HttpWebRequest和缓存策略

Question

我正在尝试在代码中使用HttpRequestCachePolicy来下载来自第三方服务的请求。我的代码目前是这样的：

    protected virtual XmlDocument Send(XmlDocument requestDoc)
    {
        // Get a SOAP request document that wraps the user request
        XmlDocument soapRequestDocument = NewSoapRequestDocument();
        InsertRequestDocumentIntoSoapRequestDocument(requestDoc, soapRequestDocument);

        Debug.WriteLine(soapRequestDocument.OuterXml);

        // Process the request
        HttpWebRequest webRequest = HttpWebRequest.Create(URI) as HttpWebRequest;
        var policy = new HttpRequestCachePolicy(HttpRequestCacheLevel.CacheIfAvailable);
        HttpWebRequest.DefaultCachePolicy = policy;

        webRequest.CachePolicy = policy;
        webRequest.Accept = "text/xml";
        webRequest.Method = "POST";
        webRequest.ContentType = "text/xml;charset=\"UTF-8\"";
        webRequest.Headers.Add("SOAPAction", string.Format("\"{0}#{1}\"", Service, Method));

        byte[] requestBytes = Encoding.UTF8.GetBytes(soapRequestDocument.OuterXml);
        webRequest.ContentLength = requestBytes.Length;
        Stream requestStream = webRequest.GetRequestStream();
        requestStream.Write(requestBytes, 0, requestBytes.Length);
        requestStream.Flush();
        requestStream.Close();

        // Process the response
        WebResponse webResponse = webRequest.GetResponse();
        Stream responseStream = webResponse.GetResponseStream();
        StreamReader responseStreamReader = new StreamReader(responseStream);

        // Create SOAP response document
        XmlDocument soapResponseDocument = new XmlDocument();
        soapResponseDocument.Load(responseStreamReader);
        responseStreamReader.Close();
        //close
        responseStream.Close();

        return GetResponseDocument(soapResponseDocument);
    }

但是，没有任何请求被缓存，我是否需要以不同方式使用它？