Object initialization syntax

Question

I'm just starting out with F# and I can't find the syntax to do object initialization like in C# 3.

I.e. given this:

public class Person {
  public DateTime BirthDate { get; set; }
  public string Name { get; set; }
}

how do I write the following in F#:

var p = new Person { Name = "John", BirthDate = DateTime.Now };

Stepup

Answer 1

You can do it like this:

let p = new Person (Name = "John", BirthDate = DateTime.Now)

Older

By convention new is only used with disposable classes. – Leading

Answer 2

the answer from CMS is definitely correct. Here is just one addition that may be also helpful. In F#, you often want to write the type just using immutable properties. When using the "object initializer" syntax, the properties have to be mutable. An alternative in F# is to use named arguments, which gives you a similar syntax, but keeps things immutable:

type Person(name:string, ?birthDate) =
  member x.Name = name
  member x.BirthDate = defaultArg birthDate System.DateTime.MinValue

Now we can write:

let p1 = new Person(name="John", birthDate=DateTime.Now)
let p2 = new Person(name="John")

The code requires you to specify the name, but birthday is an optional argument with some default value.

Funds

Thanks Tomas, but in my case Person is in another assembly and I can't change it :( – Stepup

Answer 3

You can also omit the new keyword and use less verbose syntax:

let p = Person(BirthDate = DateTime.Now, Name = "John")

https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/members/constructors

Examination

new-less syntax looks quite better in F# – Dymphia