ECMA-334: 12.3.3 Precise rules for determining definite assignment

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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 Precise rules for determining definite assignment

In order to determine that each used variable is definitely assigned, the compiler shall use a process that is equivalent to the one described in this subclause.

The compiler processes the body of each function member that has one or more initially unassigned variables. For each initially unassigned variable v, the compiler determines a definite assignment state for v at each of the following points in the function member:

• At the beginning of each statement

• At the end point (§15.1) of each statement

• On each arc which transfers control to another statement or to the end point of a statement

• At the beginning of each expression

• At the end of each expression

The definite assignment state of v can be either:

• Definitely assigned. This indicates that on all possible control flows to this point, v has been assigned a value.

• Not definitely assigned. For the state of a variable at the end of an expression of type bool, the state of a variable that isn’t definitely assigned might (but doesn’t necessarily) fall into one of the following substates:

• Definitely assigned after true expression. This state indicates that v is definitely assigned if the Boolean expression evaluated as true, but is not necessarily assigned if the Boolean expression evaluated as false.

• Definitely assigned after false expression. This state indicates that v is definitely assigned if the Boolean expression evaluated as false, but is not necessarily assigned if the Boolean expression evaluated as true.

The following rules govern how the state of a variable v is determined at each location.