Microsoft has announced details of what will be included in C# 13. The news was announced at Microsoft's recent Build conference. The new version will have enhanced parameters, extension types, and several features to enhance performance.

The most noticeable change will be that params are no longer limited to arrays. Describing the change, Microsoft says that it's still the case that when the params keyword appears before a parameter, calls to the method can provide a comma delimited list of zero or more values and those values are placed in a collection of the parameter's type.

However, starting in C# 13, the params parameter type can be any of the types used with collection expressions, like List<T>, Span<T>, and IEnumerable<T>. You can even use your own collection types if they follow special rules. The ability to use the Span type improves performance because it reduces memory use.

The next change of note is the addition of extension types, though this feature isn't included in the current preview.  Extension methods have been supported since C#3, and can be used to add methods to an underlying type, even if you cannot change its code. C# 13 will add extension types, a new kind of type that supplies extension members for an underlying type, and that are intended for uses such as having an underlying type that you can't change the code of, but you want to make changes. 

Extension types have methods, properties and other members that can be instance or static. Microsoft says there will be two kinds of extension types: implicit and explicit extensions. Implicit extension types apply to all occurrences of the underlying type – in the same way extension methods do today. Explicit extension methods and properties apply only to instances of the underlying type that have been converted to the explicit extension type.

Other improvements to C# 13 will include a lock statement pattern designed to make C# more in line with .NET 9's new dedicated System.Threading.Lock type; and improvements to the method group natural type.

C# is part of the .NET SDK.

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