For C# 13, the focus was on features to make it easier, faster, and safer for developers to write code in preferred styles. The use of the params modifier in method signatures is boosted in C# 13 with the addition of collections expressions. Thus, developers are no longer limited to using array types with params and can use any supported collection type. C# 13 also unlocks even more high-performance code by introducing new ways to use ref struct values and makes it easier to work with multi-threaded applications with System.Threading.Lock.
F# 9 offers language, library, and tool enhancements intended to make programs safer, more resilient, and performant. Nullable reference types bring type safety to interactions with C# libraries, while optimized integral ranges speed up for loops and other comprehensions. Optimized equality checks avoid boxing and increase performance of many common operations.
The ASP.NET Core web framework in .NET 9 brings improvements to help ensure apps are secure by default. It is now easier to set up a trusted development certificate on Linux to enable HTTPS during development. The framework also features improved monitoring and tracing along with performance improvements, with higher throughput, faster startup time, and less memory usage. For static web assets, like JavaScript and CSS files, ASP.NET Core in .NET 9 now optimizes these files during build and publish for efficient deployment.