ECMA-334: 12.3.3.23 short circuit AND expressions

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C# Language Specification

12. Variables 12.3 Definite assignment 12.3.3 Precise rules for determining definite assignment 12.3.3.1 General rules for statements 12.3.3.2 Block statements, checked, and unchecked statements 12.3.3.3 Expression statements 12.3.3.4 Declaration statements 12.3.3.5 If statements 12.3.3.6 Switch statements 12.3.3.7 While statements 12.3.3.8 Do statements 12.3.3.9 For statements 12.3.3.10 Break, continue, and goto statements 12.3.3.11 Throw statements 12.3.3.12 Return statements 12.3.3.13 Try-catch statements 12.3.3.14 Try-finally statements 12.3.3.15 Try-catch-finally statements 12.3.3.16 Foreach statements 12.3.3.17 Using statements 12.3.3.18 Lock statements 12.3.3.19 General rules for simple expressions 12.3.3.20 General rules for expressions with embedded expressions 12.3.3.21 Invocation expressions and object creation expressions 12.3.3.22 Simple assignment expressions 12.3.3.23 && expressions 12.3.3.24 || expressions 12.3.3.25 ! expressions 12.3.3.26 ?: expressions 12.3.3.27 Anonymous method expressions 12.3.3.28 Yield statements 12.3.3.29 ?? expressions

© 2006 ECMA International

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.

• 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:

• If the state of v after expr-first is definitely assigned, then the state of v after expr is definitely assigned.

• 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.

• 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".

• 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".

• Otherwise, the state of v after expr is not definitely assigned.

[Example: In the following code

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’s being unassigned. end example]