[Proposal]: Extensions

[333fred]

Discussed in #5496

^{Originally posted by MadsTorgersen November 30, 2021}

Extensions

• Proposed
• Prototype: Not Started
• Implementation: Not Started
• Specification: https://github.com/dotnet/csharplang/blob/main/proposals/extensions.md

LDM Meetings

https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-12-01.md#roles-and-extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-08-31.md#roles
https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-09-26.md#roles--extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-02-22.md#extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-12-11.md#extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-02-28.md#extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-06-12.md#extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-06-26.md#extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-07-22.md#extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-18.md#extensions-naming
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-30.md
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-10-02.md#extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-10-07.md
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-10-09.md
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-10-14.md
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-10-30.md#extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-11-13.md#extensions
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-11-20.md#extensions

[hez2010]

I prefer using of or something else to distinguish the type being extended and interfaces:

public extension Foo of int : IA, IB, IC, ...
{
    ...
}

Otherwise it will be too confusing if you are extending an interface:

public extension Foo : IA, IB, IC { }

vs

public extension Foo of IA : IB, IC { }

of can be relatively safely made as a keyword since it's neither a verb nor a noun, so almost nobody would choose of as an identifier.

[HaloFour]

I'm curious as to how the team weighs the relative benefits between "roles" and "extension implementation". It feels that without some additional effort in the runtime the two are somewhat incompatible with each other, so if those differences can't be reconciled which of the features might the team lean towards?

Personally, I find extension implementation much more exciting than roles, but that's just my opinion.

[FaustVX]

@hez2010
Maybe, instead of of, for would be a better name for the keyword as it's already a keyword.

public extension Foo for IA : IB, IC { }

[333fred]

I'm curious as to how the team weighs the relative benefits between "roles" and "extension implementation". It feels that without some additional effort in the runtime the two are somewhat incompatible with each other, so if those differences can't be reconciled which of the features might the team lean towards?

Who gave you an early preview of my notes? They're up now, discussion at #5500.

[sab39]

Here's a scenario that will be great fun to try to accommodate in the design:

interface IFoo { }
interface IBar { }
class Thing { }
public extension FooThing for Thing : IFoo { }
public extension BarThing for Thing : IBar { }
void Frob<T>(T t) where T : IFoo, IBar { }

Frob(new Thing());

On an unrelated bikeshedding note, what about using the existing reserved keywords explicit and implicit as modifiers, rather than treating roles and extensions as completely separate things? An extension is, more or less, just a role that gets applied implicitly based on type rather than needing to be explicitly named in the declaration. Using implicit role as the syntax would spell that correspondence out more (no pun intended) explicitly?

[CyrusNajmabadi]

@sab39 Given, as you've mentioned, how similar these two concepts are. I too am looking for a good syntactic way to convey that similarity, with a clear way to do indicate in which way they differ. Thanks for the explicit/implicit idea, definitely something we'll consider!

[TahirAhmadov]

I'm not sure if I should re-post my comments from the discussion here?
In short, I think the extensions and especially interface "adapters" make sense, but roles don't. The main motivation example - DataObject - is an anti-pattern, IMO; it runs into expensive runtime changes; and causes confusion - now you have to keep in mind when looking at an identifier in a "type receiving context" if it's a type or a role.

Here's a scenario that will be great fun to try to accommodate in the design:

interface IFoo { }
interface IBar { }
class Thing { }
public extension FooThing for Thing : IFoo { }
public extension BarThing for Thing : IBar { }
void Frob<T>(T t) where T : IFoo, IBar { }

Frob(new Thing());

This is complicated, but doable using current constraints of the framework. An anonymous type can be generated:

class <mangled>Thing_IFoo_IBar : IFoo, IBar
{
  internal <mangled>Thing_IFoo_IBar(Thing thing) { this._thing = thing; }
  readonly Thing _thing;

  void IFoo.Foo() { ... } // these member(s) are copied from, or call into, FooThing
  void IBar.Bar() { ... } // these member(s) are copied from, or call into, BarThing
}
Frob(new <mangled>Thing_IFoo_IBar(new Thing()));

The same can be done for generic types, etc. Yes, it's complicated, but unlike roles, it's very possible.

[CyrusNajmabadi]

The main motivation example

This was just one example. It's not the main motivation. We discussed in the LDM that there were definitely plenty of scenarios where you'd still want adapters in a strongly typed way that would be sensible.

[sab39]

@TahirAhmadov That works, more or less, for the specific example I gave, but what if Frob<T> had other constraints like where T : class or where T : struct or where T : new() or where T : SomeBaseClass? What about if it were Frob<T, T2> where T2 : T? In general it's not actually possible to generate an anonymous type that can meet all possible constraints that T would meet and also implement IFoo and IBar. This suggests to me that it's not possible to fully support this scenario without runtime assistance.

[TahirAhmadov]

The main motivation example

This was just one example. It's not the main motivation. We discussed in the LDM that there were definitely plenty of scenarios where you'd still want adapters in a strongly typed way that would be sensible.

If it's not the main motivation, surely it shouldn't be the one discussed in the OP, should it?

[CyrusNajmabadi]

The OP is simply showing a demonstration. This is a broad topic and we need to spend a ton more time on it prior to even getting close to a place where we could write something up that was fully fleshed out and chock full of examples and whatnot.

[TahirAhmadov]

@TahirAhmadov That works, more or less, for the specific example I gave, but what if Frob<T> had other constraints like where T : class or where T : struct or where T : new() or where T : SomeBaseClass? What about if it were Frob<T, T2> where T2 : T? In general it's not actually possible to generate an anonymous type that can meet all possible constraints that T would meet and also implement IFoo and IBar. This suggests to me that it's not possible to fully support this scenario without runtime assistance.

The where T : class constraint simply rejects Thing extensions if Thing doesn't satisfy the constraints. The where T : new() rejects all extensions outright. where T : SomeBaseClass also rejects extensions of Thing because it's a different type. Frob<T, T2> where T2 : T is completely irrelevant.

[BhaaLseN]

Back with .NET Framework, I've often ran into situations where i wanted a Math.Clamp<T>(T value, T min, T max), Math.Max(TimeSpan val1, TimeSpan val2) or Path.GetRelativePath(...) for discoverability; but there was no way for me to get this done. Same with string.Contains(string, StringComparison) etc. except as instance extension (which is somewhat taken care of by extension methods though.)
Nowadays most are in there since either .NET Core or .NET 5/6, but it would feel more natural to simply extend the existing classes (with a very high-up namespace inside the project, so its most likely in scope all the time; or with a global using for example) when another one of those situations comes up. At least compare to MathEx, MathUtilities, PathHelpers etc. which often aren't as obvious.

The only thing I don't quite get is why we need two keywords here, role and extension. They mean different things if you talk about them, sure, but does this actually matter once you write the code? I'd assume they'll be lowered to virtually the same thing during compilation after all, and I can practically hear the "what's the difference" question coming when I present this to my team during a knowledge transfer meeting.