bcquality/microsoft/knowledge/performance/case-true-of-for-long-condition-chains.md

bc-version	domain	keywords	technologies	countries	application-area
all	performance	case-statement case-true-of nested-if condition-chain guard lazy-evaluation nesting-depth	al	w1	all

Use case true of for long chains of dependent conditions

Description

Because AL gives no short-circuit guarantee for and and or, a chain of conditions that must be evaluated in order has to be sequenced with nested if statements — and past three conditions the nesting itself becomes the problem: the body drifts right, the order of evaluation is carried by indentation alone, and any shared failure path is repeated at every level. AL's case statement is the flat alternative. Its value sets "must be an expression or a range", so case true of and case false of accept arbitrary boolean expressions, and the statement "is evaluated, and the first matching value set executes the associated statement" — evaluation stops at the first matching value set, which is exactly the laziness the boolean operators do not provide. That guarantee is stated for value sets, plural: it orders evaluation across separate value sets, and says nothing about the order of the individual expressions listed inside one comma-separated value set.

Best Practice

Sequence two or three dependent conditions with nested if. Beyond that, switch to case: use case false of for a chain of guards where every condition must hold, letting control fall past end when all of them pass; use case true of for first-match dispatch, where each later probe runs only if the earlier ones did not match. Comma-separate conditions into one value set only when every one of them is a pure, order-independent test with no side effect — a field comparison, an enum check, a bound test — so it makes no difference whether AL evaluates all of them or stops early; grouping these costs nothing and removes the repeated action. A condition that guards another, or that carries a side effect or a cost of its own — a Get, a Find, a procedure call — keeps its own value set, placed immediately after the value set it depends on, so the code relies only on the ordering the documentation actually states. A value set needs no parentheses around a comparison, unlike an operand of and or or: the AL operator hierarchy places and and or above the comparison operators, so parentheses are mandatory there and the chain fills up with them. This keeps every condition at one indentation level, makes evaluation order explicit rather than implied by nesting, and preserves the stop-at-first-match behaviour it relies on. It also aligns with the AL programming convention that more than two alternatives belong in a case statement rather than an if-then-else.

See sample: case-true-of-for-long-condition-chains.good.al.

Anti Pattern

An if ladder four or more levels deep whose only purpose is sequencing guards. Detection: a chain of nested if statements with no else, each condition guarding the one below it, terminating in a single action or exit; or the same exit/error duplicated at every level of such a nested chain, purely to escape it. The second, worse form is collapsing that ladder into one and chain to escape the nesting — that trades indentation for a real defect, because the operands are still all evaluated. A third, subtler form is over-applying the comma-grouping itself: putting a guard and the condition it protects — for example Item.Get(...) and a read of a field on that same record — into one comma-separated value set. That relies on an evaluation order within a single value set that the documentation does not state; keep them in separate value sets instead. Reach for case over nested if or a collapsed and chain, and keep order-dependent conditions in their own value sets within it.

See sample: case-true-of-for-long-condition-chains.bad.al.

See also

boolean-operators-do-not-short-circuit.md covers the underlying evaluation rule that makes the sequencing necessary in the first place.