Circular reference and constructors

Question

I'm trying to build an Attribute that validates a certain instance of a type.

In order to do this I have to cast the ObjectInstance to that type.

And I need to set the attribute on the member of that type.

So we need to resort to the and keyword for the circular definition.

However in the following case I get the error that

A custom attribute must invoke an object constructor

On the line marked below.

namespace Test

open System
open System.ComponentModel.DataAnnotations

[<AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)>]
type MyAttribute() =
    class
    inherit ValidationAttribute ()

    override this.IsValid (value: Object, validationContext: ValidationContext) =
        match validationContext.ObjectInstance with
        | :? MyClass as item ->
            // TODO more validation
            ValidationResult.Success
        | _ ->
            new ValidationResult("No no no")
    end
and MyClass(someValue) =
    [<Required>]
    [<Range(1, 7)>]
  //vvvvvvvvvvvvvvv
    [<MyAttribute>]
  //^^^^^^^^^^^^^^^
    member this.SomeValue : int = someValue

I tried manually invoking the constructor, such as:

[<MyAttribute()>]
// or
[<new MyAttribute()>]

But none of them are accepted by the system.

Can an F# guru help me out here?

Answer 1

One solution would be to first describe your types in a signature files.

Since the attribute is specified in the signature file, you don't need to add it again in the implementation file:

Foo.fsi:

namespace Foo

open System

[<AttributeUsage(AttributeTargets.Property)>]
type MyAttribute =
    inherit System.Attribute

    new : unit -> MyAttribute

    member Foo : unit -> MyClass

and MyClass =
    new : someValue : int -> MyClass

    [<MyAttribute()>]
    member SomeValue : int

Foo.fs:

namespace Foo

open System

[<AttributeUsage(AttributeTargets.Property)>]
type MyAttribute() =
    inherit Attribute()

    member this.Foo () =
        new MyClass(1)

and MyClass(someValue) =
    // [<MyAttribute()>] -> specified in the fsi, still appears in compiled code
    member this.SomeValue : int = someValue

See https://msdn.microsoft.com/en-us/library/dd233196.aspx for reference