Getting the types in an F# union type II -- the perils of relying on undocumented implementation details

Recalling this example --

	open System
	open Microsoft.FSharp.Reflection
	let GetUnionTypes (t:Type) =
	seq { yield t
	if FSharpType.IsUnion t
	then yield! FSharpType.GetUnionCases t
	// implementation detail leaks here -- nested type name
	|> Seq.map (fun x -> (string t) + "+" + x.Name)
	|> Seq.map Type.GetType }
	type expr = Num of int
	| Operation of (expr * expr)
	let baseType = typeof<expr>
	GetUnionTypes baseType
	|> Seq.iter (printfn "%A")
	;;

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It was adequate for the use case I had of it at the time; but then I tried a different union type, at which point a bug showed up

	type Card = King | Queen| Jack | Spot of int;;
	GetUnionTypes typeof<Card> |> Seq.iter (fun x -> printfn "%s" x.FullName);;

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yields

FSI_0003+Card
System.NullReferenceException: Object reference not set to an instance of an object.
   at FSI_0005.it@24-1.Invoke(Type x)
   at Microsoft.FSharp.Collections.SeqModule.Iterate[T](FSharpFunc`2 action, IEnumerable`1 source)
   at .$FSI_0005.main@()
Stopped due to error

And, of course, that is exactly what I deserved for relying on the undocumented internals.

A quick decompilation reveals this to be because union cases that carry no data are actually concrete base type instances, bearing the tag number as data, like --

internal static readonly Program.Card _unique_King = new Program.Card(0);

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rather than being redundant empty subtypes.

So we need to tweak our function to be

	let GetUnionTypes (t:Type) =
	seq { yield t
	if FSharpType.IsUnion t
	then yield! FSharpType.GetUnionCases t
	// implementation detail leaks here -- nested type name
	|> Seq.map (fun x -> Type.GetType ((string t) + "+" + x.Name))
	// except when the case has no data, so there's no such class
	|> Seq.filter (fun x -> x <> null) }

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and all is well. For the moment. Until the representation changes.