(开放通用类型)没有指定参数的通用类型 [英] (open generic type) typeof generic type with no parameters specified

问题描述

有人可以在没有指定具体参数的情况下向我解释 typeof(SomeGenericType<> )的C#语言需求.

我整理了以下示例:

  var t1 = typeof(Nullable<>);var t2 = typeof(Nullable< int>);var q = 1 as int ?;var b1 = t1.IsInstanceOfType(q);//错误的var b2 = t2.IsInstanceOfType(q);//真的 

我首先想到 typeof(Nullable<>)比 t2 更通用，后者指定了通用参数 int ，但是 b1 原来是 false -因此， int?的实例不是 Nullable<> 的实例.

那么应该如何将 b1 定义为 true 的变量?它有什么实际用途?

解决方案

那么应该如何为b1定义一个变量呢?

不能.(实际上，使用 Nullable< T> 还是会遇到有趣的拳击问题，但是我们可以...)

在执行时，值始终是 closed 类型的实例. Nullable<> ， List<> 是 open 通用类型.在这种类型上调用 IsInstanceOfType 从来没有用.但这并不意味着它没用.

通常在反射中使用开放类型.例如:

  public IList CreateList(Type elementType){类型closeType = typeof(List<>).MakeGenericType(elementType);返回(IList)Activator.CreateInstance(closedType);} 

可能有较高的代码，而是通用的，但是调用较低的级别时传递的是 Type 值-然后列表可以返回堆栈并进行强制转换到 IEnumerable< T> 以获取 T 的适当值.

同样，您可能希望创建一个带有反射的封闭类型以在其上调用方法，等等.

您还可以 使用它来查找特定类型是否为 some 类型实参实现了通用接口-对于实现的每个接口，您都可以查明它是否通用，获取通用类型定义，然后查看其是否等于(例如) IEnumerable<> .

Can somebody explain me the need in C# language for typeof(SomeGenericType<>), with no concrete parameters specified.

I put together the following example:

        var t1 = typeof(Nullable<>);
        var t2 = typeof(Nullable<int>);
        var q = 1 as int?;

        var b1 = t1.IsInstanceOfType(q); //false
        var b2 = t2.IsInstanceOfType(q); //true

I first thought typeof(Nullable<>) is "more generic" than t2, which specifies generic parameter int, but b1 turns out to be false - so instance of int? is not instance of Nullable<>.

So how a variable should be defined for b1 to be true? what practical uses does it have?

解决方案

So how a variable should be defined for b1 to be true?

It can't. (In fact, with Nullable<T> you'll run into interesting boxing problems anyway, but there we go...)

At execution time, values are always instances of closed types. Nullable<>, List<> are open generic types. It's never useful to call IsInstanceOfType on such a type. That doesn't mean it's useless though.

Typically open types are used in reflection. For example:

public IList CreateList(Type elementType)
{
    Type closedType = typeof(List<>).MakeGenericType(elementType);
    return (IList) Activator.CreateInstance(closedType);
}

There can be code high up which is generic, but calls into lower levels passing in Type values instead - the list could then go back up the stack and be cast to IEnumerable<T> for the appropriate value of T.

Likewise you may want to create a closed type with reflection to call a method on it, etc.

You can also use it to find out whether a particular type implements a generic interface for some type argument - for each interface implemented, you can find out if it's generic, get the generic type definition, and see whether that's equal to (say) IEnumerable<>.

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