File Upload in ASP.Net

Private Sub Upload_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim chk As Boolean
        Dim fileString() As String
        Dim a As String
        chk = checkfile()
        If chk Then
            Dim filename As String = fileUpload.PostedFile.FileName
            fileString = filename.Split("\")
            a = fileString(UBound(fileString))
            Dim d As String = DateTime.Now
            Dim i As Integer = Application("filevar")
            i = i + 1
            fileUpload.PostedFile.SaveAs(Server.MapPath(i & a))
            Application("filevar") = i
        End If
    End Sub
    Public Function checkfile() As Boolean
        If fileUpload.PostedFile.FileName = "" Then Return False
        If fileUpload.PostedFile.ContentLength = 0 Then Return False
        Return True
    End Function

Global.asax

    Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
        Application("filevar") = 1
    End Sub