如果你的檔案下載程式，使用者反映每個文字檔的尾巴都會被塞一句「Error」，你會怎麼想？
「這一定是被駭客攻擊了...」
是的，我很想就這麼告訴使用者，但...我真的好心虛，於是，老老實實打開程式碼好好檢查一番。
Partial Class MainPage_FileDownloadSeal
Inherits CommonPage
Private Sub MainPage_FileDownloadSeal_Load(sender As Object, e As EventArgs) Handles Me.Load
Dim wsfile As New AttachFileinfo()
Try
Dim FileID As String = Request("File").ToString().Trim()
Dim ds_file As DataSet = wsfile.getFileBase(FileID)
Dim strFilePath As String = ds_file.Tables(0).Rows(0)("檔案存放路徑").ToString() + ds_file.Tables(0).Rows(0)("系統檔名").ToString()
Dim strFileName As String = ds_file.Tables(0).Rows(0)("使用者上傳檔名").ToString()
If strFilePath.Length <> 0 Then
If File.Exists(strFilePath) Then
If strFilePath.Length > 0 Then
Dim strSubName As String
strSubName = strFilePath.Substring(strFilePath.LastIndexOf(".") + 1, strFilePath.Length - strFilePath.LastIndexOf(".") - 1)
'"檔案分類"
Select Case strSubName.ToUpper()
Case "TXT"
Response.ContentType = "text/html"
Exit Select
Case "HTM"
Response.ContentType = "text/html"
Exit Select
Case "HTML"
Response.ContentType = "text/html"
Exit Select
Case "DOCX", "DOC"
Response.ContentType = "application/msword"
Exit Select
Case "XLS"
Response.ContentType = "application/vnd.ms-excel"
Exit Select
Case "GIF"
Response.ContentType = "image/gif"
Exit Select
Case "JPG"
Response.ContentType = "image/JPG"
Exit Select
Case "BMP"
Response.ContentType = "image/bmp"
Exit Select
Case "SWF"
Response.ContentType = "application/x-shockwave-flash"
Exit Select
Case "PDF"
Response.ContentType = "application/pdf"
Exit Select
Case Else
'specify that the response is a stream that cannot be read by the
'client and must be downloaded
Response.ContentType = "application/octet-stream"
Exit Select
End Select
Response.Clear()
'決定檔案存檔預設名稱
Response.AddHeader("Content-Disposition", "attachment;filename=" + Server.UrlEncode(strFileName))
Response.WriteFile(strFilePath)
Response.[End]()
End If
End If
End If
Catch ex As Exception
Response.Write("Error")
End Try
End Sub
End Class
這應該再普通不過了吧，別人有的我也有，別人沒加的，我還老實的都有加。你看看，那個 Try ... Catch ，就是我特別力求完整而補上的。
但重點是，別人沒錯，我錯了啊!?
在參考了「小筆記-避免 ThreadAbortException 的Response.End() 替代寫法」，其中一段說著：
Response.End() 會觸發 ThreadAbortException 造成中斷。若 Response.End() 被 try catch包覆，則會進入 catch 流程。
所以我的程式：
Try
Response.Clear()
'決定檔案存檔預設名稱
Response.AddHeader("Content-Disposition", "attachment;filename=" + Server.UrlEncode(strFileName))
Response.WriteFile(strFilePath)
Response.[End]()
Catch ex As Exception
Response.Write("Error")
End Try
剛好把 Response.End() 放在 Try 裡頭，當他執行到 Response.End() 時，還沒來的及結束，就先去執行了 Catch 裡面的 Response.Writer("Error")
這也就解釋了，為何我的使用者所下載的檔案尾巴，都會留下一句：Error
所以，正確寫法應該是：
Try
Response.Clear()
'決定檔案存檔預設名稱
Response.AddHeader("Content-Disposition", "attachment;filename=" + Server.UrlEncode(strFileName))
Response.WriteFile(strFilePath)
Catch ex As Exception
Response.Write("Error")
Finally
Response.[End]()
End Try
把 Response.End() 放到 Finally 裡面
