public interface IReadOnlyCollection<TKey, TValue>
    {
        bool TryGetValue(TKey key, out TValue value);
    }
    public class MyCollection<TKey, TValue>
        : Dictionary<TKey, TValue>, IReadOnlyCollection<TKey, TValue>
    { }
    public class MyClass
    {
        private MyCollection<int, string> _myCollection;
        public IReadOnlyCollection<int, string> MyCollection
        {
            get
            {
                return _myCollection;
            }
        }
        private void PrivateMethod()
        {
            _myCollection.Add(1, "test");
        }
    }
    public class SomeClass
    {
        private void SomeMethod()
        {
            MyClass myClass = new MyClass();
            string s;
            myClass.MyCollection.TryGetValue(1, out s);
        }
    }