无法将类型'BaseClass'隐式转换为'DerivedClass'

问题描述：

亲爱的开发者，

请注意以下C＃代码。

Hi dear developers,

Please observe the following C# code once.

class BaseClass
    {
        public void Method1(){}
    }

    class DerivedClass : BaseClass
    {
        public void Method2(){}
    }

    class Program
    {
        static void Main()
        {
            BaseClass bc = new BaseClass();
            DerivedClass dc = new DerivedClass();
            BaseClass bcd = new DerivedClass();
            DerivedClass dcb = new BaseClass(); /*compile time error. Cannot implicitly convert type 'BaseClass' to 'DerivedClass'. An explicit conversion exists (are you missing a cast?)*/
        }
    }

当我将基类对象分配给派生类变量编译器时抛出错误无法将类型'BaseClass'隐式转换为'DerivedClass'。但是，当我将派生类对象分配给基类对象时，我没有收到错误。

你能否澄清一下为什么会发生这种情况。

When i am assigning the base class object to the derived class variable compiler throwing the error Cannot implicitly convert type 'BaseClass' to 'DerivedClass' . But, i am not getting the error when i am assigning the derived class object to the base class object.

Can you please clarify me why it is happening.

答

您无法自动将基类实例转换为派生类实例：它将涉及系统发明数据。你可以，如果你创建一个特定的强制转换操作符，因为你可以自己填写缺失的信息，但我真的不推荐它。

想想关于它：橙子是一种水果，但不是所有水果都是橙子 - 有些是苹果。

如果你创建一个水果的实例，它不是橙色或苹果 - 它是一个通用的水果，这可能不存在。

这就是为什么你经常看到人们创建基类为 abstract 所以你甚至无法创建它们的实例。

You cannot automatically convert a base class instance into a derived class instance: it would involve the system "inventing" data. You can, if you create a specific cast operator because you can "fill in" the missing infor yourself, but I really don't recommend it at all.

Think about it: an Orange is a Fruit, but not all Fruits are Oranges - some are Apples.
If you create an instance of a Fruit, it is not an Orange or an Apple - it's a generic Fruit, which probably doesn't exist.

This is why you often see people create base classes as abstract so that you cannot even create an instance of them.

无法将类型'BaseClass'隐式转换为'DerivedClass'

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