What's new in C# 10

29th October 2021

I’m compiling information for our next Adelaide .NET User Group meetup which is focusing on the launch of .NET 6 and Visual Studio 2022.

Next up is C# 10.0. There’s a good summary on Microsoft Docs on the What’s new in C# 10.0 page. At the time of writing that page was still being updated, with the progress of documenting all features being tracked here.

Also refer to https://github.com/dotnet/roslyn/blob/main/docs/Language%20Feature%20Status.md for features implemented in Roslyn.

• Record structs
• Improvements of structure types
• Interpolated string handlers
• global using directives
• File-scoped namespace declaration
• Extended property patterns
• Allow const interpolated strings
• Record types can seal ToString()
• Allow both assignment and declaration in the same deconstruction
• Allow AsyncMethodBuilder attribute on methods

Other features still to be documented include:

• Lambda improvements - Allow lambdas with attributes, allow lambdas with explicit return type, infer a natural delegate type for lambdas and method groups
• Caller argument expression - This would aid in debugging and diagnostic scenarios. It would allow methods to capture the expression passed to a method for diagnostic purposes.
• Extend the preprocessor line directive - allow specifying full span mapping
• Improved definite assignment
• Update preprocessor symbols for .NET 6
• Documentation - Source generators
• Documentation - Best practice of when to use module initializer vs. static constructor
• Static abstract members in interfaces

These are also on the list to be documented, but I believe will be only available under a preview flag:

• Generic attributes - Generic classes can now specify System.Attribute as a base class
• Method parameter names scoping change for nameof - enable nameof to name a parameter in an attribute constructor
• Allow deconstruction of ‘default’ literal
• Simplified parameter null validation code
• List patterns
• Relax ordering constraints around ref and partial modifiers on type declarations
• Allow value tuple deconstruction with default keyword