C＃继承。从基类派生类 [英] C# inheritance. Derived class from Base class

问题描述

我有一个基类

public class A   
{
    public string s1;
    public string s2;
}

我也有一个派生类：

I also have a derived class :

public class B : A
{
    public string s3;
}

假设我的程序中创建A类的一个实例。

Suppose my program created an instance of class A.

A aClassInstance = new A();

一些参数设置：

some parameters were set:

aClassInstance.s1 = "string 1";
aClassInstance.s2 = "string 2";

在这一点上我想创建B类的一个实例，但我想B到已经。有我的A类实例的值

At this point I would like to create an instance of class B. But I would like B to already have the values of my instance of class A.

这不工作：

public B bClassInstance = new B():
bClassInstance = (B)aClassInstance;

也不是这：

NEITHER DID THIS:

发了。类中的clone方法

Made a clone method within Class A.

public B cloneA() {    
    A a = new A();
    a = (A)this.MemberwiseClone()
    return(B)a;
}

的

VS代码采用两种以上 ​​- 但我得到运行时错误

The VS code takes both of the above - but I get run-time errors

请帮忙

推荐答案

你有基本的问题是，你必须构造型 B 的一个实例（包含事业类型的属性 A ）。你的方法克隆 A 实例将无法正常工作，因为这样可以给你键入 A 的一个实例，你不能转换为 b 。

The base problem you have is, that you have to construct an instance of type B (which contains of cause the properties of type A). Your approach to clone an A instance won't work, because that gives you an instance of type A, which you can't convert to B.

我会写的类和b类的构造函数这需要类型的参数A. b类的构造函数只是传递价值，以它的基类A. A类的构造函数知道如何领域复制到其自身：

I would write constructors for class A and B which takes a parameter of type A. The constructor of class B just passes the value to its base class A. The constructor of class A knows how to copy the fields to itself:

class A {
    public A(A copyMe) {
        s1 = copyMe.s1;
        ...
    }

class B : A {

    public B(A aInstance) : base(aInstance) {
    }

}

使用这种方式：

A a = new A();
a.s1 = "...";

B b = new B(a);

修改

当你不希望有添加新字段或道具时改变 A 的构造函数，你可以使用反射来复制属性。要么使用自定义属性来装点你想要的复制，或复制的 A 刚才的所有道具/字段：

When you don't want to have to change the constructor of A when adding new fields or props, you could use reflection to copy the properties. Either use a custom attribute to decorate what you want to copy, or copy just all props/fields of A:

public A (A copyMe) {
    Type t = copyMe.GetType();
    foreach (FieldInfo fieldInf in t.GetFields())
    {
        fieldInf.SetValue(this, fieldInf.GetValue(copyMe));
    }
    foreach (PropertyInfo propInf in t.GetProperties())
    {
        propInf.SetValue(this, propInf.GetValue(copyMe));
    }
}

我没有带试过代码，但关键应该变得清晰起来。

I havn't tried the code, but the point should become clear.

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