不包含采用 0 个参数的构造函数 [英] Does not contain a constructor that takes 0 arguments

问题描述

我收到一条错误消息，指出产品不包含采用 0 个参数的构造函数"；来自以下代码:

I get an error stating "Products does not contain a constructor that takes 0 arguments" from the following code:

public class Products
{
    string id;
    string name;
    double price;
    int soldCount;
    int stockCount;

    public Products(string id, string name, double price, 
                      int soldCount, int stockCount, double tax)
    {
        this.id = id;
        this.name = name;
        this.price = price;
        this.soldCount = soldCount;
        this.stockCount = stockCount;
    }
}

//I have got some get and set values for the code above 
//but it would have been too long to put in here

public class FoodProducts : Products
{
    public FoodProduct()
    {
        Console.WriteLine("This is food product");
    }

    public void Limit()
    {
        Console.WriteLine("This is an Attribute of a Product");
    }
}

推荐答案

一些关于 C# 的规则在这里发挥作用:

Several rules about C# come into play here:

1. 每个类都必须有一个构造函数(为了更好地构造)

1. Each class must have a constructor (In order to be, well constructed)

如果您不提供构造函数，编译器会自动为您提供一个构造函数，无需更改.

If you do not provide a constructor, a constructor will be provided for you, free of change, automatically by the compiler.

这意味着类

class Demo{}

在编译时提供一个空的构造函数，成为

upon compilation is provided with an empty constructor, becoming

class Demo{
   public Demo(){}
}

我能做到

Demo instance = new Demo();

如果您确实提供了构造函数(任何具有任何签名的构造函数)，则不会生成空构造函数

If you do provide a constructor (any constructor with any signature), the empty constructor will not be generated

class Demo{
   public Demo(int parameter){}
}

Demo instance = new Demo(); //this code now fails
Demo instance = new Demo(3); //this code now succeeds

这似乎有点违反直觉，因为添加代码似乎会破坏现有的不相关代码，但这是 C# 团队的设计决定，我们必须接受它.

This can seem a bit counter-intuitive, because adding code seems to break existing unrelated code, but it's a design decision of the C# team, and we have to live with it.

调用派生类的构造函数时，如果没有指定要调用的基类构造函数，编译器会调用空的基类构造函数，所以

When you call a constructor of a derived class, if you do not specify a base class constructor to be called, the compiler calls the empty base class constructor, so

class Derived:Base {
   public Derived(){}
}

变成

class Derived:Base {
   public Derived() : base() {}
}

所以，为了构造你的派生类，你必须在基类上有一个无参数的构造函数.看你怎么给Products加了一个构造函数，编译器没有生成默认的构造函数，你需要在你的代码中显式的添加，比如:

So, in order to construct your derived class, you must have a parameterless constructor on the base class. Seeing how you added a constructor to the Products, and the compiler did not generate the default constructor, you need to explicitly add it in your code, like:

public Products()
{
}

或者从派生构造函数中显式调用它

or explicitly call it from the derived constructor

public FoodProduct()
       : base(string.Empty, string.Empty, 0, 0, 0, 0)
{
}

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