将泛型类型参数显式转换为任何接口 [英] explicitly cast generic type parameters to any interface

问题描述

在 泛型常见问题解答:最佳实践中说:

编译器允许您将泛型类型参数显式转换为任何接口，但不能转换为类:

The compiler will let you explicitly cast generic type parameters to any interface, but not to a class:

interface ISomeInterface
{...}
class SomeClass
{...}
class MyClass<T> 
{
   void SomeMethod(T t)
   {
      ISomeInterface obj1 = (ISomeInterface)t;//Compiles
      SomeClass      obj2 = (SomeClass)t;     //Does not compile
   }
}

我认为类和接口的限制都是合理的，除非类/接口未指定为约束类型.

I see limitation reasonable for both, classes and interfaces, unless the class/interface is not specified as constraint type.

那么为什么这种行为，为什么它被允许用于接口?

So why such behavior, why it is allowed for interfaces ?

推荐答案

我相信这是因为转换到 SomeClass 可能意味着任何数量的事情，这取决于可用的转换，而转换到ISomeInterface 只能是引用转换或装箱转换.

I believe this is because the cast to SomeClass can mean any number of things depending on what conversions are available, whereas the cast to ISomeInterface can only be a reference conversion or a boxing conversion.

选项:

• 首先投射到对象:

• Cast to object first:

  SomeClass obj2 = (SomeClass) (object) t;

使用 as 代替:

  SomeClass obj2 = t as SomeClass;

显然，在第二种情况下，如果 t 是 not 一个 SomeClass，您还需要在之后执行无效检查.

Obviously in the second case you would also need to perform a nullity check afterwards in case t is not a SomeClass.

C# 4 规范的第 6.2.7 节给出了原因:

The reasoning for this is given in section 6.2.7 of the C# 4 specification:

上述规则不允许从不受约束的类型参数直接显式转换为非接口类型，这可能令人惊讶.这条规则的原因是为了防止混淆并使此类转换的语义清晰.例如，考虑以下声明:

The above rules do not permit a direct explicit conversion from an unconstrained type parameter to a non-interface type, which might be surprising. The reason for this rule is to prevent confusion and make the semantics of such conversions clear. For example, consider the following declaration:

class X<T>
{
    public static long F(T t) {
        return (long)t; // Error 
    }
} 

如果允许将 t 直接显式转换为 int，人们可能很容易期望 X.F(7) 将返回 7L.但是，它不会，因为只有在绑定时已知类型为数字时才考虑标准数字转换.为了使语义清楚，上面的例子必须改为:

If the direct explicit conversion of t to int were permitted, one might easily expect that X<int>.F(7) would return 7L. However, it would not, because the standard numeric conversions are only considered when the types are known to be numeric at binding-time. In order to make the semantics clear, the above example must instead be written:

class X<T>
{
    public static long F(T t) {
        return (long)(object)t; // Ok, but will only work when T is long
    }
}

此代码现在可以编译，但执行 X.F(7) 会在运行时抛出异常，因为装箱的 int 不能直接转换为 long.>

This code will now compile but executing X<int>.F(7) would then throw an exception at run-time, since a boxed int cannot be converted directly to a long.

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