Question

I am using an instance of WebClient in my Silverlight application to retrieve an image file for display. In order to service this application, I have set up a WCF REST service. The following is a snippet of code I wrote for that service:

DateTime modSince = WebOperationContext.Current.IncomingRequest.Headers[HttpRequestHeader.IfModifiedSince] 
    == null 
    ? DateTime.MinValue 
    : DateTime.Parse(WebOperationContext.Current.IncomingRequest.Headers[HttpRequestHeader.IfModifiedSince].ToString());

FileInfo fInfo = new FileInfo(filePath);

if (fInfo.LastWriteTime > modSince)
{
    WebOperationContext.Current.OutgoingResponse.ContentType = "image/jpeg";
    WebOperationContext.Current.OutgoingResponse.StatusCode = HttpStatusCode.OK;
    WebOperationContext.Current.OutgoingResponse.LastModified = fInfo.LastWriteTime;
}
else
{
    WebOperationContext.Current.OutgoingResponse.SuppressEntityBody = true;
    WebOperationContext.Current.OutgoingResponse.StatusCode = HttpStatusCode.NotModified;
}

The service is set up such that on a request for an image file, the if-modified-since attribute of the request header is examined. If this value is less than the last time the file is modified, the file is returned. Otherwise, a 304 message is returned.

We are interested in making the transaction faster. Since the images rarely, if ever, change it is desirable to look up the cache for a copy of the image before making a request to the server, which would save on network latency. In trying to implement this for IE, I have changed the Temporary Internet Files settings to “never check for newer versions of stored pages”. When I enter in the browser’s address bar the REST service URL for an image that has a copy stored in the cache, the copy is retrieved from the cache without the server being called. However, when the URL is entered as a parameter to the WebClient’s OpenReadAsync method a GET request is sent to the server, leading to a 304 message.

Is there any way I could use the WebClient to look up the cache without a GET request being sent to the server?

Answer 1

You can’t prevent the request from code in Silverlight. The “never check” setting of IE is only used in the absence of cache control directives from the server, I would not recommend you use it.

I’m not sure why you are using WCF Rest to deliver images to the client instead of simply placing the images in a folder on the server and fetching them as standard static content?

Either way the correct approach to the your problem is for the server to specify how the images should be cached by including a Cache-Control header in the response. Something like “Cache-Control: max-age=86400”.