从 C# 类型变量初始化通用变量 [英] Initializing a Generic variable from a C# Type Variable

问题描述

我有一个类，它将泛型类型作为其初始化的一部分.

I have a class that takes a Generic Type as part of its initialization.

public class AnimalContext<T>
{
    public DoAnimalStuff()
    {
        //AnimalType Specific Code
    }
}

我现在能做的就是

AnimalContext<Donkey> donkeyContext = new AnimalContext<Donkey>();
AnimalContext<Orca> orcaContext = new AnimalContext<Orca>();

但是我需要/想要做的是能够声明一个初始化为仅在运行时已知的类型的 AnimalContext.例如，

But what I need/want to do is be able to declare an AnimalContext initialized to a type that is only known at runtime. For instance,

Animal a = MyFavoriteAnimal(); //returns an instance of a class 
                               //implementing an animal
AnimalContext<a.GetType()> a_Context = new AnimalContext<a.GetType()>();
a_Context.DoAnimalStuff();

这甚至可能吗?我似乎无法在网上找到答案.

Is this even possible? I can't seem to find an answer for this online.

推荐答案

这部分的含义是可能的:

new AnimalContext<a.GetType()>();

显然，确切的语法是错误的，我们将继续讨论，但是可以在运行时构造一个泛型类型的实例，如果您不这样做'在运行时之前不知道类型参数.

Obviously that exact syntax is wrong, and we'll get to that, but it is possible to construct an instance of a generic type at runtime when you don't know the type parameters until runtime.

这部分的意思是不是:

AnimalContext<a.GetType()> a_Context

也就是说，如果您在编译时不知道类型参数，就不可能将变量键入为泛型类型.泛型是编译时构造，依赖于在编译时提供的类型信息.鉴于此，如果您在编译时不知道类型，您将失去泛型的所有好处.

That is, it is impossible to type a variable as a generic type if you don't know the type parameters at compile-time. Generics are compile-time constructs, and rely on having the type information available at compile-time. Given this, you lose all the benefits of generics if you don't know the types at compile-time.

现在，当您直到运行时才知道类型时，要在运行时构造泛型类型的实例，您可以说:

Now, to construct an instance of a generic type at runtime when you don't know the type until runtime, you can say:

var type = typeof(AnimalContext<>).MakeGenericType(a.GetType());
var a_Context = Activator.CreateInstance(type);   

注意a_context 的编译时类型是object.您必须将 a_context 转换为定义您需要访问的方法的类型或接口.通常你会看到人们在这里做的是让泛型类型 AnimalContext 实现一些接口(比如 IAnimalContext)或从定义他们需要的方法的非通用基类(比如 AnimalContext)(这样你就可以将 a_context 转换为接口或非通用基类).另一种选择是使用 dynamic.但同样，请记住，这样做没有泛型类型的好处.

Note that the compile-time type of a_context is object. You will have to cast a_context to a type or interface that defines the methods you need to access. Often what you'll see people do here is have the generic type AnimalContext<T> implement some interface (say IAnimalContext) or inherit from a non-generic base class (say AnimalContext) that defines the methods they need (so then you can cast a_context to the interface or the non-generic base class). Another alternative is to use dynamic. But again, keep in mind, you have none of the benefits of generic types in doing this.

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