[<AbstractClass>]
type A () =
    member x.Guid with get, set
    member x.ExternalId with get, set

    override x.Equals(yobj) =
        match yobj with
        | :? A as y ->
            if (x.Guid = Guid.Empty || y.Guid = Guid.Empty) then
                // Lets look for ExternalId
                x.ExternalId = y.ExternalId
            else
                x.Guid = y.Guid
        | _ -> false

type B () =
    inherit A ()

    override x.Equals(yobj) =
        match yobj with
        | :? B as y ->
            if (x.Guid = Guid.Empty || y.Guid = Guid.Empty) then
                // Lets look for ExternalId
                x.ExternalId = y.ExternalId
            else
                x.Guid = y.Guid
        | _ -> false

type C () =
    inherit A ()

    override x.Equals(yobj) =
        match yobj with
        | :? C as y ->
            if (x.Guid = Guid.Empty || y.Guid = Guid.Empty) then
                // Lets look for ExternalId
                x.ExternalId = y.ExternalId
            else
                x.Guid = y.Guid
        | _ -> false