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When I want to make a value type read-only outside of my class I do this:

public class myClassInt
    private int m_i;
    public int i {
        get { return m_i; }
    public myClassInt(int i)
        m_i = i;

What can I do to make a List<T> type readonly (so they can't add/remove elements to/from it) outside of my class? Now I just declare it public:

public class myClassList
    public List<int> li;
    public  myClassList()
        li = new List<int>();

Best Answer:

There is limited value in attempting to hide information to such an extent. The type of the property should tell users what they're allowed to do with it. If a user decides they want to abuse your API, they will find a way. Blocking them from casting doesn't stop them:

public static class Circumventions
    public static IList<T> AsWritable<T>(this IEnumerable<T> source)
        return source.GetType()
            .GetFields(BindingFlags.Public |
                       BindingFlags.NonPublic | 
            .Select(f => f.GetValue(source))

With that one method, we can circumvent the three answers given on this question so far:

List<int> a = new List<int> {1, 2, 3, 4, 5};
IList<int> b = a.AsReadOnly(); // block modification...
IList<int> c = b.AsWritable(); // ... but unblock it again
Debug.Assert(a.Count == 6); // we've modified the original
IEnumerable<int> d = a.Select(x => x); // okay, try this...
IList<int> e = d.AsWritable(); // no, can still get round it
Debug.Assert(a.Count == 7); // modified original again


public static class AlexeyR
    public static IEnumerable<T> AsReallyReadOnly<T>(this IEnumerable<T> source)
        foreach (T t in source) yield return t;
IEnumerable<int> f = a.AsReallyReadOnly(); // really?
IList<int> g = f.AsWritable(); // apparently not!
Debug.Assert(a.Count == 8); // modified original again

To reiterate... this kind of "arms race" can go on for as long as you like!

The only way to stop this is to completely break the link with the source list, which means you have to make a complete copy of the original list. This is what the BCL does when it returns arrays. The downside of this is that you are imposing a potentially large cost on 99.9% of your users every time they want readonly access to some data, because you are worried about the hackery of 00.1% of users.

Or you could just refuse to support uses of your API that circumvent the static type system.

If you want a property to return a read-only list with random access, return something that implements:

public interface IReadOnlyList<T> : IEnumerable<T>
    int Count { get; }
    T this[int index] { get; }

If (as is much more common) it only needs to be enumerable sequentially, just return IEnumerable:

public class MyClassList
    private List<int> li = new List<int> { 1, 2, 3 };
    public IEnumerable<int> MyList
        get { return li; }

UPDATE Since I wrote this answer, C# 4.0 came out, so the above IReadOnlyList interface can take advantage of covariance:

public interface IReadOnlyList<out T>

And now .NET 4.5 has arrived and it has... guess what...

IReadOnlyList interface

So if you want to create a self-documenting API with a property that holds a read-only list, the answer is in the framework.

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