Question

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?

Answer 1

Use AsReadOnly() – see MSDN for details