使用 httpHandler(.ashx) 從特定位置下載檔案

在 ASP.NET 專案中建立一個新的 httpHandler。將以下程式碼（VB）應用於處理程式檔案：

Public Class AttachmentDownload
    Implements System.Web.IHttpHandler

    Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest

        ' pass an ID through the query string to append a unique identifer to your downloadable fileName

        Dim fileUniqueId As Integer = CInt(context.Request.QueryString("id"))

        ' file path could also be something like "C:\FolderName\FilesForUserToDownload

        Dim filePath As String = "\\ServerName\FolderName\FilesForUserToDownload"
        Dim fileName As String = "UserWillDownloadThisFile_" & fileUniqueId
        Dim fullFilePath = filePath & "\" & fileName
        Dim byteArray() As Byte = File.ReadAllBytes(fullFilePath)

        ' promt the user to download the file

        context.Response.Clear()
        context.Response.ContentType = "application/x-please-download-me" ' "application/x-unknown"
        context.Response.AppendHeader("Content-Disposition", "attachment; filename=" & fileName)
        context.Response.BinaryWrite(byteArray)
        context.Response.Flush()
        context.Response.Close()
        byteArray = Nothing

    End Sub

    ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
        Get
            Return False
        End Get
    End Property

End Class

你可以從後面的程式碼或客戶端語言呼叫處理程式。在這個例子中，我使用的是一個呼叫處理程式的 javascript。

function openAttachmentDownloadHandler(fileId) {

    // the location of your handler, and query strings to be passed to it

    var url = "..\\_Handlers\\AttachmentDownload.ashx?";
    url = url + "id=" + fileId;

    // opening the handler will run its code, and it will close automatically
    // when it is finished. 

    window.open(url);

} 

現在附加將 javascript 函式分配給 Web 表單中可單擊元素上的按鈕單擊事件。例如：

<asp:LinkButton ID="lbtnDownloadFile" runat="server" OnClientClick="openAttachmentDownloadHandler(20);">Download A File</asp:LinkButton>

或者你也可以從後面的程式碼中呼叫 javascript 函式：

ScriptManager.RegisterStartupScript(Page,
                Page.GetType(),
                "openAttachmentDownloadHandler",
                "openAttachmentDownloadHandler(" & fileId & ");",
                True)

現在，當你單擊按鈕時，httpHandler 會將你的檔案傳送到瀏覽器並詢問使用者是否要下載它。