Accessing property values in F# constructors

Question

Sorry if this is basic, but I'm still wrapping my head around F#.

I want to do this:

type Person(user:User) as this =
 member val Name = "" with get, set
 user.UserId <- this.Name

But that gives me the error Unexpected identifier in member definition.

This compiles:

type Person(user:User) as this =
 do 
 user.UserId <- this.Name
 member val Name = "" with get, set

But gives me an InvalidOperationException at runtime.

I've settled for this:

type Person(user:User) =
 member val Name = "" with get, set
 member this.Setup() = 
 user.UserId <- this.Name

But I don't like having to call person.Setup() each time.

What's the best F-Sharpy way to do this?

Answer 1

You can do this with an explicit constructor to force the code for initializing the member functions to be run before your other code as follows

type User() = 
 member val UserId = "" with get,set 
type Person private () =
 member val Name = "" with get, set
 new (user:User) as this =
 Person()
 then user.UserId <- this.Name

This is due to the fact that the initializer code is run in the constructor, so you must force it to run to completion before acccessing the properties

• Ken Smith: Is there a way to do that and only allow the second constructor (passing in the User object) to be called? I.e., some way of making the first (explicit) constructor private?