I often use the following analogy when explaining these concepts.
Imagine that an object is a balloon. A variable is a person. Every person is either in the value type team or in the reference type team. And they all play a little game with the following rules:
Rules for value types:
- You hold in your arms a balloon filled with air. (Value type variables store the object.)
- You must always be holding exactly one balloon. (Value types are not nullable.)
- When someone else wants your balloon, they can blow up their own identical one, and hold that in their arms. (In value types, the object is copied.)
- Two people can't hold the same balloon. (Value types are not shared.)
- If you want to hold a different balloon, you have to pop the one you're already holding and grab another. (A value type object is destroyed when replaced.)
Rules for reference types:
- You may hold a piece of string that leads to a balloon filled with helium. (Reference type variables store a reference to the object.)
- You are allowed to hold one piece of string, or no piece of string at all. (Reference type variables are nullable.)
- When someone else wants your balloon, they can get their own piece of string and tie it to the same balloon as you have. (In reference types, the reference is copied.)
- Multiple people can hold pieces of string that all lead to the same balloon. (Reference type objects can be shared.)
- As long as there is at least one person still holding the string to a particular balloon, the balloon is safe. (A reference type object is alive as long as it is reachable.)
- For any particular balloon, if everyone eventually lets go of it, then that balloon flies away and nobody can reach it anymore. (A reference type object may become unreachable at some point.)
- At some later point before the game ends, a lost balloon may pop by itself due to atmospheric pressure. (Unreachable objects are eligible for garbage collection, which is non-deterministic.)
