// A type that is defined as a class is a reference type.

//[access modifier] - [class] - [identifier]
public class Customer
{
  // Fields, properties, methods and events go here...
}

Customer object1 = new Customer(); // object1 is a reference to an allocated space that will know where the object exists.
Customer object2; // Reference to null

Customer object3 = new Customer(); 
Customer object4 = object3; // object4 has the same reference as object3. If any of both instances changes, the other one does as well. Not recommended to do that.

public class Container
{
  private int _capacity;

  public Container(int capacity) => _capacity = capacity;
}

public class Container(int capacity)
{
  private int _capacity = capacity;
}

public class Person
{
  public required string LastName { get; set; }
  public required string FirstName { get; set; }
}

var p1 = new Person(); // Error! Required properties not set
var p2 = new Person() { FirstName = "Grace", LastName = "Hopper" };