属性值会如同变数或常数参与运算式的方式参与运算式，或是储存于指派陈述式左边的变数或属性中。
The value of the property participates in the expression just as a variable or constant would, or it is stored in the variable or property on the left side of the assignment statement.
加入、移除，或变更条件式编译陈述式的评估运算式。
Adding, removing, or changing the evaluation expression of a conditional compilation statement.
依据运算式的值，有条件的执行一组陈述式。
Conditionally executes a group of statements, depending on the value of an expression.
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