Properly exposing a List<T>?

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

Exposing a List<T> as a property isn't actually the root of all evil; especially if it allows expected usage such as foo.Items.Add(...).

You could write a cast-safe alternative to AsEnumerable():

public static IEnumerable<T> AsSafeEnumerable<T>(this IEnumerable<T> data) {
    foreach(T item in data) yield return item;
}

But your biggest problem at the moment is thread safety. As a static member, you might have big problems here, especially if it is in something like ASP.NET. Even ReadOnlyCollection over an existing list would suffer from this:

        List<int> ints = new List<int> { 1, 2, 3 };
        var ro = ints.AsReadOnly();
        Console.WriteLine(ro.Count); // 3
        ints.Add(4);
        Console.WriteLine(ro.Count); // 4

So simply wrapping with AsReadOnly is not enough to make your object thread-safe; it merely protects against the consumer adding data (but they could still be enumerating it while your other thread adds data, unless you synchronize or make copies).

Answer 2

Yes and No. Yes, there is a performance overhead, because a new object is created. No, your list is not cloned, it is wrapped by the ReadOnlyCollection.

Answer 3

If the class has no other purpose you could inherit from list and override the add method and have it throw an exception.