Caching of XML output in my HttpHandler don;t work for me “If-Modified-Since” is always null

Question

I have a custom HttpHandler which return XML. I want to add cache condition and return 304 when there is no changes but "If-Modified-Since" header never appear in my Request always is null can somebody help me.

public class ResourceHandler : IHttpHandler
{
  public void ProcessRequest(HttpContext context)
  {
        TimeSpan freshness = new TimeSpan(0, 0, 0, 3);
        context.Response.Cache.SetExpires(DateTime.Now.Add(freshness));
        context.Response.Cache.SetMaxAge(freshness);
        context.Response.Cache.SetCacheability(HttpCacheability.Private);


???? Always is NULL
    string rawIfModifiedSince = context.Request.Headers.Get("If-Modified-Since");
    if (string.IsNullOrEmpty(rawIfModifiedSince))
    {
      // Set Last Modified time
      context.Response.Cache.SetLastModified(XML last modified date);
    }
    else
    {
      DateTime ifModifiedSince = DateTime.Parse(rawIfModifiedSince);

      // HTTP does not provide milliseconds, so remove it from the comparison
      if (resource.LastModifiedDate.AddMilliseconds(
                  -resource.LastModifiedDate.Millisecond) >= ifModifiedSince)
      {
          // The requested file has not changed
          context.Response.StatusCode = 304;
          return;
      }
    }

    ….. XML output 
            XmlSerializer xr = new XmlSerializer(typeof(XElement));
            MemoryStream mr = new MemoryStream();

            XElement xml = OperatorXmlManager.Load(usr.SDEID, lang);


            xr.Serialize(mr, xml);
            byte[] OutXmlByte = mr.ToArray();

            context.Response.OutputStream.Write(OutXmlByte, 0, OutXmlByte.Length/4);
            context.Response.ContentType = "text/xml";

  }

  public bool IsReusable
  {
    get { return true; }
  }
}

Answer 1

According to RFC 2616 HTTP/1.1 section 14.25 :

To get best results when sending an If-Modified-Since header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header field whenever possible.

Hence, try to set Last-Modified header in you response.