I just started to learn Mono Develop

Installed Mono Develop 2.4.1 and trial version of Mono Touch.

my Code:

using System;
using System.Xml.Linq;
using System.Collections.Generic;


namespace RSSReader
{
public static class RSSRepository
{
    public static IList<FeedItem> GetFeeds(string url)
    {
        XDocument rssFeed = XDocument.Load(url);
        Console.Write(rssFeed.ToString());
        var feeds = new List<FeedItem>();

        try {
                var query  = from item in rssFeed.Descendants("item")
                     select new FeedItem
                     {
                        Title = item.Element("title").Value,
                        Published = DateTime.Parse(item.Element("pubDate").Value),
                        Url = item.Element("link").Value
                    };
            feeds = query.ToList();
        }
        catch (Exception ex){
            Console.WriteLine(ex.Message);
        }
        return feeds;
    }
}
}

This is throwing an error: An implementation of 'select' query expression pattern could not be found. Are you missing 'System.linq' using directive or 'System.Core.dll' assembly reference?

I got both references to System.Xml.Linq and System.Core

What am i missing ?