c# - Choosing between immutable objects and structs for value objects

Question

How do you choose between implementing a value object (the canonical example being an address) as an immutable object or a struct?

Are there performance, semantic or any other benefits of choosing one over the other?

Answer 1

There are a few things to consider:

A struct is allocated on the stack (usually). It is a value type, so passing the data around across methods can be costly if it is too large.

A class is allocated on the heap. It is a reference type, so passing the object around through methods is not as costly.

Generally, I use structs for immutable objects that are not very large. I only use them when there is a limited amount of data being held in them or I want immutability. An example is the DateTime struct. I like to think that if my object is not as lightweight as something like a DateTime, it is probably not worth being used as a struct. Also, if my object makes no sense being passed around as a value type (also like DateTime), then it may not be useful to use as a struct. Immutability is key here though. Also, I want to stress that structs are not immutable by default. You have to make them immutable by design.

In 99% of situations I encounter, a class is the proper thing to use. I find myself not needing immutable classes very often. It's more natural for me to think of classes as mutable in most cases.