更改集合中一项的值会影响所有重复项 [英] Changing value of one item in a collection affects all duplicate items

问题描述

我似乎遇到了一个奇怪的问题，即每次尝试更改集合中某项的值时，它都会影响所有包含相同初始值的其他项.

I seem to be having an odd issue whereby every time I try to change a value of an item in a collection, it affects all others that contain the same initial values.

下面是一个示例:

public class Product : ICloneable
{
  public int Id { get; set; }
  public string Name { get; set; }
  public int Quantity { get; set; }

  public Product()
  {
    Id = 0;
    Quantity = 0;
  }

  public Clone()
  {
    return (Product)this.MemberwiseClone();
  }
}

...

private static IEnumerable<Product> GetProducts(Product product, int quantity)
{
  for (int i = 0; i < quantity; i++)
  {
    yield return product.Clone();
  }
}

...

IEnumerable<Product> myProducts = Enumerable.Empty<Product>();
Product product1 = new Product() { Id = 0, Name = "Buzz Cola" };
Product product2 = new Product() { Id = 1, Name = "Choco Bites" };

myProducts = myProducts.Concat(GetProducts(product1, 2));
myProducts = myProducts.Concat(GetProducts(product2, 1));

//Now set the quantity of the first product to be 1.
myProducts.ElementAt(0).Quantity = 1;

foreach(Product product in myProducts)
{
  Console.WriteLine(string.Format("Id: {0}  Quantity: {1}", product.Id, product.Quantity));
}

//Output:
//Id: 0  Quantity: 1
//Id: 0  Quantity: 1 //NO!
//Id: 1  Quantity: 0

有什么想法吗?

非常感谢！

更新 我已按照建议将问题更新为包括Clone().输出仍然是相同的.

Update I have updated the question to include the Clone() as suggested. The output is still the same however.

推荐答案

您需要类似克隆方法或复制构造函数的内容.

You need something like a clone method or a copy constructor.

public class Product
{
  public int Id { get; set; }
  public string Name { get; set; }
  public int Quantity { get; set; }

  public Product()
  {
      this.Id = 0;
      this.Name = null;
      this.Quantity = 0;
  }

  public Product(Product product)
  {
      this.Id = product.id;
      this.Name = product.Name;
      this.Quantity = product.Quantity;
  }
}

IList<Product> myProducts = new List<Product>();

Product product1 = new Product() { Id = 0, Name = "Buzz Cola" };
Product product2 = new Product() { Id = 1, Name = "Choco Bites" };
Product product3 = new Product(product1); // Use copy-constructor.

myProducts.Add(product1);
myProducts.Add(product2);
myProducts.Add(product3);

myProducts[0].Quantity = 1;

现在一切都很好.您可以将其与c​​loniung方法一起使用，一次生成大量克隆.

And now everything should be fine. You can use this together with your cloniung method to produce a large number of clones at once.

仅需注意，此代码的味道仍然很差-您正在创建具有相同ID的不同产品实例.我只能猜测，但是您是否要构建像购物车这样的东西，其中包含具有数量和产品的购物车项目?如果是，那么您应该真正考虑将产品类别分为两类.并考虑您的属性aganin的可访问性.

Just to note, this code has still a very bad taste - you are creating different product instances with equal ids. I can just guess, but do you want to build something like a shopping cart with cart items having a quantity and a product? If yes, you should really think about splitting the product class into two classes. And think about the accessibility of your properties aganin.

public class Product
{
  public Int32 Id { get; private set; }
  public String Name { get; private set; }
}

public class ShoppingCartItem
{
  public Product Product { get; private set; }
  public Int32 Quantity { get; set; }
}

public class ShoppingCart
{
  public IList<ShoppingCartItem> Items { get; private set; }
}

这将解决您当前的问题，因为不再需要克隆产品.

This solves your current problems because there is no longer a need for cloning products.

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