Proposal: Default Generic Type Parameter

[alrz]

The ability to specify a default type for a generic type parameter:

class Foo<TBar = Bar, TBaz = Baz> {}

As a real world example, the IdentityDbContext class in ASP.NET Identity:

class IdentityDbContext<TUser = IdentityUser, TRole = IdentityRole, TKey = string, TUserLogin = IdentityUserLogin, TUserRole = IdentityUserRole , TUserClaim = IdentityUserClaim> where ...

to avoid the need to declare different subtypes for different default types like

class IdentityDbContext : IdentityDbContext<IdentityUser, IdentityRole, string, IdentityUserLogin, IdentityUserRole, IdentityUserClaim> { ... }
class IdentityDbContext<TUser> : IdentityDbContext<TUser, IdentityRole, string, IdentityUserLogin, IdentityUserRole, IdentityUserClaim> where TUser : IdentityUser { ... }

In companion with improved type aliases can avoid this for god's sake.

To use default type parameters, one may skip the type parameter as follow:

var foo = new Foo<,>(); // new Foo<Bar, Baz>()
var foo = new Foo<A,>(); // new Foo <A, Baz>()

This has some interactions with generic type argument inference proposals like #5429 or #2319.

[gafter]

@alrz Lucky for us you're not aware of the work to add higher kinded types

oops 🙊

[alrz]

@gafter you mean like #2212? what about it, how is that relevant? :speak_evil:

[orthoxerox]

@alrz how will you disambiguate between creating instances of classes Foo and Foo<T = Bar> via new Foo()?

[alrz]

@orthoxerox new Foo() and new Foo<>. see the examples.

[aluanhaddad]

@alrz I don't think this adds very much value.

@gafter

Luckily for us you're not aware of the work to add higher kinded types

You don't say? 😀

[alrz]

I don't think this adds very much value.

That's it? Well that was not helpful either.

[BeeWarloc]

Foo<> already have a meaning inside typeof(), which obviously can't change.
Having Foo<> mean different things inside and outside of typeof() could be a bit confusing.

[MgSam]

@gafter Any further details forthcoming on this work?

I do think this is a useful idea. It would allow users to use generic methods/types without necessarily having to specify each type if they're happy with the default, thus potentially making consumption far more straightforward.

TypeScript has a similar proposal currently.

[gafter]

@MgSam Any further details on what work? Higher-kinded types? If anything it is years away, and would be led by changes to F# and CLR.

[alrz]

I think this can eliminate the need for generic/non generic variations of types without breaking existing code,

interface IEnumerator<T = object> {
  T Current { get; }
  bool MoveNext();
  ...
}
interface IEnumerable<T = object> : IEnumerable {
  IEnumerator<T> GetEnumerator();
}

[aluanhaddad]

I think this can eliminate the need for generic/non generic variations of types without breaking existing code

I'm not sure what that means. Are you saying that there would be type equivalence between

interface IEnumerator<T = object> {
    T Current;
    bool MoveNext();
}

and

interface IEnumerator {
    object Current;
    bool MoveNext();
}

?
If so that would be a breaking change because

new ArrayList() is IEnumerable == true

and

new ArrayList() is IEnumerable<object> == false

and

typeof(IEnumerator<>) != typeof(IEnumerator)

[alrz]

@aluanhaddad I'm saying that we could get rid of non-generic IEnumerable.

[alrz]

Moved to dotnet/csharplang#278