是运营商检查如果一个对象可以转换为特定类型。

is

The is operator checks if an object can be cast to a specific type.

例如：

if (someObject is StringBuilder) ...

为

为运营商尝试将对象强制转换为特定类型，并返回null如果失败。

as

The as operator attempts to cast an object to a specific type, and returns null if it fails.

例如：

StringBuilder b = someObject as StringBuilder;
if (b != null) ...

还有相关的：

Also related:

借助转换运算符试图将对象转换为特定类型，并抛出exeption如果失败。

The cast operator attempts to cast an object to a specific type, and throws an exeption if it fails.

例如：

StringBuilder b = (StringBuilder)someObject.