Say you have an IEnumerable<T> of some sort and you want to check if it contains any duplicates. How do you do that?
Terrible solution
I needed to do this a while ago and the first solution that hit me wasn’t exactly good. Linq has method called Distinct, which returns the distinct items in a sequence (weeds out duplicates). Using that to check if duplicates exists you can do something like this:
subjects.Count() != subjects.Distinct().Count();
This is of course a really bad solution. So don’t use it! 
Good solution
The much better solution is to use a HashSet<T>.
The HashSet<T> class provides high performance set operations. A set is a collection that contains no duplicate elements, and whose elements are in no particular order.
In addition to the fact that it cannot contain duplicate items, it also has a very nice add-method which returns false if the item is already in the set.
To check for duplicates using a hash set we can start out by assuming there is no duplicates and then add items to the hash set until we either run out of items, or we try to add an item that already exists. In the first case our assumption was right and in the second it turned out to be wrong.
var hasDuplicates = false;
foreach (var s in subjects)
if (!set.Add(s))
{
hasDuplicates = true;
break;
}
Good and handy solution
Since we don’t want to type in that stuff over and over again we want this snippet in a nice extension method. Code snippet coming up
namespace Geekality.Snippets
{
public static class EnumerableExtensions
{
public static bool HasDuplicates<T>(this IEnumerable<T> subjects)
{
return HasDuplicates(subjects, EqualityComparer<T>.Default);
}
public static bool HasDuplicates<T>(this IEnumerable<T> subjects, IEqualityComparer<T> comparer)
{
if(subjects == null)
throw new ArgumentNullException("subjects");
if(comparer == null)
throw new ArgumentNullException("comparer");
var set = new HashSet<T>(comparer);
foreach (var s in subjects)
if (!set.Add(s))
return true;
return false;
}
}
}
Simple and handy. You use it for example like this:
{
// Do something
}
Hope it can help someone, somewhere, below and/or over the rainbow
Let me know what you think. I’m always open for improvements and feedback
I earlier wrote about the book, 
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and 
.
, is the first term to contain three digits.
Greetings! I am Torleif Berger, 24 years old, a Seventh-Day Adventist and currently working as a software developer. Otherwise, not much to tell. Although I do juggle a bit...
Interlude
Last Friday (January, 29th) I had my last day at SMS Development & Support AS where I have been working for the last 1.5 years. It was my very first job as a full-time software developer and I learned a lot. Both about developing software and about myself as a developer. Good stuff!
Next up I will return to LifeStyleTV where I spent a year as a Media Missionary. This time I will be working there as a regular employee and help out with various things. Mainly I will be developing some internal software systems with some of the others there, but I will most likely also help out with the TV production. I am going down there the 11th of February and I am looking forward to get started.
What am I doing now, during this interlude? Not a lot. Very little. Extremely little. My computer is dead. I’m sick. My energy reserves are non-existent. I’m sleeping, eating, watching tv, reading… and that’s pretty much it… To be honest I am actually kind of happy I am sick now instead of earlier or later. Just hope I will be back to normal before Thursday…