c# - 无法引发事件

Question

我正在尝试完成给定的作业，但我没有这样做。我写了一个产品类并用它做了一朵花。然后我想在鲜花数量低于 20 时引发一个事件，它应该给我一个警告。我想我在发起这个事件时遇到了困难。我确定我对委托(delegate)和事件的声明是正确的，但缺少一些东西。先感谢您。

这一行

flower.StockDecreased();  

给我这个错误:

Error   3   The event 'StokTakip.Product.StockDecreased' can only appear on the left hand side of += or -= (except when used from within the type 'StokTakip.Product')  


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace StokTakip
{
class Program
{
    static void FlowerStockDecreased()
    {
        Console.WriteLine("Flower stock decreased");
    }

    static void Main(string[] args)
    {
        Product flower = new Product("Flower", 50);
        Console.WriteLine(flower);

        flower.StockDecreased += new Product.FlowerEventHandler(FlowerStockDecreased);

        while (true)
        {
            Console.WriteLine("What is your choice");
            Console.WriteLine("[1] Stock entry quantity ");
            Console.WriteLine("[2] Stock exit quantity: ");                

            int choice = Convert.ToInt32(Console.ReadLine());

            if (choice == 1)
            {
                Console.Write("Enter stock entry quantity: ");
                flower.quantity += Convert.ToInt32(Console.ReadLine());

            }

            else if (choice == 2)
            {
                Console.Write("Enter stock exit quantity: ");
                flower.quantity -= Convert.ToInt32(Console.ReadLine());
            }              

            Console.WriteLine(flower);

            if (flower.quantity<20)
            {
                flower.StockDecreased();  //????

            }              
        }
    }       
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace StokTakip
{
public class Product
{
    public string name;
    public int quantity;

    public Product(string a, int m)
    {
        name = a;
        quantity = m;
    }       

    public override string ToString()
    {
        return "Name: "+ this.name + " Stock Quantity: " + this.quantity;
    }

    public delegate void FlowerEventHandler();
    public event FlowerEventHandler StockDecreased; 
}

Answer 1

错误信息很清楚。你不能引发这样的事件。只有声明类才能像这样调用事件并引发事件。所有其他类只能从事件中添加 (+=) 或删除 (-=) 事件处理程序。

您可以将一个公共(public)方法放入引发事件的 Product 类中，如下所示:

public void RaiseStockDecreased()
{
    if (StockDecreased != null)
        StockDecreased();
}

您可以在外部调用它。

但是话又说回来，这与正确的设计相矛盾，因为我希望 Product 类本身确定库存是增加还是减少并引发正确的事件。否则，您必须在每个想要收到库存变化通知的地方实现该逻辑。