Best way for constructing a unique list of objects in C#

Question

I'm wondering whether it will be quicker to follow one pattern or another for constructing a unique list of objects in C#:

Option 1

• Add all the items into a generic list
• Call the list.Distinct function on it

Option 2

• Iterate over each item
• Check whether the item already exists in the list and if not add it

Pyrazole

Answer 1

You can use HashSet<T>:

The HashSet class provides high-performance set operations. A set is a collection that contains no duplicate elements, and whose elements are in no particular order.

You can provide custom IEqualityComparer<T> via constructor.

Throaty

Thanks, though it's a quote from MSDN. – Throaty

A HashSet<T> isn't actually a list -- it doesn't preserve order and doesn't provide indexed access. (Not that the OP seems to be aware of the distinction, either.) – Viddah

@DavidMoles indeed HashSet<T> isn't a List<T>. It seems that OP simply wasn't aware of HashSet, so this is why I've proposed it as a solution. Because of this, I think, that we can assume that word "list" (in this case) means something being a collection of items. – Throaty

Answer 2

This is one of those "should I use a shoe or a brick to pound a nail into the wood" questions. You should use the appropriate data structure for the job, which based on your requirement of "constructing a unique list of objects", the HashSet<T> class satisfies.

If you require the items in list format, you can always call ToList() on the set.

Priapitis

Actually it is a Shoe or glass bottle. – Unscientific

A HashSet<T> isn't actually a list -- it doesn't preserve order and doesn't provide indexed access. (Not that the OP seems to be aware of the distinction, either.) – Viddah

Answer 3

If you are concerned about the performance of looking up unique items, use a Dictionary<TKey, TVale>. Also, a dictionary requires unique keys, so you will never have duplicates.

Indoor

Based on the description he doesn't have a key-value relationship, so instead of a Map (one implementation in C# is a Dictionary) he needs a Set (such as a HashSet). – Lugsail

Indeed, I forgot about Hashset. – Indoor