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将基类转换为派生类[重复]

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这个问题在这里已有答案:

在C#中是否可以将基类对象显式转换为其派生类之一?目前我认为我必须为我的派生类创建一个构造函数,它接受一个基类对象作为参数并复制属性值 . 我真的不喜欢这个想法,所以我想尽可能避免它 .

这似乎不应该工作(对象被实例化为新的基础,因此不应该为派生类的额外成员分配内存)但是C#似乎允许我这样做:

class BaseClass
{
  ... some stuff ...
}

class DerivedClass : BaseClass
{
    public bool MyDerivedProperty{ get; set; }
}


static void Main(string[] args)
{
    BaseClass myBaseObject = new BaseClass();
    DerivedClass myDerivedObject = myBaseObject as DerivedClass;

    myDerivedObject.MyDerivedProperty = true;
}

6 回答

  • 1

    不,没有像你说的那样转换类的内置方法 . 最简单的方法是执行您的建议:创建一个 DerivedClass(BaseClass) 构造函数 . 其他选项基本上可以自动地将属性从基础复制到派生实例,例如,使用反射 .

    您使用 as 发布的代码将编译,如我所见,但在运行时会抛出空引用异常,因为 myBaseObject as DerivedClass 将评估为 null ,因为它不是 DerivedClass 的实例 .

  • 1

    那是不可能的 . 但你可以使用像AutoMapper这样的Object Mapper

    例:

    class A
    {
        public int IntProp { get; set; }
    }
    class B
    {
        public int IntProp { get; set; }
        public string StrProp { get; set; }
    }
    

    在global.asax或应用程序启动中:

    AutoMapper.Mapper.CreateMap<A, B>();
    

    用法:

    var b = AutoMapper.Mapper.Map<B>(a);
    

    它可以通过流畅的API轻松配置 .

  • 6

    我找到了一个解决方案,并不是说它是最好的解决方案,但它对我来说很干净,并且不需要对我的代码进行任何重大更改 . 我的代码看起来与你的相似,直到我意识到它不起作用 .

    My Base Class

    public class MyBaseClass
    {
       public string BaseProperty1 { get; set; }
       public string BaseProperty2 { get; set; }
       public string BaseProperty3 { get; set; }
       public string BaseProperty4 { get; set; }
       public string BaseProperty5 { get; set; }
    }
    

    My Derived Class

    public class MyDerivedClass : MyBaseClass
    {
       public string DerivedProperty1 { get; set; }
       public string DerivedProperty2 { get; set; }
       public string DerivedProperty3 { get; set; }
    }
    

    Previous method to get a populated base class

    public MyBaseClass GetPopulatedBaseClass()
    {
       var myBaseClass = new MyBaseClass();
    
       myBaseClass.BaseProperty1 = "Something"
       myBaseClass.BaseProperty2 = "Something else"
       myBaseClass.BaseProperty3 = "Something more"
       //etc...
    
       return myBaseClass;
    }
    

    Before I was trying this, which gave me a unable to cast error

    public MyDerivedClass GetPopulatedDerivedClass()
    {
       var newDerivedClass = (MyDerivedClass)GetPopulatedBaseClass();
    
       newDerivedClass.UniqueProperty1 = "Some One";
       newDerivedClass.UniqueProperty2 = "Some Thing";
       newDerivedClass.UniqueProperty3 = "Some Thing Else";
    
       return newDerivedClass;
    }
    

    我改变了我的代码,如下所示,它似乎工作,现在更有意义:

    Old

    public MyBaseClass GetPopulatedBaseClass()
    {
       var myBaseClass = new MyBaseClass();
    
       myBaseClass.BaseProperty1 = "Something"
       myBaseClass.BaseProperty2 = "Something else"
       myBaseClass.BaseProperty3 = "Something more"
       //etc...
    
       return myBaseClass;
    }
    

    New

    public void FillBaseClass(MyBaseClass myBaseClass)
    {
       myBaseClass.BaseProperty1 = "Something"
       myBaseClass.BaseProperty2 = "Something else"
       myBaseClass.BaseProperty3 = "Something more"
       //etc...
    }
    

    Old

    public MyDerivedClass GetPopulatedDerivedClass()
    {
       var newDerivedClass = (MyDerivedClass)GetPopulatedBaseClass();
    
       newDerivedClass.UniqueProperty1 = "Some One";
       newDerivedClass.UniqueProperty2 = "Some Thing";
       newDerivedClass.UniqueProperty3 = "Some Thing Else";
    
       return newDerivedClass;
    }
    

    New

    public MyDerivedClass GetPopulatedDerivedClass()
    {
       var newDerivedClass = new MyDerivedClass();
    
       FillBaseClass(newDerivedClass);
    
       newDerivedClass.UniqueProperty1 = "Some One";
       newDerivedClass.UniqueProperty2 = "Some Thing";
       newDerivedClass.UniqueProperty3 = "Some Thing Else";
    
       return newDerivedClass;
    }
    
  • 9

    您可以自己实现转换,但我不建议这样做 . 如果要执行此操作,请查看Decorator Pattern以扩展现有对象的功能 .

  • 43

    不,没有内置的转换 . 您需要创建一个构造函数,就像您提到的那样,或者其他一些转换方法 .

    此外,由于BaseClass不是DerivedClass,myDerivedObject将为null,并且上面的最后一行将抛出空引用异常 .

  • 81

    不,这是不可能的 . 唯一可行的方法是

    static void Main(string[] args)
    {
        BaseClass myBaseObject = new DerivedClass();
        DerivedClass myDerivedObject = myBaseObject as DerivedClass;
    
        myDerivedObject.MyDerivedProperty = true;
    }
    

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