114

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 };
abatishchev
  • 98,240
  • 88
  • 296
  • 433
Mauricio Scheffer
  • 98,863
  • 23
  • 192
  • 275

3 Answers3

154

You can do it like this:

let p = new Person (Name = "John", BirthDate = DateTime.Now)
Maslow
  • 18,464
  • 20
  • 106
  • 193
Christian C. Salvadó
  • 807,428
  • 183
  • 922
  • 838
99

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.

Tomas Petricek
  • 240,744
  • 19
  • 378
  • 553
5

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

Vladimir Efimenko
  • 71
  • 2
  • 7