custom class attribute for discriminated union case

Question

With the code below, Success and Failure are compiled into 2 separate classes. How can I provide custom attributes for Success and Failure?

type Result<'TSuccess,'TFailure> = 
    | Success of 'TSuccess
    | Failure of 'TFailure

EDIT Since Success and Failure result in classes I need to decorate them with class-compatible attribute AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum | AttributeTargets.Interface)

Can I get this to work? If not, why not?

[<ClassAttribute>]
type Result<'TSuccess,'TFailure> = 
    | [<ClassAttribute>] Success of 'TSuccess
    | [<ClassAttribute>] Failure of 'TFailure

Answer 1

Attributes can be placed between the | and the name of the union case.

Here is a simple example

open System
type t =  |[<Obsolete("hello")>]A

Answer 2

An attribute on a union case gets associated with the static New<Case> method returning the subclass for that case or with the Case property for a valueless case. With this definition in F# ...

type Result<'TSuccess,'TFailure> = 
    | [<Obsolete>] Success of 'TSuccess
    | Failure of 'TFailure
    | [<Obsolete>] NotSure

Use this C# snippet to invoke the decompilation ...

var y = Result<string, string>.NewSuccess("yay");
var z = Result<string, string>.NotSure;

Going to the definition of NewSuccess will bring up the C# definition of Result from metadata showing the obsolete attribute.

[Obsolete]
public static Result<TSuccess, TFailure> NewSuccess(TSuccess item);

[Serializable]
[DebuggerDisplay("{__DebugDisplay(),nq}")]
public class Success : Result<TSuccess, TFailure>
{
  [CompilationMapping(SourceConstructFlags.Field, 0, 0)]
  [CompilerGenerated]
  [DebuggerNonUserCode]
  public TSuccess Item { get; }
}

For a case identifier like NotSure above that has no values, the case becomes a property rather than a subclass and the obsolete attribute attached to the case is compiled into thin air as far as the metadata decompilation goes ...

[CompilerGenerated]
[DebuggerBrowsable(DebuggerBrowsableState.Never)]
[DebuggerNonUserCode]
public static Result<TSuccess, TFailure> NotSure { get; }

Looking through ILdasm, the attributes are there alright on both ...

Method #3 (06000003) 
-------------------------------------------------------
MethodName: NewSuccess (06000003)
Flags     : [Public] [Static] [ReuseSlot]  (00000016)
RVA       : 0x00002064
ImplFlags : [IL] [Managed]  (00000000)
CallCnvntn: [DEFAULT]
ReturnType: GenericInst Class Result`2< Var!0, Var!1>
1 Arguments
    Argument #1:  Var!0
1 Parameters
    (1) ParamToken : (08000001) Name : item flags: [none] (00000000)
CustomAttribute #1 (0c000011)
-------------------------------------------------------
    CustomAttribute Type: 0a00000b
    CustomAttributeName: System.ObsoleteAttribute :: instance void .ctor()
    Length: 4
    Value : 01 00 00 00                                      >                <
    ctor args: ()

CustomAttribute #2 (0c000012)
-------------------------------------------------------
    CustomAttribute Type: 0a00000c
    CustomAttributeName: Microsoft.FSharp.Core.CompilationMappingAttribute :: instance void .ctor(value class Microsoft.FSharp.Core.SourceConstructFlags,int32)
    Length: 12
    Value : 01 00 08 00 00 00 00 00  00 00 00 00             >                <
    ctor args: ( <can not decode> )


Method #7 (06000007) 
-------------------------------------------------------
MethodName: get_NotSure (06000007)
Flags     : [Public] [Static] [ReuseSlot]  (00000016)
RVA       : 0x0000208c
ImplFlags : [IL] [Managed]  (00000000)
CallCnvntn: [DEFAULT]
ReturnType: GenericInst Class Result`2< Var!0, Var!1>
No arguments.
CustomAttribute #1 (0c000030)
-------------------------------------------------------
    CustomAttribute Type: 0a00000b
    CustomAttributeName: System.ObsoleteAttribute :: instance void .ctor()
    Length: 4
    Value : 01 00 00 00                                      >                <
    ctor args: ()

CustomAttribute #2 (0c000031)
-------------------------------------------------------
    CustomAttribute Type: 0a00000c
    CustomAttributeName: Microsoft.FSharp.Core.CompilationMappingAttribute :: instance void .ctor(value class Microsoft.FSharp.Core.SourceConstructFlags,int32)
    Length: 12
    Value : 01 00 08 00 00 00 02 00  00 00 00 00             >                <
    ctor args: ( <can not decode> )