c# - .NET - 使用 "as"显式转换与 (someType)someobject 有何不同(在内部)，为什么？

Question

我理解当您使用这样的显式强制转换时:

(someType)someobject

如果 someobject 不是真正的 someType，您可能会得到一个无效的转换异常。

据我所知，当您像这样使用 as 进行转换时:

myObject = someObject as someType

如果 someObject 不是真正的 someType，

myObject 将被呈现为 null。

如何对这些进行不同的评估，为什么？

Answer 1

John Skeet 有一个 C# faq他解释了两个运营商之间的差异。请参阅“使用强制转换语法和 as 运算符有什么区别？”段落。

引用:

Using the as operator differs from a cast in C# in three important ways:

1. It returns null when the variable you are trying to convert is not of the requested type or in its inheritance chain, instead of throwing an exception.
2. It can only be applied to reference type variables converting to reference types.
3. Using as will not perform user-defined conversions, such as implicit or explicit conversion operators, which casting syntax will do.

There are in fact two completely different operations defined in IL that handle these two keywords (the castclass and isinst instructions) - it's not just "syntactic sugar" written by C# to get this different behavior. The as operator appears to be slightly faster in v1.0 and v1.1 of Microsoft's CLR compared to casting (even in cases where there are no invalid casts which would severely lower casting's performance due to exceptions).