Linq rocks, except when it doesn’t. Can anyone spot the bug in this code?

Derik Whittaker — Fri, 21 Jan 2011 23:10:59 GMT

I am in the middle of building out some wcf endpoints to support the Dimecasts.net WP7 application I am going to publish. The code below is some of my main logic which helps grab the episodes. Because the data for Dimecasts is not very volatile I am going to do some basic (aka primitive) caching.

What I am looking to do is save the amount of data which needs to be gathered from the db. The intent of the code below is to get all data from cache which is greater then the last known episode which is cached on the phone. If data is found in the cache we will adjust the ‘last known episode’ in order to again reduce the db hit. However, there is a slight bug.

Who can spot the bug?

        public IList<EpisodeModel> FetchEpisodes( Int32 lastKnownEpisodeId )
        {
            var episodes = new List();
            var cachedEpisodes = Cache.FetchEpisodes()
                .Where( x => x.EpisodeNumber > lastKnownEpisodeId )
                .OrderBy( x => x.Id );

            if ( cachedEpisodes.Count() != 0 )
            {
                // reset the last known id in order to reset the reset
                lastKnownEpisodeId = cachedEpisodes.LastOrDefault().Id;

                episodes.AddRange( cachedEpisodes );
            }

            // more code which does not matter here.....

            return episodes;
        }

If you know the issue post it in the comments. If it is not ‘solved’ I will post an update with the solution and the reason.

Till next time,

Sorting List's the .Net 3.5 Way

Derik Whittaker — Sat, 29 Mar 2008 02:09:13 GMT

This post is another in the recent string of How-to's. But it is just so hard to resist. .Net 3.5 has so many cool features, it is just fun to explore them all.

Ok, so here is the issue we are going to solve today. We have a list of entities and we need to sort them. In this post I will show you 3 different ways to accomplish this task, all producing the same result, but using different techniques (hey Peter, go ahead and show me a 4th and better way :) ).

A little up front declarations. Each method will use the same list, so to save space I will declare this now.

List userList = new List
    {
        new User {FirstName = "Andy", LastName = "Tayler"},
        new User {FirstName = "Opie", LastName = "Tayler"},
        new User {FirstName = "Bee", LastName = "Tayler"},
        new User {FirstName = "Barney", LastName = "Fife"},
        new User {FirstName = "Goomer", LastName = "Pile"}
};

Sort Method 1 -- Using the Sort with IComarer

public void Sort_OldSchool()
{
    userList.Sort(new UserCompare());
}

// Sort class
private class UserCompare : IComparer<User>
 {
    public int Compare( User x, User y )
    {
    return x.LastName.CompareTo( y.LastName );
}

The above works, but requires you to create a custom class that implements the IComparer<User> interface to do the trick.

Sort Method 2 -- Using Lambda and extension methods

[Test]
public void Sort_UsingLambda()
{
	userList = userList.OrderBy( user => user.LastName ).ToList();
}

This way works great, it is clean and very readable.

Sort Method 3 -- Use Linq2Objects

[Test]
public void Sort_UsingLinq()
{
	userList = ( from u in userList
        	orderby u.LastName, u.FirstName
                select u ).ToList();
}

By using Linq you open yourself up to a whole new level of flexibility. You can not only sort (OrderBy), you can filter, return only a select number of rows, etc, etc. Basically you can do anything that Linq provides.

As you can see from the above, they all accomplish the same goal but method 2 & 3 are just more fun. Besides, if you are going to be using .Net 3.5 you might as well use the new features.

BTW, all the methods (except 3 since it sorts by LastName and FirstName) above produce the following output.
Barney Fife
Goomer Pile
Andy Tayler
Opie Tayler
Bee Tayler

Till next time,

Derik Whittaker : Linq, Development

Linq rocks, except when it doesn’t. Can anyone spot the bug in this code?

Sorting List's the .Net 3.5 Way