Question

I ran across an unexpected problem with the following code.

List<string> items = new List<string>();
items = items.OrderBy(item => item);

This code generates the error:

Cannot implicitly convert type ‘System.Linq.IOrderedEnumerable’ to ‘System.Collections.Generic.List’. An explicit conversion exists (are you missing a cast?)

It appears I can change items to be of type IEnumerable<string> and the error goes away. But I need to be able to add items to the list, which IEnumerable doesn’t support.

Can someone help me understand this error, and what the easiest fix is? Is it safe to simply cast the result?

Answer 1

Why not just sort the list in place using the Sort() instance method; then you can add items to it later if you like:

List<string> items = GetSomeItems();
items.Sort();

Or, use an ordered collection like a binary search tree. SortedSet<T> might fit the bill, depending on your needs.

The solution suggested by the others:

items = items.OrderBy(item => item).ToList(); 

… creates another list with the original items in a new order. This is only useful if you need to preserve the original ordering for some other purpose; it’s rather more wasteful of memory than sorting the list in place.

As far as understanding the error, it’s simple: List<T> isn’t a subtype of IOrderedEnumerable<T>, so there’s no implicit reference conversion between the two. The explicit cast that the compiler suggests will satisfy the compiler, but it will fail at run time because the object returned by OrderBy<T> does not inherit from List<T>.

EDIT

An example of List<T>.Sort(Comparison<T>), assuming the type MyType has a Key property of some type type T where T : IComparable<T>:

List<MyType> items = GetSomeItems();
items.Sort((a, b) => a.Key.CompareTo(b.Key));