How can I get a KeyedCollection to be read-only?

Question

First let me explain why I'm using a KeyedCollection. I'm building a DLL and I have a list of items that I need to add to a collection and have them stay in the order I placed them but I also need to access them by both their index and by key (the key is a property of an object which I already defined). If there is any other simpler collection that does this, then please let me know.

Ok now, I need to be able to add items to this collection internally in the DLL but I need it to be publicly available to the DLL's end-user as read-only because I don't want them removing/altering the items I added.

I've searched all over this site, other sites, google in general and I have not been able to find a way to get some sort of read-only KeyedCollection. The closest I came was this page (http://www.koders.com/csharp/fid27249B31BFB645825BD9E0AFEA6A2CCDDAF5A382.aspx?s=keyedcollection#L28) but I couldn't quite get it to work.

UPDATE:

I took a look at those C5 classes. That, along with your other comments, helped me better understand how to create my own read-only class and it seems to work. However I have a problem when I try to cast the regular one to the read-only one. I get a compile-time cannot convert error. Here's the code I created (the first small class is what I originally had):

public class FieldCollection : KeyedCollection<string, Field>
{
    protected override string GetKeyForItem(Field field)
    {
        return field.Name;
    }
}

public class ReadOnlyFieldCollection : KeyedCollection<string, Field>
{
    protected override string GetKeyForItem(Field field)
    { return field.Name; }

    new public void Add(Field field)
    { throw new ReadOnlyCollectionException("This collection is read-only."); }

    new public void Clear()
    { throw new ReadOnlyCollectionException("This collection is read-only."); }

    new public void Insert(int index, Field field)
    { throw new ReadOnlyCollectionException("This collection is read-only."); }

    new public bool Remove(string key)
    { throw new ReadOnlyCollectionException("This collection is read-only."); }

    new public bool Remove(Field field)
    { throw new ReadOnlyCollectionException("This collection is read-only."); }

    new public bool RemoveAt(int index)
    { throw new ReadOnlyCollectionException("This collection is read-only."); }
}

If I have this variable defined:

private FieldCollection _fields;

then do this:

public ReadOnlyFieldCollection Fields;
Fields = (ReadOnlyFieldCollection)_fields;

it fails to compile. They're both inheriting from the same class, I thought they would be "compatible". How can I cast (or expose) the collection as the read-only type I just created?

Answer 1

I don't know of any built-in solution as well. This is an example of the suggestion I gave on the comment:

    public class LockedDictionary : Dictionary<string, string>
    {
        public override void Add(string key, string value)
        {
            //do nothing or log it somewhere
            //or simply change it to private
        }

        //and so on to Add* and Remove*

        public override string this[string i]
        {
            get
            {
                return base[i];
            }
            private set
            {
                //...
            }
        }
    }

You'll be able to iterate through the KeyValuePair list and everything else, but it won't be available for writing operations. Just be sure to type it according to your needs.

---

EDIT In order to have a sorted list, we can change the base class from Dictionary<T,T> to a Hashtable.

It would look like this:

    public class LockedKeyCollection : System.Collections.Hashtable
    {
        public override void Add(object key, object value)
        {
            //do nothing or throw exception?
        }

        //and so on to Add* and Remove*

        public override object this[object i]
        {
            get
            {
                return base[i];
            }
            set
            {
                //do nothing or throw exception?
            }
        }
    }

Usage:

        LockedKeyCollection aLockedList = new LockedKeyCollection();

        foreach (System.Collections.DictionaryEntry entry in aLockedList)
        {
            //entry.Key
            //entry.Value
        }

Unfortunately, we can't change the access modifier of the methods, but we can override them to do nothing.