F# code for getting attribute data off discriminated union cases

Errors.fs

type FatalError(message: string) =
    inherit System.Attribute()
    member __.Message = message

type Errors =
    | [<FatalError("The value specified is currently not available")>] UnknownValue
    | NotAnError

let PrintErrorMessage : Errors -> string =
    fun err ->
        // Get UnionCaseInfo value from the F# reflection tools
        let (case, _args) = Microsoft.FSharp.Reflection.FSharpValue.GetUnionFields(err, typeof<Errors>)
        // Pull all attributes
        let attributes = case.GetCustomAttributes()
        // Find the attributes that have an error message
        let haveError =
            attributes
            |> Seq.ofArray
            // Filter for the FatalError values
            |> Seq.filter (fun x -> x :? FatalError)
            // Cast each value
            |> Seq.map (fun x -> x :?> FatalError)
            // Seq.take breaks if there aren't enough, so use Seq.truncate instead
            |> Seq.truncate 1
            |> List.ofSeq

        // If the FatalError attribute wasn't found, give a default message
        if haveError.IsEmpty then "(no error message available)"
        else haveError.Head.Message

printfn "%s" <| PrintErrorMessage Errors.UnknownValue
printfn "%s" <| PrintErrorMessage Errors.NotAnError


// Alternative version of PrintErrorMessage which returns the attribute
// Allows providing a default or "backup" value
let GetAttribute<'T> : 'T -> obj -> 'T =
    fun backup value ->
        let (case, _args) = Microsoft.FSharp.Reflection.FSharpValue.GetUnionFields(value, value.GetType())
        let attributeValue =
            case.GetCustomAttributes()
            |> Seq.ofArray
            |> Seq.filter (fun x -> x :? 'T)
            |> Seq.truncate 1
            |> Seq.map (fun x -> x :?> 'T)
            |> List.ofSeq

        if attributeValue.IsEmpty then backup
        else attributeValue.Head

let GetFatalErrorMessage = GetAttribute <| FatalError("(no error message available)")
let unknownValue = GetFatalErrorMessage Errors.UnknownValue