open System

open FSharp.Core

​

type C = {

    Id: string

    Value: string

}

​

let c1 = [{ Id = ""; Value = ""}; { Id = ""; Value = ""}; { Id = ""; Value = ""}]

let c2 = [{ Id = ""; Value = ""}; { Id = ""; Value = ""}; { Id = ""; Value = ""}]

​

c1

    |> List.append c2

    |> List.mapFold 

        (fun state el -> 

            if not List.exists (fun x-> x.Id = el.Id) then state @ el

            else

                if el.Value = null then state

                else List.updateAt (List.findIndex (fun x -> x.Id = el.Id)) el state

        ) []