Freely convert between List<T> and IEnumerable<T>

List<string> myList = new List<string>();
IEnumerable<string> myEnumerable = myList;
List<string> listAgain = myEnumerable.ToList();

A List<T> is an IEnumerable<T>, so actually, there's no need to 'convert' a List<T> to an IEnumerable<T>. Since a List<T> is an IEnumerable<T>, you can simply assign a List<T> to a variable of type IEnumerable<T>.

The other way around, not every IEnumerable<T> is a List<T> offcourse, so then you'll have to call the ToList() member method of the IEnumerable<T>.

A List<T> is already an IEnumerable<T>, so you can run LINQ statements directly on your List<T> variable.

If you don't see the LINQ extension methods like OrderBy() I'm guessing it's because you don't have a using System.Linq directive in your source file.

You do need to convert the LINQ expression result back to a List<T> explicitly, though:

List<Customer> list = ...
list = list.OrderBy(customer => customer.Name).ToList()