使用UserAgent在ASP.Net中检测ipad / iphone

Question

我正在尝试使用以下内容检测iphone / ipad等：

    Dim iphone As Boolean = False

    Dim userAgent = HttpContext.Current.Request.UserAgent.ToLower()
    ' iPhone is there
    If userAgent.Contains("iphone") Then
        iphone = True
    ElseIf userAgent.Contains("ipad") Then
        iphone = True
    ElseIf userAgent.Contains("itouch") Then
        iphone = True
    End If

但是，我注意到很多失败/例外：

Dim userAgent = HttpContext.Current.Request.UserAgent.ToLower()

任何人都可以告诉我可能导致异常的原因（详情如下）？

谢谢，Mark

System.NullReferenceException

Object reference not set to an instance of an object.

[System.NullReferenceException]
Inner Exception
 Exception Message: Object reference not set to an instance of an object.

   at book.Page_Load(Object sender, EventArgs e) in xxxxxxxxx\xxxx.aspx.vb:line 110
   at System.Web.UI.Control.LoadRecursive()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

 [System.Web.HttpUnhandledException]

 Exception Message: Exception of type 'System.Web.HttpUnhandledException' was thrown.

 at System.Web.UI.Page.HandleError(Exception e)
 at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
 at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
 at System.Web.UI.Page.ProcessRequest()
 at System.Web.UI.Page.ProcessRequest(HttpContext context)
 at ASP.book_aspx.ProcessRequest(HttpContext context)
 at     System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
 at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)