Can we have default properties, and base web interfaces too inside the interfaces?

[Nefcanto]

Default methods inside interfaces provide a neat way for composition. Since C# does not have multiple inheritance, we can extract common functionality into separate interfaces and add them to existing classes easily.

But they are very limited. Two points:

1. A lot of time we need type-level state management (fields and properties) to be used amongst multiple methods.
2. We can't use them for the web. We don't have access to HttpContext or return Ok in our default methods.

Can these two new features be added to the C# language? Can we have ISeoController which has a default property called PathSeo and two default methods using that property, and also they access the HttpContext?

[HaloFour]

You can define properties on interfaces, including with default implementations. However, interfaces cannot contain state. They have no storage for it, and attempting to support that would bring with it all of the complexity (and problems) of multiple inheritance.

As for the web, that sounds like a question for ASP.NET or whatever framework you use. There's nothing about the language that forbids their use in such cases.

[Nefcanto]

@CyrusNajmabadi, is there an example of that? I mean the syntax is very good.

[Nefcanto]

@CyrusNajmabadi, please guide me on how can I use attributes on adding methods and properties via code generation.

[davidfowl]

You would define your own attributes and build a source generator:
https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/source-generators-overview

Your controllers would all need to be partial and you would emit another partial that would code spit the implementation based on the attribute. This isn't a trivial thing to do but it is pretty reasonable given your constraints.

The other approach would be to extend the MVC framework itself to detect those attributes and inject more action methods into the model. You can use application model conventions to customize the framework https://www.stevejgordon.co.uk/customising-asp-net-mvc-core-behaviour-with-an-iapplicationmodelconvention.

Again, non-trivial but doable given your constraints.

[CyrusNajmabadi]

Again, non-trivial but doable given your constraints.

Given taht there are 500 APIs over 50 projects, the investment here would certainly be worthwhile.

[Nefcanto]

@davidfowl, thank you for your help and guidance. I will test them and come back with the results.

[huoyaoyuan]

Multiple inheritance is just one way to compose code. I'd like to explore better and friendly way to compose codes in more dimensions.
For example, currently in CoreLib, we need to declare each functionality for every numeric types. Composing may be used to reduce the whole matrix.