When should I use a struct rather than a class in C#?

Question

When should you use struct and not class in C#? My conceptual model is that structs are used in times when the item is merely a collection of value types. A way to

Answer 1

In addition to the "it is a value" answer, one specific scenario for using structs is when you know that you have a set of data that is causing garbage collection issues, and you have lots of objects. For example, a large list/array of Person instances. The natural metaphor here is a class, but if you have large number of long-lived Person instance, they can end up clogging GEN-2 and causing GC stalls. If the scenario warrants it, one potential approach here is to use an array (not list) of Person structs, i.e. Person[]. Now, instead of having millions of objects in GEN-2, you have a single chunk on the LOH (I'm assuming no strings etc here - i.e. a pure value without any references). This has very little GC impact.

Working with this data is awkward, as the data is probably over-sized for a struct, and you don't want to copy fat values all the time. However, accessing it directly in an array does not copy the struct - it is in-place (contrast to a list indexer, which does copy). This means lots of work with indexes:

int index = ...
int id = peopleArray[index].Id;

Note that keeping the values themselves immutable will help here. For more complex logic, use a method with a by-ref parameter:

void Foo(ref Person person) {...}
...
Foo(ref peopleArray[index]);

Again, this is in-place - we have not copied the value.

In very specific scenarios, this tactic can be very successful; however, it is a fairly advanced scernario that should be attempted only if you know what you are doing and why. The default here would be a class.