C# 14 Extension Members: Extension Everything

Extension methods have worked the same way since .NET 3.5: a static method with a this-modified first parameter, callable either as a static call or, more commonly, as an instance method on the receiver type. Requests for extension properties and static extension methods go back to C# 3.0. An attempt at “extension everything” for C# 4.0 failed outright, and a 2016 language design meeting revisited the idea and shelved it again, until C# 14 finally shipped it, under the name extension members.

The new syntax wraps a method or property in an extension(){ } block that takes the receiver type, and for instance members a named receiver parameter, instead of putting this on the first argument. It adds three new kinds of member beyond the old instance extension method: static extension methods, static extension properties, and instance extension properties, with extension operators following in a later preview. Lock ported his own NuGet package, NetEscapades.EnumGenerators, to the new syntax and found it trivial: the generator’s existing static helper methods just needed wrapping in an extension(global::MyColours) { } block to become callable directly on the enum type itself.

Fichamento

#

• The extension member syntax is optional. Existing this-parameter extension methods keep compiling exactly as before, to the same IL either way.
• Converting one means wrapping it in an extension<T>(IEnumerable<T> target) { } block, moving the type parameters and receiver into that block, and dropping static from the method itself.
• Static extension methods, such as a HasValue added to string itself and called as string.HasValue(someValue), don’t take a named receiver parameter. The member attaches to the type, not to a variable.
• Instance extension properties keep the receiver parameter and read like ordinary properties (someValue.IsAscii), compiling down to get_-prefixed methods the same way normal C# properties always have.
• Disambiguating a clash falls back to the same shapes extension methods always used: a static call with the instance as first argument, or a get_-prefixed call for properties.
• Porting NetEscapades.EnumGenerators to the new syntax took wrapping its existing static helpers, IsDefined, Parse, in an extension(global::MyColours) { } block, after which they became callable as MyColours.Parse("Red") instead of through the generated extensions class name.