rows(rowIndex).Cells(1).Text)
    Dim format As String = "yyyy-MM-dd"
    Dim convertedDate As String = presentationDate.ToString(format)
    Dim dir As String = "\" + gvPresentations.Rows(rowIndex).Cells(0).Text & "\" & convertedDate
    Dim publisherDir As String = "D:\Presentations" & dir

    If Directory.Exists(publisherDir) Then
        Dim materials() As String = Directory.GetFiles(publisherDir)
        Dim newDir As String = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments) + dir
        messageLabel.Text = newDir

        Try
            If Not Directory.Exists(newDir) Then
                Directory.CreateDirectory(newDir)
            End If
        Catch ex As Exception
            messageLabel.Text = "Could not download materials"
        End Try

        For Each material As String In materials
            Dim fileName As String = Path.GetFileName(material)

            If Not File.Exists(newDir & "\" & fileName) Then
                File.Copy(material, newDir & "\" & fileName)
            End If
        Next

        messageLabel.Text = "All files have been saved to " + newDir
    Else
        messageLabel.Text = "No materials available for this presentation"
    End If
End Sub