Making a SortedList readonly

Question

I often have classes exposing lists as ReadOnlyCollection<T>s, i.e.

public class Class
{
  List<string> list;

  public ReadOnlyCollection<string> TheList
  {
    get { return list.AsReadOnly(); }
  }
}

What's the best way to do this for an IDictionary<T,U> such as a SortedList<string, string>?

Answer 1

public class ReadOnlyDictionary<TKey, TValue> : IDictionary<TKey, TValue>
{
    private readonly IDictionary<TKey, TValue> sourceDictionary;

    public ICollection<TKey> Keys
    {
        get { return sourceDictionary.Keys; }
    }

    public ICollection<TValue> Values
    {
        get { return sourceDictionary.Values; }
    }

    public TValue this[TKey key]
    {
        get { return sourceDictionary[key]; }
        set { throw new NotSupportedException(); }
    }

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

    public bool IsReadOnly
    {
        get { return true; }
    }

    public ReadOnlyDictionary(IDictionary<TKey, TValue> sourceDictionary)
    {
        AssertUtilities.ArgumentNotNull(sourceDictionary, "sourceDictionary");

        this.sourceDictionary = sourceDictionary;
    }

    void IDictionary<TKey, TValue>.Add(TKey key, TValue value)
    {
        throw new NotSupportedException();
    }

    public bool ContainsKey(TKey key)
    {
        return sourceDictionary.ContainsKey(key);
    }

    bool IDictionary<TKey, TValue>.Remove(TKey key)
    {
        throw new NotSupportedException();
    }

    public bool TryGetValue(TKey key, out TValue value)
    {
        return sourceDictionary.TryGetValue(key, out value);
    }

    void ICollection<KeyValuePair<TKey, TValue>>.Add(KeyValuePair<TKey, TValue> item)
    {
        throw new NotSupportedException();
    }

    void ICollection<KeyValuePair<TKey, TValue>>.Clear()
    {
        throw new NotSupportedException();
    }

    public bool Contains(KeyValuePair<TKey, TValue> item)
    {
        return sourceDictionary.Contains(item);
    }

    public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
    {
        sourceDictionary.CopyTo(array, arrayIndex);
    }

    bool ICollection<KeyValuePair<TKey, TValue>>.Remove(KeyValuePair<TKey, TValue> item)
    {
        throw new NotSupportedException();
    }

    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
    {
        return sourceDictionary.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return ((IEnumerable)sourceDictionary).GetEnumerator();
    }
}

[Edit] @Simon Buchan and @Cory Nelson pointed out that it is better to use implicit interface implementation for those methods that are not supported. Updated the code accordingly.