Working on a project, I found myself wanting to match different discriminated union cases with text for output. Normally I write this out as a discriminated union with a separate function that uses pattern matches to determine which value to return, but I'd really like to attach the text directly to the different union cases. If I could find a way to do that, I wouldn't have to dig around my code as much to find out where the different messages came from.
As I've been poking at .NET's Reflection abilities recently, I figured the
easiest way for me to make that happen would be to add my own attribute type,
and apply them to the different union cases. Should be easy enough to get the
attribute back and pull the text from there, right? Not so fast! Union cases are
internally defined and hidden from F# code in such a way that you'd have to go
through the union's type definition, grab values from
System.Type.GetMembers(), and match them by name; at that point, I may as well
have just matched the values by union case and not used attributes at all.
The solution is down in Microsoft.FSharp.Reflection, which provides a handful
of useful functions for discriminated unions. The process therefore becomes:
- Get the
Microsoft.FSharp.Reflection.UnionCaseInfoby passing in the union value and atypeof<X>to the union's type - Call
GetCustomAttributes()on theUnionCaseInfovalue - Filter the returned attribute values (they're an
objarray) to find the attribute you want - Return the contents of the attribute
This is the full contents of how I did it: