What is the purpose of the enclosing static class for extension members?

[Gavin-Williams]

Is there a reason for this enclosing class when designing extension members? What does it express?

public static class AccountExtensions
{

... When this code seems to express what the code is intended for...

extension(ref Account account)
{

I don't see a need for an enclosing class. The extension members are fully described and only have meaning in relation to the Account type. The AccountExtensions type and the requirement to type that out, as well as the nesting is meaningless.

[CyrusNajmabadi]

It gives you access to the underlying static members, should you need to call them explicitly (for example for disambiguation purposes).

[HaloFour]

JSON.NET, AutoMapper, Dapper, FluentAssertions, DotNext, MoreLinq, etc. The .NET runtime exposes a bunch of extension methods, too. It's how LINQ was implemented.

[CyrusNajmabadi]

ASP.Net... and more.

[dlebansais]

I should have been more specific. Of course I know about LINQ and other libraries extending stuff outside what the library provides. I use many of them and never encountered the need to disambiguate between two extensions.

I was thinking of a library that provides a type, say Foo and also extensions over Foo. Seems like a dubious use of extension to me.

[CyrusNajmabadi]

This is very common. As trivial examples, you often only want to expose functionality for particular instantiations of your generic types.

Furthermore, it's a normal pattern to say "this is better as an extension, since it can be built entirely on the public surface area, without need to access internals".

Roslyn does this in many locations. We have lots of extensions that are on our own types.

[jnm2]

Right, so in a general case, if you declare Foo and you declare extension methods on Foo, you can be reasonably sure that a library won't declare a conflicting extension method on Foo. (It may declare a conflicting extension method on object, though that's generally frowned on.) However, this is a really special case. Most extension methods aren't declared for a type declared in the same assembly.