Remain path of FileUpload control after postback

Question

I have a FileUpload control and a DropDownlist control in an UpdatePanel and when user select a file for the FileUpload control (no upload yet), in the meanwhile the user select an option from the DropDownList control which will cause a postback! Once the page postback, the path selected in the FileUpload control will gone. How can i remain the path in the FileUpload control? File uploading function was working. I hope can remain the path in the FileUpload control during postback.

I have tried the solution below but the "FileUpload1.HasFile" will return false to me.

            If Session("FileUpload1") Is Nothing AndAlso Upload.HasFile Then
                Session("FileUpload1") = Upload
                lblPhotoUploadErr.Text = Upload.FileName
            ElseIf Session("FileUpload1") IsNot Nothing AndAlso (Not Upload.HasFile) Then
                Upload = DirectCast(Session("FileUpload1"), FileUpload)
                lblPhotoUploadErr.Text = Upload.FileName
            ElseIf Upload.HasFile Then
                Session("FileUpload1") = Upload
                lblPhotoUploadErr.Text = Upload.FileName
            End If

but the "Upload.HasFile" in the uploading function below will true when it was executed.

Public Sub uploadPhoto()
    Dim FileOK As Boolean = False
    Dim FileSaved As Boolean = False
    Dim CandidateCode As String = Nothing
    Dim newFileName As String = Nothing

    Dim extension As String = Nothing
    Dim fileNameWithoutExt As String = Nothing

    If txtCandidateCode.Text.Trim <> "" Then
        CandidateCode = txtCandidateCode.Text.Trim
    End If

    If Upload.HasFile Then
        Dim FileExtension As String = Path.GetExtension(Upload.FileName).ToLower
        Dim allowedExtensions() As String = {".png", ".jpeg", ".jpg", ".gif"}

        Dim i As Integer = 0
        Do While (i < allowedExtensions.Length)
            If (FileExtension = allowedExtensions(i)) Then
                FileOK = True
            End If
            i = (i + 1)
        Loop
    End If

    If FileOK Then
        Try
            fileNameWithoutExt = Path.GetFileNameWithoutExtension(Upload.FileName)
            extension = Path.GetExtension(Upload.FileName)
            newFileName = fileNameWithoutExt + "_" + CandidateCode + extension

            Upload.PostedFile.SaveAs((path1 + newFileName))
            FileSaved = True
        Catch ex As Exception
            lblPhotoUploadErr.Text = ("File could not be uploaded." + ex.Message.ToString)
            FileSaved = False
        End Try
    Else
        lblPhotoUploadErr.Text = "Cannot accept files of this type."
    End If

    If FileSaved Then
        pnlUpload.Visible = False
        imgPhoto.ImageUrl = ("~/images/" + newFileName)
        hfPhotoUploadPath.Value = ("~/images/" + newFileName)

        hfFileExtension.Value = extension
        hfPhotoUploadFileName.Value = fileNameWithoutExt
    End If
End Sub

Answer 1

The FileUpload will only keep it's value if you take it out of the UpdatePanel. That way you can still do everything with the DropDownList and its AutoPostBack but the ajax-postback won't refresh the FileUpload causing it to become empty. This way you don't need the postbacktriggers any more.

Put the UpdatePanel only around the DropDownList and any controls the postback has to change. If these controls are not next to each other you can use multiple UpdatePanels, the AutoPostBack will refresh all of them (default behavior, you can even change that).