.NET (Core and Framework)

George L. Jackson wrote:

Use an event (delegate).

public class MyDateTime {
   private DateTime _value;
   public event EventHandler ValueChanged;
   public MyDateTime(DateTime value) {
      _value = value;
   }
   public DateTime Value {
      get { return _value; }
      set {
         _value = value;
         OnValueChanged(EventArgs.Empty);
      }
   }
   protected virtual void OnValueChanged(EventArgs ea) {
      if (ValueChanged != null)
         ValueChanged(this, ea);
   }
}

class SomeClass
{
  public MyDateTime Field
  {
    get
    {
       MyDateTime result = new MyDateTime(new System.DateTime());
       result.ValueChanged += new EventHandler(OnFieldValueChanged);
       return result;
    }
  }
  private void OnFieldValueChanged(object sender, EventArgs ea) {
     //value has changed so do something
  }
}

class SomeClass : IDisposable
{
  private MyDateTime _field;
  public SomeClass() {
     _field = new MyDateTime(new System.DateTime());
     _field.ValueChanged += new EventHandler(OnFieldValueChanged);
  }
  public MyDateTime Field
  {
     get { return _field; }
  }
  public void Dispose() {
     //unbind the eventhandler when the object should be disposed
     _field.ValueChanged -= new EventHandler(OnFieldValueChanged);
  }
  private void OnFieldValueChanged(object sender, EventArgs ea) {
     //value has changed so do something
  }
}