C# What is the best way to compute a hash of an xml feed

Question

I want to detect if a feed has changed, the only way I can think of would be to hash the contents of the xml document and compare that to the last hash of the feed.

I am using XmlReader because SyndicationFeed uses it, so idealy I don't want to load the syndication feed unless the feed has been updated.

XmlReader reader = XmlReader.Create("http://www.extremetech.com/feed");
SyndicationFeed feed = SyndicationFeed.Load(reader);

Answer 1

A hash approach won't work in this case due to an XML comment added by some server side caching which constantly very frequently even when the actual feed never changes.

One thing you can do which works for this feed is use HTTP conditional requests to ask the server to give you the data only if its actually been modified since the last time you requested.

For example:

You'd have a global/member variable to hold the last modified datetime from your feed

    var lastModified = DateTime.MinValue;

Then each time you'd make a request like the following

    var request = (HttpWebRequest)WebRequest.Create( "http://www.extremetech.com/feed" );
    request.IfModifiedSince = lastModified; 
    try {

      using ( var response = (HttpWebResponse)request.GetResponse() ) {

        lastModified  = response.LastModified;

        using ( var stream = response.GetResponseStream() ) {

          //*** parsing the stream
          var reader = XmlReader.Create( stream );
          SyndicationFeed feed = SyndicationFeed.Load( reader );
          }
        }
      }
    catch ( WebException e ) {
      var response = (HttpWebResponse)e.Response;
      if ( response.StatusCode != HttpStatusCode.NotModified )
        throw; // rethrow an unexpected web exception
      }