Tag Helpers: Add enum attribute support.

[NTaylorMullen]

Right now if a TagHelper has an Enum property the user has to type in the full Enum value:

<mytaghelper someattr="MyEnum.Foo" />

We can simplify this and make it:

<mytaghelper someattr="Foo" />

By assuming the Enum.

[rynowak]

See TODO in the MVC repro in ValidationSummaryTagHelper for a string that should be changed to an enum when this is fixed.

[DamianEdwards]

We discussed this again and can't find a reason not to do it and it it's a nice improvement.

[NTaylorMullen]

Note: we should also add the @ override to not specify the MyEnum. automagically (like we do for ModelExpressions today).

[DamianEdwards]

Agreed

[ghost]

nit: missing issue tag TagHelper.

[NTaylorMullen]

22ecf85
aspnet/Mvc@33b37d6
aspnet/RazorTooling@747b1b3

[tomshane]

Is it possible to convert enum value names to lower case when used in html? The same way how tags and attributes are converted, so it is more inline with html conventions.

enum Type { Number, Text }

To be used like:

<tag type="number"/>

[NTaylorMullen]

@R74NN sorry it is not. The values exposed in the attributes today are pure C# so they must match.

If you'd like to work around it in your own implementations you can always have HTML specific enums with lower case names that you interop between.

[DamianEdwards]

It would certainly be possible to modify the code gen to lowercase enum value names. We don't have a great extensibility point for that today, but it might be feasible with a tree rewriter. We could investigate adding some kind of option for this in the future but I think it would likely be part of a more in depth set of features enabling customization of bound property code generation.