将对象转换为派生类中的对象 [英] Cast an object into an object from a derived class

问题描述

我有一个很好的类Record:

public class Record 
{
    protected string table;
    protected string idcolumn;
    public Record(string _table, string _idcol, int _id) 
    {
        table = _table;
        idcolumn = _idcol;
        Id = _id;
    }
}

我还有一个类Order，它是从Record派生的，该类实现了额外的方法，仅适用于某种记录类型:

I also have a class Order that is derived from Record, that implements extra methods, only applicable on a certain type of record:

class Order : Record 
{
    public void Start() 
    {
    }
}

在我的应用程序中，我有一个类型为Record的对象theRecord，我想将其强制转换为Order，以便可以在其上调用Start方法.

In my application, I have an object theRecord of type Record that I want to cast to Order, so that I can call the Start method on it.

我尝试投射它:

Order r = (Order)theRecord;

但是会抛出一个InvalidCastException.

我认为我可以为Order创建一个新的构造函数，该构造函数采用Record，但是我已经有了该对象(通过从数据库中获取记录来构建该对象).

I am thinking that I could create a new constructor for Order that takes a Record, but I already have the object (which is built by fetching a record from the database).

我该如何正确实现呢?

推荐答案

我在评论中说

您可以将一只狗投向动物，但不能将动物投向狗(除非该动物是狗).更清晰所有的狗都是动物，但并非所有的动物都是狗

You can cast a Dog to Animal but not Animal to Dog(unless that animal is Dog). More clearer All Dogs are Animal but not all Animals are Dogs

Record record = new Record();
Order order = (Order)record;//wont work since record is some record not an Order

要使其正常工作，您必须执行以下操作

To make it work you've to do something like this

Record record = new Order();

然后您可以像这样投射它

Then you can cast it like this

Order order = (Order)record;//works since record is Order

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