Activator.CreateInstance（）烦恼 [英] Activator.CreateInstance() troubles

问题描述

我有一个工厂，它应该创建在运行时继承自Foo类的对象。我认为System.Activator.CreateInstance的返回类型与它创建的对象的类型相同，但从以下错误消息判断，其返回类型是Object。

< blockquote>

错误1无法将类型object隐式转换为cs_sandbox.Foo。显式转换存在（是否缺少转换？）F：\projects\cs_sandbox\Form1.cs 46 24 cs_sandbox

OK，所以也许我缺少一个转换，但

  return（t）System.Activator .CreateInstance（t）; 
  

会导致另一个错误消息，我必须承认 - 对我来说没有意义：

错误1无法找到类型或名称空间名称't'（是否缺少using指令或程序集引用？）F ：\projects\cs_sandbox\Form1.cs 45 25 cs_sandbox

这里是我的代码：

class Foo {}
class FooChild1：Foo {}
class FooChild2：Foo {}

MyFactory
{
public static Foo CreateInstance（string s）
{
Type t;
if（s.StartsWith（abcdef））
{
t = typeof（FooChild1）;
return System.Activator.CreateInstance（t）;
}
else
{
t = typeof（FooChild2）;
return System.Activator.CreateInstance（t）;
}
}
}


如何修复此代码？

解决方案

或者，如果不可修复，还有什么其他方法来创建在运行时继承自特定类的对象？您需要将返回的对象转换为 Foo 类型。将它转换为在变量中定义的类型没有意义。它应该是编译器知道的，因为通过继承层次结构的整个点通过满足编译器的静态类型检查。

  return （Foo）System.Activator.CreateInstance（t）; 
  

有一个通用版本， System.Activator.CreateInstance< T& code>，它创建一个已知的类型（不是类型变量，而是类型参数或静态已知的类型，在后一种情况下，它没有什么意义）：

  return System.Activator.CreateInstance< FooChild1>（）; 
  




I have a factory that is supposed to create objects that inherit from class Foo at run-time. I would think that System.Activator.CreateInstance's return type was the same as the type of an object it's creating, but judging from the following error message, its return type is Object.

Error 1 Cannot implicitly convert type 'object' to 'cs_sandbox.Foo'. An explicit conversion exists (are you missing a cast?) F:\projects\cs_sandbox\Form1.cs 46 24 cs_sandbox

OK, so maybe I am missing a cast, but

return (t)System.Activator.CreateInstance(t);

results in yet another error message, which -- I must admit -- makes no sense to me:

Error 1 The type or namespace name 't' could not be found (are you missing a using directive or an assembly reference?) F:\projects\cs_sandbox\Form1.cs 45 25 cs_sandbox

And here's my code:

class Foo { }
class FooChild1 : Foo { }
class FooChild2 : Foo { }

class MyFactory
{
    public static Foo CreateInstance(string s)
    {
        Type t;
        if (s.StartsWith("abcdef"))
        {
            t = typeof(FooChild1);
            return System.Activator.CreateInstance(t);
        }
        else
        {
            t = typeof(FooChild2);
            return System.Activator.CreateInstance(t);
        }
    }
}

How can I fix this code? Or, if it's not fixable, what are other ways of creating objects that inherit from a specific class at run-time?

解决方案

You need to cast the returned object to Foo type. It doesn't make sense to cast it to a type defined in a variable. It should be known by the compiler, as the whole point of casting through the inheritance hierarchy is satisfying compiler's static type checking.

return (Foo)System.Activator.CreateInstance(t);

There's a generic version, System.Activator.CreateInstance<T>, which creates a known type (not a type variable but a type argument or a statically known type, in the latter case, it doesn't make much sense though):

return System.Activator.CreateInstance<FooChild1>();

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