Values, references and safety.
WHAT VALUE TYPES ARE
Structures and enumerations, copied when assigned or passed.
WHAT REFERENCE TYPES ARE
Classes, shared when assigned or passed.
WHY THAT DISTINCTION MATTERS MOST
It determines whether a change in one place is visible in another.
WHAT THE LANGUAGE PREFERS
Value types, by default.
WHY
Copies cannot be changed unexpectedly by other code, which eliminates a class of bug.
WHAT COPY-ON-WRITE PROVIDES
Values behaving as copies while sharing storage until one is modified.
WHY THAT MATTERS
Copying large collections is cheap until something changes.
WHEN TO USE A CLASS
When identity matters, or when the object must be shared deliberately.
WHAT AN ENUMERATION WITH ASSOCIATED VALUES PROVIDES
A type representing one of several cases, each carrying different data.
WHY THAT IS POWERFUL
It models state precisely, and the compiler requires every case to be handled.
WHAT THAT PREVENTS
States that should be impossible being representable.
WHAT TO AIM FOR
Making invalid states impossible to express.
