Proposal: support an enum constraint on generic type parameters

[gafter]

Allow the constraints on a type parameter to include enum. Such a constrained type parameter could be instantiated with an enum type or with another type parameter that is so constrained.

This probably needs more design work.

[mikedn]

For reference: long, too long unfortunately, discussion from codeplex: http://roslyn.codeplex.com/discussions/572464

[HaloFour]

Ultimately what we could distill from that is that thread is the following:

1. The CLR supports three forms of enum constraints:
   1. System.Enum by itself which allows the generic type argument to be either System.Enum or a specific enum type
   2. System.Enum and class, which forces the generic type to be only System.Enum
   3. System.Enum and struct, which forces the generic type to be a specific enum type
2. Supporting operators like & or | would not be possible given that the IL generated would have to be differently depending on the underlying type of the enum.
3. F# supports an enum constraint that also constrains the underlying type to a specific integral type which would allow this functionality but it is not supported by the CLR.

type Class9<'T when 'T : enum<uint32>> =

In my opinion out of the three potential CLR constraint combinations 1.iii. is the one that would be the most useful. I don't know how one would write a generic method or type accepting System.Enum in place of a real enum type and what use that would be. Of course 1.ii. just seems completely useless.

Unfortunately, without support for 2.i. the use cases for this feature in general are mostly for the purposes of reflection. Generally improved versions of existing CLR functions, such as parsing the enum from a string, which is something that I have written myself to be both faster and more capable (e.g. can parse a flags value from a comma- or pipe-separated string.)

Given that these scenarios largely involve reflection that does pose a small problem if compiling to .NET Native as some of the existing methods of Enum are not friendly with .NET Native, specifically Enum.GetValues(Type) as internally it attempts to create a generic array. There are other less efficient ways to get to the values, such as calling Enum.GetNames(Type) and enumerating over that using Enum.Parse(Type, string).

[gafter]

See also https://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/2557231-enums-constraint-for-generics

[HaloFour]

I was toying with .NET Native in Visual Studio 2015 CTP6 and it seems that the issue with reflection and enums are no longer a problem as Type.GetEnumValues() no longer throws due to the call to Array.UnsafeCreateInstance.

[Ultrahead]

Another reference: http://roslyn.codeplex.com/discussions/543871