Original
The as operator
The as operator performs
checked typecasts. The expression
object as class
returns a reference to the same object as object,
but with the type given by class. At runtime, object must be an
instance of the class denoted by class or one of its descendants, or be nil;
otherwise an exception is raised. If the declared type of object is
unrelated to class, that is, if the types are distinct and one is not an
ancestor of the other, compilation error results. For example,
with
Sender as TButton do
begin
Caption := '&Ok';
OnClick := OkClick;
end;
The rules of operator precedence
often require as typecasts to be enclosed in parentheses. For example,
(Sender as TButton).Caption :=
'&Ok';
See also
译文
as运算符
as运算符执行类型转换检查。其用法表达式如下：
object as class
表达式返回与object相同的对象的引用，但是是以class给出的类型。在运行时，object必需是class表示的类或其后裔类的实例，或者是nil；否则引发异常。如果object的声明类型与class无关，即它们二者各自独立并且其中任意一个不是另一个的祖先，那么将导致编译错误。例如，
with
Sender as TButton do
begin
Caption := '&Ok';
OnClick := OkClick;
end;
运算符优先规则通常要求as类型转换被封装在圆括号中。例如，
(Sender as TButton).Caption :=
'&Ok';