How to get around lack of covariance with IReadOnlyDictionary?

Question

I'm trying to expose a read-only dictionary that holds objects with a read-only interface. Internally, the dictionary is write-able, and so are the objects within (see below example code). My problem is that IReadOnlyDictionary doesn't support covariant conversions because of the reason outlined in the question here. This means I can't just expose my internal dictionary as a read only one.

So my question is, is there an efficient way to convert my internal dictionary to an IReadOnlyDictionary, or some other way to handle this? The options I can think of are:

1. Hold two internal dictionaries and keep them in sync.
2. Create a new dictionary when the property is accessed and cast all the objects within.
3. Cast the IReadOnly's back to NotReadOnly when using it internally.

1 seems like a pain, 2 seems highly inefficient. 3 sounds like the most promising at the moment, but is still ugly. Do I have any other options?

public class ExposesReadOnly
{
    private Dictionary<int, NotReadOnly> InternalDict { get; set; }
    public IReadOnlyDictionary<int, IReadOnly> PublicList
    {
        get
        {
            // This doesn't work...
            return this.InternalDict;
        }
    }

    // This class can be modified internally, but I don't want
    // to expose this functionality.
    private class NotReadOnly : IReadOnly
    {
        public string Name { get; set; }
    }
}

public interface IReadOnly
{
    string Name { get; }
}

Answer 1

You could write your own read-only wrapper for the dictionary, e.g.:

public class ReadOnlyDictionaryWrapper<TKey, TValue, TReadOnlyValue> : IReadOnlyDictionary<TKey, TReadOnlyValue> where TValue : TReadOnlyValue
{
    private IDictionary<TKey, TValue> _dictionary;

    public ReadOnlyDictionaryWrapper(IDictionary<TKey, TValue> dictionary)
    {
        if (dictionary == null) throw new ArgumentNullException("dictionary");
        _dictionary = dictionary;
    }
    public bool ContainsKey(TKey key) { return _dictionary.ContainsKey(key); }

    public IEnumerable<TKey> Keys { get { return _dictionary.Keys; } }

    public bool TryGetValue(TKey key, out TReadOnlyValue value)
    {
        TValue v;
        var result = _dictionary.TryGetValue(key, out v);
        value = v;
        return result;
    }

    public IEnumerable<TReadOnlyValue> Values { get { return _dictionary.Values.Cast<TReadOnlyValue>(); } }

    public TReadOnlyValue this[TKey key] { get { return _dictionary[key]; } }

    public int Count { get { return _dictionary.Count; } }

    public IEnumerator<KeyValuePair<TKey, TReadOnlyValue>> GetEnumerator()
    {
        return _dictionary
                    .Select(x => new KeyValuePair<TKey, TReadOnlyValue>(x.Key, x.Value))
                    .GetEnumerator();
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return this.GetEnumerator();
    }
}

Answer 2

I would suggest that you might want to define your own covariant interfaces, and include covariant access methods as well as a method which will create a read-only wrapper object which implements either IDictionary or IReadonlyDictionary with the desired types. Simply ignore IEnumerable<KeyValuePair<TKey,TValue>> within your interface.

Depending upon what you're doing, it may be helpful to define an IFetchByKey<out TValue> which is inherited by IFetchByKey<in TKey, out TValue>, with the former accepting queries for any type of object (given an object instance, a collection of Cat should be able to say whether it contains that instance, even if it's a type Dog or ToyotaPrius; the collection won't contain any instances of the latter types, and should be able to say so).

Answer 3

Maybe this solutions works for you:

public class ExposesReadOnly
{
    private IDictionary<int, IReadOnly> InternalDict { get; set; }
    public IReadOnlyDictionary<int, IReadOnly> PublicList
    {
        get
        {
            IReadOnlyDictionary<int, IReadOnly> dictionary = new ReadOnlyDictionary<int, IReadOnly>(InternalDict);

            return dictionary;
        }
    }

    private class NotReadOnly : IReadOnly
    {
        public string Name { get; set; }
    }

    public void AddSomeValue()
    {
        InternalDict = new Dictionary<int, NotReadOnly>();
        InternalDict.Add(1, new NotReadOnly() { Name = "SomeValue" });
    }
}

public interface IReadOnly
{
    string Name { get; }
}

class Program
{
    static void Main(string[] args)
    {
        ExposesReadOnly exposesReadOnly = new ExposesReadOnly();
        exposesReadOnly.AddSomeValue();

        Console.WriteLine(exposesReadOnly.PublicList[1].Name);
        Console.ReadLine();

        exposesReadOnly.PublicList[1].Name = "This is not possible!";
    }
}

Hope this helps!

Greets

Answer 4

Depending on your use case, you might be able to get away with exposing a Func<int,IReadOnly>.

public class ExposesReadOnly
{
    private Dictionary<int, NotReadOnly> InternalDict { get; set; }
    public Func<int,IReadOnly> PublicDictionaryAccess
    {
        get
        {
            return (x)=>this.InternalDict[x];
        }
    }

    // This class can be modified internally, but I don't want
    // to expose this functionality.
    private class NotReadOnly : IReadOnly
    {
        public string Name { get; set; }
    }
}

public interface IReadOnly
{
    string Name { get; }
}

Answer 5

Another approach for a specific lack of covariance:

A work around for a specific type of useful covariance on idictionary

public static class DictionaryExtensions
    {
        public static IReadOnlyDictionary<TKey, IEnumerable<TValue>> ToReadOnlyDictionary<TKey, TValue>(
            this IDictionary<TKey, List<TValue>> toWrap)
        {
            var intermediate = toWrap.ToDictionary(a => a.Key, a =>a.Value!=null? a.Value.ToArray().AsEnumerable():null);
            var wrapper = new ReadOnlyDictionary<TKey, IEnumerable<TValue>>(intermediate);
            return wrapper;
        }   
    }