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Compile error with Discriminated Union and Custom Attribute
f#

Hi there,

I receive the following error when attempting to use a discriminated union as a parameter to a custom attribute:

error FS0191: invalid custom attribute value (not a constant or literal)

I've included some code below that recreates the error.

Is there any workaround to this issue or am I doing something really stupid? I tried using an enum too, but still received the same compile error.

I'd appreciate any help you can give as I'm about to resort to declaring constants!

Cheers,
Ryan

Here's an FSI session that will demonstrate the error:

> type MyColour =
| Red
| Green
| Blue
| Yellow;;

type MyColour =
| Red
| Green
| Blue
| Yellow

> open System;;
> type MyCustomAttribute(colour: MyColour) =
inherit Attribute()
member mca.Colour = colour;;

type MyCustomAttribute =
class
inherit Attribute
new : colour:MyColour -> MyCustomAttribute
member Colour : MyColour
end

> type MyRecord = {
[<MyCustom(Red)>] Name : string
[<MyCustom(Green)>] Age : int
};;

[<MyCustom(Red)>] Name : string
---------------^^^^

stdin(11,16): error FS0191: invalid custom attribute value (not a constant or literal).
>

.

It will compile for enums, as in

1
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10
11

#light

type Color = 
    | Red=1 
    | Green=2

type MyAttribute(color:Color) =
    inherit System.Attribute()

[<My(Color.Red)>]
type Yadda() = class end

It does not work for discriminated unions, as they are objects, and .Net attributes cannot take arbitrary objects (I forget the exact list, but only something like integers/enums, strings, and typeof()s are allowed as values passed to an attribute).

By on 3/10/2009 9:54 AM ()

Perfect! That fixed it.

When I tried changing to an Enum before, I didn't qualify with the type (i.e. in your example it would have been [<My(Red)>]) which gave the same error as before. Fully qualifying the enum resolved it though.

Thanks very much for the speedy response.

Cheers,
Ryan

By on 3/10/2009 10:01 AM ()

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