将变量转换为由另一个Type变量表示的类型？ [英] Cast a variable to a type represented by another Type variable?

问题描述

我知道这样的问题之前已被问过，我怀疑是可能的，但我只是想确保不是这样。

I'm aware that questions like this have been asked before and I doubt it's possible, but I just wanted to make 100% sure that it isn't.

在VB.net或C＃（任一种语言，没有关系），我想把一个变量转换为另一个类型变量表示的类型。下面是C＃中需要的代码类型的示例：

In VB.net or C# (either language, it doesn't matter), I want to cast a variable to a type represented by another Type variable. Here's an example of the kind of code that would be needed in C#:

Object castMe = new Foo();
Type castTo = typeof(Foo);
Foo beenCast = (castTo.GetRepresentedType())castMe;
beenCast.MethodInFoo();

...或在VB中，类似：

... or in VB, something like:

Dim castMe As Object = New Foo()
Dim castTo As Type = GetType(Foo)
Dim beenCast As Foo = CType(castMe, castTo.GetRepresentedType())
beenCast.MethodInFoo()

它将在运行时为cast类型参数返回一个Type，而不是一个实际的编译时类型（即 CType（castMe，Foo））。我不太明白为什么你不能这样做，虽然...肯定，运行时转换错误可能会导致，但你也可以得到他们，当你指定一个编译时类型。如果castMe不是Foo的实例，那么即使 CType（castMe，Foo）仍然会抛出 InvalidCastException 。 / p>

The big problem is obviously specifying a method which will return a Type at runtime for the cast type argument, rather than an actual compile-time type (ie. CType(castMe, Foo)). I don't quite understand why you can't do this, though... sure, runtime cast errors may result, but you can also get them when you DO specify a compile-time type. If castMe is not an instance of Foo, then even CType(castMe, Foo) will still throw an InvalidCastException.

推荐答案

如果您确实知道要投射的类型，像这样：

If you do know to which type you want to cast you will have to end up with something like this:

public static T Cast<T>(object o) {
  return (T)o;
}

T现在是您的类型参数。

T is now your type argument.

如果不要知道所投递的类型，编译器也无法知道。这是一个静态类型语言为你。如果你不喜欢这样的事情，你可能想要有一个漫步，例如。 ruby。

If you don't know what the type is that you cast to, the compiler can't know either. That's a statically typed language for you. If you don't like such things you may want to have a wander through e.g. ruby.

在这种情况下，通常的响应是通过基类或接口的abstactions - 将常见的行为提取到可以在静态类型语言中使用的属性，事件和方法，即使你不知道在给定情况下的具体类型。

In cases like that, usual responses are abstactions via base classes or interfaces - extract common behaviour into properties, events and methods that can be used in a statically typed language, even though you don't know the specific type in a given situation.

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