CODEWITHSUNDEEP
f#
Alexander Zeitler
Alexander Zeitler

Reputation: 13089

Access values of implemented interface members in function

I have this interface:

interface IHaveHeaders {
  Dictionary<string, object>? Headers { get; set; }
  void SetHeader(string key, object value);
  object? GetHeader(string key);
}

I'm trying to implement it in F# and access the Headers Dictionary in the SetHeader and GetHeader functions but I don't know how to access the Headers member from the functions:

type HeadersImpl =
  interface IHaveHeaders with
  member val Headers = Dictionary<string, obj>() with get, set
  member this.SetHeader (key, value) = () // how do I add a new `Headers` entry?
  member this.GetHeader (key) = null // how do I get an entry from `Headers`

Upvotes: 1

Views: 65

Answers (2)

Tomas Petricek
Tomas Petricek

Reputation: 243051

The answer from Philip will certainly do the trick. An alternative approach would be to not use automatic property for Headers, but instead define a local field explicitly. This makes your property definition a bit longer, but it makes the access code a lot simpler:

type HeadersImpl() =
  let mutable headers = Dictionary<string, obj>()
  interface IHaveHeaders with
    member this.Headers with get() = headers and set(v) = headers <- v
    member this.SetHeader (key, value) = headers.Add(key, value)
    member this.GetHeader (key) = headers.[key]

Upvotes: 4

Phillip Carter
Phillip Carter

Reputation: 5005

Since F# doesn't support implicit upcasting as of this answer, you need to explicitly upcast to the interface type on this.

type HeadersImpl =
  interface IHaveHeaders with
      member val Headers = Dictionary<string, obj>() with get, set
      member this.SetHeader (key, value) =
          let headers = (this :> IHaveHeaders).Headers
          headers.Add(key, value)
      member this.GetHeader (key) =
          let headers = (this :> IHaveHeaders).Headers
          headers.[key]

Upvotes: 4

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