Normally when I need to have a list of ints/strings/etc. I create a list like:
var list = new List<string> And then I create a hashtable that contains all the strings, and I don't insert into the list unless it isn't in the hashtable i.e. to enforce unique items in the list.
Is there a datatype that can satisfy both of these requirements for me?
There is. Use HashSet:
var set = new HashSet<int>(); set.Add(4); set.Add(4); // there is already such an element in the set, no new elements added Keep in mind, though, that it does not guarantee you the order of elements.
Do you just mean HashSet<string> ?
All elements in a HashSet<T> are unique; the Add() method returns a bool to indicate if a new item was actually added, or whether it was a no-op.
Is there a datatype that can satisfy both of these requirements for me?
No. A hashtable will provide you a direct access to an element given its unique key, whereas in a list you don't need a key and you could definitely have duplicates.
If you are after a set of unique values only (and don't subsequently care about ordering) then you should look at a HashSet<T>
Technically, there is System.Collections.Specialized.OrderedDictionary. However, this is an old non-updated (non-generic) class and I would generally recommend avoiding it ;-)
Represents a collection of key/value pairs that are accessible by the key or index.
In practice I would create a minimal wrapper class that exposes the required operations. (I would likely use a HashSet<T> (for existence) and a List<T> (for ordering), although just a single List<T> is far than sufficient for a relatively small n in most cases -- remember Big-O is about limits.)
Happy coding.
HashSet<string> set = new HashSet<string>(); bool inserted = set.Add("Item"); bool insertedDuplicate = set.Add("Item"); inserted.Dump("Result1"); insertedDuplicate.Dump("Result2"); //Result //Result1 = true //Result2 = false You can run this in LinqPad to see the functionality and how it works.
