ExceptionParam

function ExceptionParam: string;

Example

procedure ScriptEvent(var Value: Variant);
begin
  try
    RaiseException(erCustomError, 'Fictional validation failure');
  except
    Value := ExceptionParam;  // 'Fictional validation failure'
  end;
end;

Usage

ExceptionParam returns the parameter text attached to the current Velox scripting runtime exception.

Returns

The saved exception parameter as a script string. For erCustomError, this is normally the complete custom message. Other exception types may store a procedure name, conversion detail, host exception message or an empty string.

Behaviour

• The function reads VM state; it does not inspect a Velox Exception object and does not create a scoped E.Message variable.
• ExceptionParam is meaningful only in relation to ExceptionType. The same text can be interpreted differently for different TIFException values.
• Calling it while merely inside a protected try region, before an exception has entered the handler, reads the top handler frame's initially empty parameter.
• Reading the value does not mark the exception handled, clear it or re-raise it.

Errors

There is no separate missing-value exception. String conversion or allocation failures can propagate through the script runtime.

Usage notes

Prefer ExceptionToString(ExceptionType, ExceptionParam) when logging a general VM failure because many exception types use a fixed description in addition to, or instead of, the parameter. Keep ExceptionParam for logic that deliberately understands the corresponding type.

Additional Technical Info

ExceptionParam returns the text parameter saved with the current PascalScript virtual-machine exception. Use it inside a try..except handler when the parameter itself is useful; use ExceptionToString when you need the runtime's type-aware display text.

The example is source-reviewed and was not executed by the documentation workflow.

Implementation

The PascalScript compiler registers ExceptionParam as a built-in exception function. Runtime dispatch reads TPSExec.LastExParam:

1. When an exception-handler frame is active, it returns the ExceptionParam field from the top frame.
2. When no frame is active, it returns the executor-level ExParam field.
3. The dispatch writes the value through PascalScript's ANSI-string stack path before returning it as the declared script string.

When control transfers into an except block, the VM copies the current exception parameter into that handler frame and clears the executor's active error code. The handler copy therefore remains available while the block runs.

Edge cases and quirks

• Empty text is a valid result. It can mean that the exception type has no parameter, that the producer supplied an empty parameter, or that no exception is saved; it is not a success indicator on its own.
• The current Delphi build declares script string as Unicode, but these exception built-ins use PascalScript's AnsiString runtime path. Characters that the active ANSI code page cannot represent may be changed during the conversion.
• Runtime and host errors can place data from the failed operation in the parameter. Do not expose it to an external party without applying the Flow's information-disclosure policy.
• The parameter is not guaranteed to equal a native Delphi exception message. The VM maps and stores several error categories of its own.

Side effects

None. The function reads the current executor or handler frame.

Performance and concurrency

The lookup is constant-time and copies one managed string. The state belongs to the currently executing PascalScript executor; the function does not synchronise shared objects or make concurrent use of an executor safe.

Related entries

• ExceptionType returns the TIFException value that gives this parameter its meaning.
• ExceptionToString applies the runtime's type-specific formatting rules.
• RaiseException supplies the type and parameter stored by a deliberately raised VM error.
• try..except explains the handler state in which these values are normally read.

Created 2026-07-15