"default" Attribute to give to interface implementations.

[MkazemAkhgary]

I have a custom list like

 class MyList<T> : IEnumerable<IList<T>>,  IEnumerable<IReadOnlyList<T>>,  IEnumerable<Foo<T>>, ...

because multiple interfaces are implemented, I cant use linq easily. so I have to write something like

mylist.Where<Foo<T>>(foo => true);

It would be nice if I could give priority to interface implementations which tells compiler, if you cant infer type from usage, pick this one.

 [DefaultInterface(typeof(IEnumerable<Foo<T>>))]
 class MyList<T> : IEnumerable<IList<T>>,  IEnumerable<IReadOnlyList<T>>,  IEnumerable<Foo<T>>

this probably is hard to implement, but I think its worth it to help inferring generic type arguments from usage.

By the way I can always cast my list into preferred ienumerable before writing query, but since its part of API, putting this limitation away would be nice.

[iam3yal]

@MkazemAkhgary Related #278.

[jnm2]

Just curious: why not make it IEnumerable<MyCustomType<T>> which implements IList and IReadOnlyList etc? I've never been in this situation without being able to find a more desirable design.

[ufcpp]

different but similar to #964

[Mafii]

@jnm2 I agree. The problem that op is trying to solve with the default attribute can be solved by proper design, I don't think this feature (DefaultInterface) would benefit a lot of people:

class Foot<T> : IList<T>, IReadOnlyList<T>
{
    ... 
}

and then just:

class MyList<T> : IEnumerable<Foo<T>> 
{
  ...
}

should work.

[MkazemAkhgary]

You are right, this is something that can be fixed with correct design.

[MkazemAkhgary]

I reopen this issue because of this scenario.

class Foo : IFoo<T>
interface IFoo<T> : IReadOnlyFoo<T>, IEnumerable<IBar<T>>
interface IReadOnlyFoo<out T> : IEnumerable<IReadOnlyBar<T>>

how can I improve my design?

if I deprecate enumerable from readonly interface it works, but I want that. (enumerating is basic function)
if I deprecate readonly interface all together it works, but I want covariance from it.
if I deprecate enumerable from IFoo then I will only have enumerable on readonly Bars.