Provide "static virtual interface members" with examples and description of static virtual methods

[BartoszKlonowski]

This pull request closes #41668
It adds the section presenting the static virtual methods of an interface.

This PR adds the separate implementation files for IGetDuplicated interface containing the static virtual example as well as the Duplicate type implementing that interface.
The implementation of the Duplicate struct is (on purpose) very similar to already existing RepeatSequence type, so that it bases on the reader's understanding and provides them with the comparison between static abstract and static virtual scenarios.

The description is brief, with only required details, and follows the same style/layout as the ## Static abstract interface methods section.
The only addition is the note regarding the pitfall of leaving the static virtual method unimplemented. Compiler won't scream about lack of it explicitly, so I wanted to highlight that having the default implementation does not mean that this default implementation will be called.

This is broad topic, so any concerns or remarks will be highly appreciated.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/whats-new/tutorials/static-virtual-interface-members.md	Tutorial: Explore C# 11 feature - static virtual members in interfaces

[BillWagner]

@BartoszKlonowski Do you want to come back to this one? I think it's close.

[BartoszKlonowski]

@BillWagner Yes, let me come back to this considering the last input you made here 👍

[BillWagner]

@BartoszKlonowski Thanks. Also, as it's now mid-August, I have less concerns about using the field feature. November and .NET 10 are coming soon.

[BillWagner]

I really like this @BartoszKlonowski

I had one wording suggestion. Let me know what you think, and then I'll merge this.

[BillWagner]

This opening sentence seemed out of place somehow. How about this alternative:

Suggested change

	Similar can be achieved using the static virtual methods of an interface. This is done with the syntax of `static` and `virtual` modifiers added to any member that should be implemented in the type implementing that interface. The following example defines the `IGetDuplicated<T>` interface, providing any type with a constraint that a type will implement the `operator ++`:
	You can also provide a default implementation for static methods defined in an interface. This is done with the syntax of `static` and `virtual` modifiers added to any member that should be implemented in the type implementing that interface. The following example defines the `IGetDuplicated<T>` interface, providing any type with a constraint that a type will implement the `operator ++`:

[BillWagner]

I like this.

There's one build error that we need to fix. I have a suggestion that emphasizes the fact that default interface members must be accessed through a reference of the interface type.

LMK what you think, and then I'll merge.

[BillWagner]

For this to work, you'll need to use the interface type:

Suggested change

	var instance = UniqueInstance.Instance;
	var instance = (UniqueInstance as ISingleton)?.Instance;

[BartoszKlonowski]

@BillWagner I committed this suggestion, although locally, with VS still on version 17.4 I do have issues with your suggestion: CS0119 for UniqueInstance and CS0305 for ISingleton (which can be fixed by specifying the T type for this cast).
Let's see if remotely it will also fail and then we can check.

[BartoszKlonowski]

@BillWagner I should mention, that locally, with VS v17.4 I can also see the initial error for Instance not being a part of UniqueInstance.
Somewhere I read that static virtual feature requires:

• .NET v8.0 or above - locally I'm running on .NET 9, and I see this staticinterfaces project runs on .NET 8, which should be fine?
• preview enabled as LangVersion - we have that.
• VS 17.6 or above - this is what I'm missing locally, but can't say regarding the CI checks.

[BillWagner]

Hi @BartoszKlonowski

I see the new errors. To answer your question:

.NET v8.0 or above - locally I'm running on .NET 9, and I see this staticinterfaces project runs on .NET 8, which should be fine?

Yes, it should be. As a normal practice, we're happy to move to the latest GA release (.NET 9 now).

preview enabled as LangVersion - we have that.

Check.

VS 17.6 or above - this is what I'm missing locally, but can't say regarding the CI checks.

We should be using the .NET 10 preview build (because some of the docs are for the preview).

Now, this example might be tricky to get right unless the type overrides the interface method (even if it just calls the interface implementation.) Why? Well, an ISingleton<int> could be implemented by more than one class type. So, which one should be used? Without an instance of the type, that's hard to express.