Question

在使用关键字＆＃34; as＆＃34;时，如何将基类数据放入子类对象中。我尝试了下面的代码，但它返回了空数据。

class BaseC
{
    public int BaseId { get; set; }
    public string BaseName { get; set; }
}

class DerivedC: BaseC
{
    public int DerivedId { get; set; }
    public string DerivedName { get; set; }
}
class Program
{
    static void Main(string[] args)
    {

        BaseC baseC = new BaseC();
        baseC.BaseId = 1;
        baseC.BaseName = "base class name ";
        var derivedC = baseC as DerivedC;
    }
}

Answer 1

这是的行为：

您的代码（简化）：

 BaseC baseC = new BaseC();

 // null: result is a BaseC instance and not DerivedC one
 DerivedC result = baseC as DerivedC;

反向代码（可能是您期望看到的）：

 BaseC baseC = new DerivedC(); // <- Now, DerivedC instance created

 // not null: result is in fact a DerivedC instance: new DerivedC()
 DerivedC result = baseC as DerivedC; 

 // not null: DerivedC is inherired from BaseC
 // and that's why any DerivedC instances are BaseC instances as well
 BaseC result2 = baseC as BaseC;

Answer 2

这不会奏效。将BaseC替换为Animal，将DerivedC替换为Cow，您就会知道原因。

运行时无法从Cow的实例创建Animal，因为缺少信息。这将有效：

BaseC baseC = new DerivedC();

因为实例实际上是一个DerivedC。

Answer 3

baseC不是DerivedC的实例，因此as运算符将始终返回null。

但如果您将Main的第一行更改为：

，则会有效

BaseC baseC = new DerivedC();

Answer 4

至于一个实用的解决方案（也许你在尝试 as 关键字之前已经这样做了，但我想我会把它扔出去）：< / em>的

如果您所有可用的是基本实例（并且由于某种原因无法更改），并且您想要填充派生实例，您可以考虑添加一个接受该实例的构造函数基类作为参数：

class DerivedC : BaseC { public DerivedC() {} // req'd so you can still create an instance without a BaseC public DerivedC(BaseC baseC) { BaseId = baseC.BaseId; BaseName = baseC.BaseName; } public int DerivedId { get; set; } public string DerivedName { get; set; } }

然后这样称呼：

var derivedC = new DerivedC(baseC);

至少可以减少代码重复，因此您不会在多个位置手动分配值。

＆＃34;如＆＃34; c＃中的关键字返回空数据

4 个答案: