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

My rule is

1, Always use class;

2, If there is any performance issue, I try to change some class to struct depending on the rules which @IAbstract mentioned, and then do a test to see if these changes can improve performance.