List readonly with a private set

Question

How can I expose a List so that it is readonly, but can be set privately?

This doesn\'t work:

public List myList          


        

Answer 1

private List<string> myList;

public string this[int i]
{
    get { return myList[i]; }
    set { myList[i] = value; }
}

Answer 2

Why are you using a List. Sounds like what you really want to expose is IEnumerable

public IEnumerable<string> myList { get; private set; }

Now users of the class can read the items but not chagnge the list.

Answer 3

I didn't see this option mentioned yet:

private List<string> myList;

public List<string> MyList
{
    get { return myList.AsReadOnly().ToList(); }
}

This should allow you to expose a read-only List.

Answer 4

I think you are mixing concepts.

public List<string> myList {get; private set;}

is already "read-only". That is, outside this class, nothing can set myList to a different instance of List<string>

However, if you want a readonly list as in "I don't want people to be able to modify the list contents", then you need to expose a ReadOnlyCollection<string>. You can do this via:

private List<string> actualList = new List<string>();
public ReadOnlyCollection<string> myList
{
  get{ return actualList.AsReadOnly();}
}

Note that in the first code snippet, others can manipulate the List, but can not change what list you have. In the second snippet, others will get a read-only list that they can not modify.

Answer 5

private List<string> my_list;

public ReadOnlyCollection<string> myList
{
    get { return my_list.AsReadOnly(); }
    private set { my_list = value; }
}

Answer 6

A bit late, but nevertheless: I don't like using the ReadOnlyCollection wrapper because it still exposes all the methods for modifying the collection, which all throw a NotSupportedException when accessed in run-time. In other words, it implements the IList interface, but then violates this same contract in run-time.

To express that I am really exposing an immutable list, I usually resort to a custom IIndexable interface, which adds Length and an indexer to an IEnumerable (described in this CodeProject article). It is a wrapper as it should have been done in the first place IMHO.