C#中事件只能在内部调用的原因分析

public class Customer
    {
     	// 定义事件
        public event  OrderEventHandler Order;
        public string? name;
        public float? price;
        protected void onOrder(OrderEventArgs orderEventArgs)
        {
            if(Order != null)
            {
                Order.Invoke(this, orderEventArgs);
            }
        }
     ......

public class Program{
        public static void Main(string[] args)
        {
            var customer = new Customer();
            customer.name = "1";
            Waiter waiter = new Waiter();
            customer.Order += waiter.Serve;
           //  customer.Order.Invoke() 这样写无法编译
            customer.Think();
            customer.Pay();
        }
    }

// 事件的简明定义方式
public event  OrderEventHandler Order;
而C#中事件完整的定义方式是这样：
        private OrderEventHandler orderEventHandler;//委托，使用private 修饰
        public event  OrderEventHandler Order // 事件，对委托方法进行增减
        {
            add
            {
                this.orderEventHandler += value;
            }
            remove 
            { 
                this.orderEventHandler -= value; 
            }
        }

     protected void onOrder(OrderEventArgs orderEventArgs)
        {
            if(this.orderEventHandler != null)
            {
                // 调用委托
                this.orderEventHandler.Invoke(this, orderEventArgs);
            }
        }