How do you Implement an interface in F#

Question

I have an interface in C# like this:

public interface IMessageOptions
{
    int ReceiveTimeout { get; set; }
    int PeekTimeout { get; set; }
    bool Transactional { get; set; }
}

and I'm trying to implement it in F# like this:

type Settings() =
    interface IMessageOptions with
        member this.PeekTimeout: int with get, set
        member this.ReceiveTimeout: int with get, set // error at start of this line
        member this.Transactional: bool with get, set

But I get an error after the first property saying:

"Incomplete structured construct at or before this point in pattern"

How should this be written?

Are auto properties not allowed in this context?

Answer 1

Here is the syntax for that:

type Settings() =
   interface IMessageOptions with
      member val PeekTimeout = 0 with get, set
      member val ReceiveTimeout = 0 with get, set
      member val Transactional = false with get, set

The differences are:

• Use val instead of this.
• Define the default values explicitly
• You don't have to specify the types (although you can)

For the reference, here is how you would define the interface itself in F#:

type IMessageOptions =
  abstract member PeekTimeout : int with get, set
  abstract member ReceiveTimeout : int with get, set
  abstract member Transactional : bool with get, set

Answer 2

If you want automatically implemented properties, then this is the syntax:

type Settings() =
    interface IMessageOptions with
        member val PeekTimeout : int = 0 with get, set
        member val ReceiveTimeout : int = 0 with get, set
        member val Transactional : bool = false with get, set

Notice that you explicitly have to define the default value.