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I know I shouldn't be exposing a List<T> in a property, but I wonder what the proper way to do it is? For example, doing this: public static class Class1 { private readonly static List<string> _list; public static IEnumerable<string> List { get { return _list; //return _list.AsEnumerable<string>(); behaves the same } } static Class1() { _list = new List<string>(); _list.Add("One"); _list.Add("Two"); _list.Add("Three"); } } would allow my caller to simply cast back to List<T> : private void button1_Click(object sender, EventArgs e) { var test = Class1.List as List<string>; test.Add("Four"); // This really modifies Class1._list, which is bad™ } So if I want a really immutable List<T> would I always have to create a new list? For example, this seems to work (test is null after the cast): public static IEnumerable<string> List { get { return new ReadOnlyCollection<string>(_list); } } But I'm worried if there is a performance overhead as my list is cloned every time someone tries to access it?

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