5.4.2.13 On...GoTo Statement

Note

This section describes the implementation of MS-VBAL §5.4.2.13 On...GoTo Statement.

Syntax

AST node	Instruction kind	Notes
OnGoToStatementNode	`JumpTable`	Instruction`.Targets` holds one offset per label, in source order.

An On expression GoTo label, ... statement is represented by OnGoToStatementNode, and lowers to the InstructionKind JumpTable (RD-VBAL §3.5.2 Instruction).

OnGoToStatementNode holds its targets in a Labels list. Each target is an expression naming or numbering a label or line, with no static link to the label node: the same convention as GoTo (RD-VBAL §3.4.2 Simple Statements).

OnGoToStatementNode and OnGoSubStatementNode are separate node types, rather than one shared node with a Kind.

Static Semantics

Lowering resolves each label in the list into Instruction.Targets (RD-VBAL §3.5.3 Lowering Block Statements).

Condition	Result
The procedure defines the label.	Its `Targets` entry is the label's offset.
The procedure does not define the label, e.g. `Missing` in `On n GoTo A, Missing`.	Its `Targets` entry is `null`, and lowering reports VBC09309 Label not defined. Each undefined label in the list is reported on its own.

Runtime Semantics

On…GoTo and On…GoSub share one selector algorithm (RD-VBAL §5.4.2.16 On...GoSub Statement). The executor applies it to a JumpTable instruction as follows:

The selector expression is evaluated once.
The evaluated value is Let-coerced to Integer. Let n be the coerced value.
If n is zero, or greater than the number of labels, execution falls through to the next instruction without branching.
Otherwise, if n is negative or greater than 255, run-time error 5 is raised.
Otherwise, execution branches to the n'th label (1-based) in Instruction.Targets.

Step 3 takes precedence over step 4, in the order MS-VBAL §5.4.2.13 lists them.

Condition	Run-time error
n is negative, or greater than 255 (step 4).	5 — Invalid procedure call or argument

A label with no statement after it, including the end-label of the lexically enclosing procedure declaration, resolves to the offset right past the last instruction. A branch to it completes the activation as if execution had reached the end of the procedure body (RD-VBAL §3.5.1 InstructionList).

Implementation

RDCore.Runtime.Execution.ProcedureExecutor dispatches JumpTable instructions (RD-VBAL §3.5.4 Execution).
RDCore.Runtime.Execution.JumpTableEvaluator evaluates the selector and Let-coerces it to Integer (steps 1 and 2). ProcedureExecutor applies steps 3 to 5, in one code path shared by JumpTable and GoSubTable.
JumpTableEvaluator Let-coerces the selector by calling VBNumericLetCoercionTypeRuntimeSemantics directly, bypassing the let-coercion provider. ConditionEvaluator likewise bypasses the provider to force a condition to Boolean (RD-VBAL §3.5.4 Execution).