Implementing dictionary subclass in F#

Question

I'm new to the .Net world and I'm trying to figure out how inheritance and interfaces work exactly. I am trying to implement a Dictionary<string,string> that keeps all the keys as upper case strings. In F# my first naive stab was

type UpperDictionary1() = 
    inherit Dictionary<string, string>()
    override this.Add (key: string, value : string) =
        base.Add(key.ToUpper(), value)

That doesn't work because Dictionary Add is sealed. However dropping the override (and shadowing Add instead?) works:

type UpperDictionary2() = 
    inherit Dictionary<string, string>()
    member this.Add (key: string, value : string) = 
        base.Add(key.ToUpper(), value)

What is the quickest and most appropriate way to implement a special form of dictionary class?

Also, does this relate to Google's ForwardingSet in Java. Are forwarding sets simply an easier way to add this type of functionality or am I missing the point completely?

Answer 1

I think what you want here is to implement the IDictionary interface. This will allow you to interact with your type as an IDictionary (Add, Remove, etc.) and you can just forward most of the calls to a private Dictionary member.

Check this out, too.

Answer 2

I really don't see implementing all of this interface as best practice and I highly doubt it's all correct even though it compiles. I'd suggest just making your own interface.

type UpperDictionary() = 
    let dictionary = new Dictionary<String, String>()

    interface IDictionary<string, string> with
        member this.Add(key: string, value : string) = dictionary.Add(key.ToUpper(), value)
        member this.Add(kvp) = dictionary.Add(kvp.Key.ToUpper(), kvp.Value)
        member this.ContainsKey(key) = dictionary.ContainsKey(key.ToUpper())
        member this.Contains(kvp) = dictionary.TryGetValue(kvp.Key.ToUpper(), ref kvp.Value)
        member this.Item with get key = dictionary.Item(key) and set key value = dictionary.Item(key) <- value
        member this.Count with get() = dictionary.Count
        member this.IsReadOnly with get() = false
        member this.Keys = dictionary.Keys :> ICollection<String>
        member this.Remove key = dictionary.Remove(key)
        member this.Remove(kvp : KeyValuePair<String,String>) = dictionary.Remove(kvp.Key.ToUpper())
        member this.TryGetValue(key, value) = dictionary.TryGetValue(key, ref value)
        member this.Values = dictionary.Values :> ICollection<String>
        member this.Clear() = dictionary.Clear()
        member this.CopyTo(array, arrayIndex) = (dictionary :> IDictionary<string, string>).CopyTo(array, arrayIndex)
        member this.GetEnumerator() = dictionary.GetEnumerator() :> IEnumerator
        member this.GetEnumerator() = dictionary.GetEnumerator() :> IEnumerator<KeyValuePair<String,String>>