显式转换泛型类型参数的任何界面 [英] explicitly cast generic type parameters to any interface

问题描述

在泛型常见问题解答：最佳实践说：

编译器将让您显式转换泛型类型参数的任何接口，而不是一类：

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
   }
}

我看到不限于合​​理的为，类和接口，除非类/接口没有指定为约束类型。

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

那么，为什么这样的行为，为什么它是允许的接口？

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

推荐答案

我相信这是因为剧组到 SomeClass的可根据什么转换意味着许多东西都可用，而投地 ISomeInterface 只能是一个参考转换或装箱转换。

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：

• 演员到第一个对象：

• Cast to object first:

SomeClass obj2 = (SomeClass) (object) t;

使用为而不是：

SomeClass obj2 = t as SomeClass;

显然，在第二种情况下，你还需要的情况下，请稍后执行无效检查 T 终止的不的一个 SomeClass的。

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

编辑：推理为此在C＃4规范第6.2.7所示：

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

上面规则不允许从不受约束的类型参数到非接口类型，这可能是令人惊讶的直接显式转换。这样做的原因法则是prevent混乱，使这种转换的语义清晰。例如，请考虑下面的声明：

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T的直接显式转换为int被允许，人们可能很容易想到的是 X＆LT; INT＆GT; .F（7）将返回7L。但是，它不会，因为当类型是已知的数字在结合时的标准数字转换只考虑。为了使语义清楚，上面的例子而必须被写成：

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
    }
}

这code现在可以编译，但执行 X＆LT; INT＆GT; .F（7）然后将引发在运行时异常，因为装箱的int不能直接转换为长

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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