Classes, properties, and invariants

The idea

A class groups state with operations that maintain it. Properties offer field-like access syntax while allowing the class to control reading and writing. A property with a private setter can be updated by the class but not freely assigned by callers. Constructors establish a valid starting state, and methods preserve the conditions that define a valid object.

Class variables hold references. Assigning one to another copies the reference, not the object, so both names can observe the same mutation. Do not assume every C# type behaves this way: structs have value semantics. Decide whether your model requires shared identity or independent values before choosing its representation.

using System;
var meter = new Meter();
var alias = meter;
alias.Add(3);
Console.WriteLine(meter.Value);
class Meter {
  public int Value { get; private set; }
  public void Add(int amount) {
    if (amount < 0) throw new ArgumentOutOfRangeException(nameof(amount));
    Value += amount;
  }
}

3

meter and alias refer to the same Meter instance. Its public method validates the update, while the private setter prevents direct external assignment.