Combining two SyndicationFeeds

Question

What's a simple way to combine feed and feed2? I want the items from feed2 to be added to feed. Also I want to avoid duplicates as feed might already have items when a question is tagged with both WPF and Silverlight.

Uri feedUri = new Uri("http://stackoverflow.com/feeds/tag/silverlight");
XmlReader reader = XmlReader.Create(feedUri.AbsoluteUri);
SyndicationFeed feed = SyndicationFeed.Load(reader);    

Uri feed2Uri = new Uri("http://stackoverflow.com/feeds/tag/wpf");
XmlReader reader2 = XmlReader.Create(feed2Uri.AbsoluteUri);
SyndicationFeed feed2 = SyndicationFeed.Load(reader2);

Answer 1

If it's solely for stackoverflow, you can use this :
http://stackoverflow.com/feeds/tag/silverlight%20wpf
This will do an union of the two tags.

For a more general solution, I don't know. You'd probably have to manually iterate the elements of the two feeds and join them together. You can compare the <id> elements of <entry>s to see if they are duplicates.

Answer 2

Well, one possibility is to create a new syndication feed that is a clone of the first feed, and then simply iterate through each post on the second one, check the first for its existence, and add it if it doesn't exist.

Something along the lines of:

SyndicationFeed newFeed = feed.clone;
foreach(SyndicationItem item in feed2.items)
{
  if (!newFeed.contains(item))
    newFeed.items.Add(item);
}

might be able to do it. It looks like 'items' is a simple enumberable list of syndication items, so theres not reason you can't simply add them.

Answer 3

You can use LINQ to simplify the code to join two lists (don't forget to put System.Linq in your usings and if necessary reference System.Core in your project) Here's a Main that does the union and prints them to console (with proper cleanup of the Reader).

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.ServiceModel.Syndication;

namespace FeedUnion
{
    class Program
    {
        static void Main(string[] args)
        {
            Uri feedUri = new Uri("http://stackoverflow.com/feeds/tag/silverlight"); 
            SyndicationFeed feed;
            SyndicationFeed feed2;
            using(XmlReader reader = XmlReader.Create(feedUri.AbsoluteUri))
            {
                feed= SyndicationFeed.Load(reader); 
            }
            Uri feed2Uri = new Uri("http://stackoverflow.com/feeds/tag/wpf"); 
            using (XmlReader reader2 = XmlReader.Create(feed2Uri.AbsoluteUri))
            {
            feed2 = SyndicationFeed.Load(reader2);
            }
            SyndicationFeed feed3 = new SyndicationFeed(feed.Items.Union(feed2.Items));
            StringBuilder builder = new StringBuilder();
            using (XmlWriter writer = XmlWriter.Create(builder))
            {
                feed3.SaveAsRss20(writer);
                System.Console.Write(builder.ToString());
                System.Console.Read();
            }
        }
    }
}