本节内容

表达式的定义

• 表达式是一种专门用来求值的语法实体。

• Expressions, together eith commands and declarations,are one of the basic components of every programming language.We can say that expressions are the essential commponent of every language.

• An expressions is a syntatic entity whose evaluation eithrt produces a value or fails to terminate, inwhich case the expression is undefined.

• 各种编程语言堆表达式的实现不尽相同，但大体上都符合这个定义。

• An expression is a sequence of one or more operators and zerp pr more operator that can be evaluated to a single value,object,method, or namespace. Expressions can consist of literal value, a method invocation, an operator and its operands, or a simple name. Simple names can be the name od a variable, type member, method parameter, namespace or type.

• 算法逻辑的最基本（最小）单元，表达一定的算法意图

• 因为操作符有优先级，所以表达式也就有了优先级

各类表达式概览

• A value. Every value has an associated type.任何能得到值的运算（回顾操作符和结果类型）

• A variable。Every variable has an associated type.

• A namespace.

• A type.

• A method group.例如：Console.WriteLine，这是一组方法，重载决策决定调用哪一个

• A null literal.

• An anonymous function.

• A property access.

• An event access.

• An indexer access.

• Nothing. 对返回值为void的方法的调用。

• 注意操作符的优先级和同优先级操作符的运算方向

• 仅做参考，不必深究–毕竟我们是在学习语言，不是去实现这门语言