Patrick Smacchia walks us through record types in C# 9:
The second core property of string and record value-based semantic is immutability. Basically, an object is immutable if its state cannot change once the object has been created. Consequently, a class is immutable if it is declared in such way that all its instances are immutable.
I remember a discussion with a developer that got nervous about immutability. It looked like an unnatural constraint to him: he wanted his object’s state to change. But he didn’t realized that something he used everyday – string operations – relied on immutability. When you are modifying a string actually a new string object gets created. Records behave the same way. Moreover a clean new syntax based on the keyword
withhas been introduced with C#9.
They aren’t as fancy as F# record types, but it is fun to watch C# move slowly to being a functional-friendlier language—something which has been the case since Don Syme helped implement generics in C#.