5.4.4.1 On Error Statement

Note

This section describes the implementation of MS-VBAL §5.4.4.1 On Error Statement.

Syntax

AST node	Instruction kind(s)	Notes
OnErrorGoToStatementNode	`OnErrorGoTo`	`On Error GoTo <label>`. `Target`: the label's offset.
`OnErrorGoToStatementNode`	`OnErrorDisable`	`On Error GoTo 0`, and the undocumented VBA6/VBA7 form `On Error GoTo -1`.
OnErrorResumeStatementNode	`OnErrorResumeNext`	`On Error Resume Next`.

On Error Resume Next and On Error GoTo are parsed by the same grammar rule, disambiguated by the keyword that follows On Error.
The 0 of On Error GoTo 0 is the same MS-VBAL §5.4.4.2 Resume Statement sentinel that Resume 0 uses; see RD-VBAL §5.4.4.2 Resume Statement.

See RD-VBAL §3.4.2 Simple Statements and RD-VBAL §3.5.2 Instruction.

Static Semantics

Condition	Compile error
`On Error GoTo` names a handler label that is not defined, e.g. `On Error GoTo Handler` with no `Handler:`.	VBC09309 — Label not defined

On Error GoTo 0 and On Error GoTo -1 do not refer to a label. The 0 of On Error GoTo 0 and the -1 of On Error GoTo -1 are never looked up as labels: VBC09309 is not raised for them, whether or not the procedure defines a line 0.

Runtime Semantics

An On Error statement sets the error-handling policy of the current activation. The policy is an ErrorHandlingMode:

Policy (`ErrorHandlingMode`)	Set by	When an error is raised
`Disabled` (the policy MS-VBAL §5.4.4 Error Handling Statements calls Default)	`On Error GoTo 0`, `On Error GoTo -1`; an error caught under `GoTo`	The error is not caught: it propagates out of the activation (see Propagation).
`ResumeNext`	`On Error Resume Next`	The error is caught silently. Execution continues at the statement that follows the one that raised the error. The policy is not reset: every later error in the same activation is caught the same way.
`GoTo`	`On Error GoTo <label>`	The error branches to the label. The policy is reset to disabled: a second, unhandled error inside the handler body propagates rather than re-entering the handler.

The asymmetry between On Error Resume Next (policy kept) and On Error GoTo <label> (policy reset to disabled) follows MS-VBAL.

In MS-VBA, On Error GoTo 0 turns error handling off, and On Error GoTo -1 clears the active error. RD-VBA treats the two identically: both lower as OnErrorDisable, which turns the activation's error handling off and clears any active error. MS-VBAL does not document On Error GoTo -1, so treating it as On Error GoTo 0 does not diverge from the specification.

Error interception

When a statement raises a run-time error:

The executor routes the error through ProcedureExecutor.InterceptError, before the loop decides whether to stop.
InterceptError applies the activation's error-handling policy, as the table above shows.
When the policy does not catch the error, the error propagates out of the activation.

Every runtime error the executor can raise, from any subsystem, is catchable by an error handler: for example TypeMismatch, SubscriptOutOfRange and ForLoopNotInitialized. See RD-VBAL §3.5.4 Execution and RD-VBAL §2.6.3 Runtime Errors.

Propagation

An error that propagates out of an activation (no handler caught it) is returned as the ProcedureExecutor.Run call's own return value.

In a called procedure, that value reaches the caller as the result of the call. A nested call's runtime error propagates like any other runtime error, so the caller's own error-handling policy applies to it. See RD-VBAL §5.3.1.11 Procedure Invocation Argument Processing.

Per-activation state

activation.ErrorHandler, of type ErrorHandlerState, is a single mutable value per activation, like Pc. It holds the error-handling mode, the handler target, the active error, and the fault-statement offset.

The error-handler state is per activation, not per-offset hidden state the way With, Select, For and For Each state is. An On Error statement changes the activation's error-handling policy going forward; it is not scoped to one block.

Implementation

Type or member	Role
`RDCore.Runtime.Execution.ProcedureExecutor.InterceptError`	The one point every run-time error passes through; applies the activation's error-handling policy.
`ErrorHandlerState`	The activation's error-handling mode, handler target, active error and fault-statement offset (RDCore.SDK).
`ErrorHandlingMode`	`Disabled`, `ResumeNext`, `GoTo` (RDCore.SDK).
ICallStackFrame`.ErrorHandler`	Reads the activation's `ErrorHandlerState`; read-only on the SDK interface, and written only by the executor through `CallStackFrame.ErrorHandler`. See RD-VBAL §3.5.5 Placement and Licensing.
InstructionListLowering	Lowers `On Error GoTo 0` and `On Error GoTo -1` as `OnErrorDisable`, and `On Error GoTo <label>` as `OnErrorGoTo` (RDCore.SDK).

⏮️ RD-VBAL §5.4.4 Error Handling Statements | ⏭️ RD-VBAL §5.4.4.2 Resume Statement

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