gistfile1.ml

type O<'T>(v : 'T, isSet : bool) =
    struct
    static member op_Implicit(value) : O<'T> = O(value, true)
    member this.ValueOrDefault(defaultValue) = if isSet then v else defaultValue 
    end
module O = 
    let get (o : O<_>) v = o.ValueOrDefault v

type Optional = System.Runtime.InteropServices.OptionalAttribute
 
type Record = 
    {
        A : int
        B : string
    }
    with 
    member this.With ([<Optional>] a, [<Optional>]b) = 
        {
            A = O.get a this.A
            B = O.get b this.B
        }
    override this.ToString() = 
        sprintf "A=%d, B=%s" this.A this.B

gistfile2.cs

var a = new global::Program.Record(1, "2");
Console.WriteLine(a); // A = 1, B = 2
var b = a.With(b: "123");
Console.WriteLine(b); // A = 1, B = 123