Do C# collections always enforce order?

Question

I.E. If I want to select from an array, is the resultant IEnumerable<object> object necessarily in order?

public class Student { public string FullName, ... }
public class School { public string Name, public Student[] Students, ... }

public void StudentListWork(School thisSchool)
{
    IEnumerable<string> StudentNames = thisSchool.Students.Select(student => student.FullName);

    // IS StudentNames GUARANTEED TO BE IN THE SAME ORDER AS thisSchool.Students?
}

Thanks!

What is Students? A list? a sorted list? an array? a dictionary? "collections" is a broad term, and they have different characteristics. — Erik Funkenbusch, Aug 14 '11 at 22:42
IEnumerable implementations of ordered collections do not go out of their way to return items in an un-ordered sequence. Not guaranteed, just horse sense. You can take a dependency on that, it won't be broken. — Hans Passant, Aug 14 '11 at 23:01

Jon Skeet · Accepted Answer · 2011-08-14T22:50:53.230

Yes, in this case:

Arrays returns items in the natural order
Enumerable.Select returns items in the order of the original sequence (after projection, of course)

Some collections do not retain order, however. In particular:

Collections such as HashSet<T> and Dictionary<TKey, TValue> make no guarantees about the order in which values are returned
Collections such as SortedSet<T> and SortedDictionary<TKey, TValue> apply guaranteed ordering based on the items placed within them

Do C# collections always enforce order?

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