I have been learning about collection in .NET, and I have come across the terms, Ordered and Unordered. I do not know what the difference is between them. Could someone please explain what exactly is the difference, and when one is preferred over the other.

CodePudding user response：

In mathematical terms, an ordered collection is like a sequence, an unordered collection is like a set.

In practical terms, you use an ordered collection if, well... order matters, and an unordered collection if it doesn't.

As a concrete example:

• I want to store which file extensions are fine for my file uploading service. I'd use an unordered collection, since I only care whether an entry is in the collection or not. I don't care about the order.

  var imageExtensions = new HashSet<string>() { "gif", "jpg", "png" };
  
  // I might just as well have written { "jpg", "png", "gif" } -- it doesn't matter
  

• I want to store the words of a sentence. The sentence might have duplicate words, and I might want to refer to the "third" word by index number. Here, I use an ordered collection.

  var words = new List<string>() { "one", "small", "step", "for", "man", "one", "giant", "leap", "for", "mankind" };
  

In the .NET base class library, unordered collections are usually optimized for efficiently finding elements of the collection. For example, HashSet<T>.Contains is an O(1) operation. On the other hand, finding an element in a List<T> requires traversing the list and is, thus, an O(n) operation.