Copying a value and copying a reference are different

First, build a mental model

A struct variable contains a value whose fields are copied on ordinary assignment. A class variable contains a reference; copying that reference gives another route to the same object.

1. Value versus reference describes copy and identity semantics. It is not a reliable shortcut for declaring that everything always lives on a particular memory region.
2. Changing a copied struct's numeric field does not change the original struct.
3. Changing a class instance through one copied reference is visible through the other reference.
4. A struct containing a List still contains a reference to that list. Copying the struct does not recursively clone its referenced objects.

var point = new Point { X = 1 };
var pointCopy = point;
pointCopy.X = 9;
var player = new Player { Score = 1 };
var alias = player;
alias.Score = 9;
Console.WriteLine(point.X);
Console.WriteLine(player.Score);

struct Point { public int X; }
sealed class Player { public int Score { get; set; } }

1. Struct assignment

   pointCopy starts with its own copied X value.

2. Reference assignment

   player and alias point to the same Player object.

3. Mutation

   The original point stays at 1; the shared Player now has Score 9.