Question

我正在尝试做一项任务，但我没有这样做。我写了一个产品类，并用它做了一朵花。然后我想在花量低于20时举起一个事件，它应该给我一个警告。我想我很难举起这个活动。我确信我对代表和事件的判断是正确的，但缺少某些东西。先感谢您。

这一行

flower.StockDecreased();

给了我这个错误：

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

错误信息非常清楚。你不能举办这样的活动。只有声明类可以像这样调用事件，从而引发事件。所有其他类只能从事件中添加（+=）或删除（-=）事件处理程序。

你可以做的是将一个公共方法放入Product类，引发这样的事件：

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

你可以在外面打电话。

但话又说回来，这与正确的设计相矛盾，因为我希望Product类本身可以确定股票是增加还是减少，并提出适当的事件。否则，您必须在希望收到库存变化通知的每个地方实施该逻辑。

Answer 2

您的产品类应如下所示：

public class Product
{
    public event FlowerEventHandler StockDecreased; 

    private int _quantity;
    public int Quantity
    {
        get { return _quantity; }
        set
        {
            int newQuantity = value;
            if (newQuantity < _quantity)
            {
               if (StockDecreased != null) StockDecreased();
            }
            _quantity = newQuantity;
        }
    }

    // Other stuff
}

虽然更好的模式可能是StockChange方法可以检查更改是正面还是负面，并引发相应的事件。

不能举起活动

2 个答案: