public void Unregister(ICurrencyObserver observer)
 {
     if (this.observers.Contains(observer))
     {
         this.observers.Remove(observer);
     }
 }
 public void Unregister(ICurrencyObserver observer)
 {
     if (this.observers.Contains(observer))
     {
         this.observers.Remove(observer);
     }
 }
 public void Register(ICurrencyObserver observer)
 {
     if (!this.observers.Contains(observer))
     {
         this.observers.Add(observer);
     }
 }
 public void Register(ICurrencyObserver observer)
 {
     if (!this.observers.Contains(observer))
     {
         this.observers.Add(observer);
     }
 }