.NET 9 brings a number of performance improvements and minor changes. However, this version doesn't bring much to the table that it radically changes the way we write code. In this, the 3 previous versions were sufficiently implemented. Even one more significant change – implicit extension types, which had the potential to significantly change the way we write code, was eventually removed from C# 13 and will probably not see it for a year.
What’s new in Base Class Library?
QUIC and HTTP/3
You may know that the HTTP/3 protocol is built on the QUIC protocol, which has been in .NET since version 5 as an internal implementation. In .NET 7, the implementation was already experimental and could be tested by developers. However, this has changed since version 9 and HTTP/3 is enabled by default.