C# WebClient disable cache

Question

Good day.

I'm using the WebClient class in my C# application in order to download the same file every minute, and then the application performs a simple check to see if the file has been changed, and if it does do something with it.

Well since this file is downloaded every minute the WebClient caching system is caching the file, and not downloading the file again, just simply getting it from the cache, and that gets in the way of checking if the file downloaded is new.

So i would like to know how can disable the caching system of the WebClient class.

I've tried.

Client.CachePolicy = new System.Net.Cache.RequestCachePolicy(System.Net.Cache.RequestCacheLevel.BypassCache);

I also tried headers.

WebClient.Headers.Add("Cache-Control", "no-cache");

Didn't work as well. So how can i disable the cache for good?

Thanks.

EDIT

I also tried the following CacheLevels: NoCacheNoStore, BypassCache, Reload. No effect, however if i reboot my computer the cache seems to be cleared, but i can't be rebooting the computer every time.

Answer 1

Try NoCacheNoStore:

Never satisfies a request by using resources from the cache and does not cache resources. If the resource is present in the local cache, it is removed. This policy level indicates to intermediate caches that they should remove the resource. In the HTTP caching protocol, this is achieved using the no-cache cache control directive.

client.CachePolicy = new System.Net.Cache.RequestCachePolicy(System.Net.Cache.RequestCacheLevel.NoCacheNoStore); 

Answer 2

I Guess you will have to use webrequest/webresponse rather than webclient

    WebRequest request = WebRequest.Create(uri);
     // Define a cache policy for this request only. 
     HttpRequestCachePolicy noCachePolicy = new HttpRequestCachePolicy(HttpRequestCacheLevel.NoCacheNoStore);
     request.CachePolicy = noCachePolicy;
     WebResponse response = request.GetResponse();

//below is the function for downloading the file

   public static int DownloadFile(String remoteFilename,
                           String localFilename)
    {
        // Function will return the number of bytes processed
        // to the caller. Initialize to 0 here.
        int bytesProcessed = 0;

        // Assign values to these objects here so that they can
        // be referenced in the finally block
        Stream remoteStream = null;
        Stream localStream = null;
        WebResponse response = null;

        // Use a try/catch/finally block as both the WebRequest and Stream
        // classes throw exceptions upon error
        try
        {
            // Create a request for the specified remote file name
            WebRequest request = WebRequest.Create(remoteFilename);
            // Define a cache policy for this request only. 
            HttpRequestCachePolicy noCachePolicy = new HttpRequestCachePolicy(HttpRequestCacheLevel.NoCacheNoStore);
            request.CachePolicy = noCachePolicy;
            if (request != null)
            {
                // Send the request to the server and retrieve the
                // WebResponse object 
                response = request.GetResponse();

                if (response != null)
                {
                    if (response.IsFromCache)
                        //do what you want

                    // Once the WebResponse object has been retrieved,
                    // get the stream object associated with the response's data
                    remoteStream = response.GetResponseStream();

                    // Create the local file
                    localStream = File.Create(localFilename);

                    // Allocate a 1k buffer
                    byte[] buffer = new byte[1024];
                    int bytesRead;

                    // Simple do/while loop to read from stream until
                    // no bytes are returned
                    do
                    {
                        // Read data (up to 1k) from the stream
                        bytesRead = remoteStream.Read(buffer, 0, buffer.Length);

                        // Write the data to the local file
                        localStream.Write(buffer, 0, bytesRead);

                        // Increment total bytes processed
                        bytesProcessed += bytesRead;
                    } while (bytesRead > 0);
                }
            }
        }
        catch (Exception e)
        {
            Console.WriteLine(e.Message);
        }
        finally
        {
            // Close the response and streams objects here 
            // to make sure they're closed even if an exception
            // is thrown at some point
            if (response != null) response.Close();
            if (remoteStream != null) remoteStream.Close();
            if (localStream != null) localStream.Close();
        }

        // Return total bytes processed to caller.
        return bytesProcessed;
    }

Answer 3

client.CachePolicy = new RequestCachePolicy(RequestCacheLevel.BypassCache);

Should work. Just make sure you clear the cache and delete any temporary downloaded files in Internet Explorer before running the code as System.Net and IE both use the same cache.

Answer 4

In some scenarios, network debugging software can cause this issue. To make sure your url is not cached, you can append a random number as last parameter to make url unique. This random parameter in most cases is ignored by servers (which try to read parameters sent as name value pairs).

Example: http://www.someserver.com/?param1=val1&ThisIsRandom=RandomValue

Where ThisIsRandom=RandomValue is the new parameter added.

Answer 5

You could try appending some random number to your url as part of a querystring each time you download the file. This ensures that urls are unique each time.

For ex-

Random random = new Random();
string url = originalUrl + "?random=" + random.Next().ToString();
webclient.DownloadFile(url, downloadedfileurl);

Answer 6

From the above I would guess that you have problem somewhere else. Can you log http requests on server side? What do you get when you alter some random seed parameter?

Maybe SERVER caches the file (if the log shows that request is really triggered every minute.

Do you use ISA or SQUID?

What is http response code for your request?

I know that answering with answers might not be popular, but comment doesn't allow me this much text :)

EDIT:

Anyway, use HttpRequest object instead of WebClient, and hopefully (if you place your doubts in WebClient) everything will be solved. If it wasn't solved with HttpRequest, then the problem really IS somewhere else.

Further refinement:

Go even lower: http://stackoverflow.com/questions/2109695/how-do-i-create-an-http-request-manually-in-net

This is pure sockets, and if the problem still persists, then open a new question and tag it WTF :)