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:

public class B : A
{
    public string s3;
}

假设我的程序创建了一个类 A 的实例.

Suppose my program created an instance of class A.

A aClassInstance = new A();

设置了一些参数:

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

此时我想创建一个 B 类的实例.但我希望 B 已经拥有我的 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.

这不起作用:

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

没有这样做:

在 A 类中创建了一个克隆方法.

Made a clone method within Class A.

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

VS 代码采用上述两种方式 - 但我遇到运行时错误

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

请帮忙

推荐答案

您遇到的基本问题是，您必须构造一个 B 类型的实例(其中包含原因类型的属性A).您克隆 A 实例的方法不起作用，因为这会为您提供 A 类型的实例，您无法将其转换为 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.

我会为 A 类和 B 类编写构造函数，它接受 A 类型的参数.B 类的构造函数只是将值传递给它的基类 A.A 类的构造函数知道如何将字段复制到自身:

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) {
    }

}

这样使用:

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

B b = new B(a);

编辑

当您不想在添加新字段或道具时更改 A 的构造函数时，可以使用反射来复制属性.使用自定义属性来装饰您要复制的内容，或仅复制 A 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));
    }
}

我没有尝试过代码，但重点应该很清楚了.

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

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