web-services - webrequest.create(URL) 处的 NULL 引用异常

Question

我在该行中得到一个空引用异常webrequest.create(v_strURL)

Dim objHttpWebRequest As HttpWebRequest
        Dim objHttpWebResponse As HttpWebResponse
        Dim objRequestStream As Stream = Nothing
        Dim objResponseStream As Stream = Nothing

        Dim objXMLReader As XmlTextReader

        objHttpWebRequest = CType((WebRequest.Create(v_strURL)), HttpWebRequest) ''This line throws exception.

这Url是正确的，并且确实给出了正确的响应。