12.3.3.23 && expressions

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本文围绕C#中形式为expr-first && expr-second的表达式展开，详细阐述了变量v在expr-first前后、expr-second前后以及整个expr前后的明确赋值状态，并通过示例说明变量i在if语句不同嵌入语句中的赋值情况。

12.3.3.23 && expressions
For an expression expr of the form expr-first && expr-second:
?The definite assignment state of v before expr-first is the same as the
definite assignment state of v
before expr.
C# LANGUAGE SPECIFICATION
110
?The definite assignment state of v before expr-second is definitely
assigned if the state of v after
expr-first is either definitely assigned or .definitely assigned after true
expression.. Otherwise, it is
not definitely assigned.
?The definite assignment state of v after expr is determined by:
o If the state of v after expr-first is definitely assigned, then the state
of v after expr is
definitely assigned.
o Otherwise, if the state of v after expr-second is definitely assigned,
and the state of v after
expr-first is .definitely assigned after false expression., then the state
of v after expr is
definitely assigned.
o Otherwise, if the state of v after expr-second is definitely assigned or
.definitely assigned
after true expression., then the state of v after expr is .definitely
assigned after true
expression..
o Otherwise, if the state of v after expr-first is .definitely assigned
after false expression., and
the state of v after expr-second is .definitely assigned after false
expression., then the state
of v after expr is .definitely assigned after false expression..
o Otherwise, the state of v after expr is not definitely assigned.
[Example: In the example
class A
{
static void F(int x, int y) {
int i;
if (x >= 0 && (i = y) >= 0) {
// i definitely assigned
}
else {
// i not definitely assigned
}
// i not definitely assigned
}
}
the variable i is considered definitely assigned in one of the embedded
statements of an if statement but not
in the other. In the if statement in method F, the variable i is definitely
assigned in the first embedded
statement because execution of the expression (i = y) always precedes
execution of this embedded
statement. In contrast, the variable i is not definitely assigned in the
second embedded statement, since
x >= 0 might have tested false, resulting in the variable i抯 being
unassigned. end example]