Windows error 1551 (ERROR_INVALID_TASK

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: A Windows component received a task index outside its current task collection.

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ERROR_INVALID_TASK_NAME	ERROR_THREAD_ALREADY_IN_TASK

ERROR_INVALID_TASK_INDEX

The supplied task index is not valid in the current task context.

ERROR_INVALID_TASK_INDEX is Win32 error 1551 (0x60F). The public error text does not define the collection, index base, or API that owns the task. It may identify an internal task slot rather than a Task Scheduler entry. Diagnosis must begin with the function that returned the code and the current collection generation; treating the integer as globally meaningful invites off-by-one and stale-state errors.

Common index failures

the index is negative, beyond the current count, or uses one-based numbering incorrectly
a task was removed or reordered after the caller cached its position
the index belongs to another parent object, session, or version of the collection
integer truncation or signed/unsigned conversion changed a large value
concurrent enumeration and mutation invalidated a previously valid slot

Evidence needed for diagnosis

Log the exact API and component, supplied index in decimal and hexadecimal, current task count, collection or parent identity, generation number, thread ID, and the operation that produced the index. Include whether the collection can change concurrently. Avoid logging only “index invalid”; without count and generation the fault cannot be distinguished from a race.

Investigation sequence

Inspect the documented index base and range at the point of use. Validate before conversion to narrower integer types. Trace how the index was obtained: enumeration result, configuration file, user selection, or protocol message. If the collection is mutable, reproduce under synchronization and check whether a deletion or reorder occurs between lookup and use.

Prefer a stable task identifier or handle when the subsystem offers one. If only indexes are available, pair them with a snapshot or generation token and repeat enumeration after 1551 instead of decrementing or wrapping the value. Confirm client and server agree on collection ordering across version boundaries.

Corrective design

Reject out-of-range input early, refresh stale collections, and serialize mutation where required by the API. Do not silently clamp an invalid index to the last task; that can execute work against the wrong object. Expose the supported range in diagnostics and tests, including empty, first, last, and concurrent-removal cases.

Difference from an invalid task name

ERROR_INVALID_TASK_NAME rejects a textual identifier. Error 1551 rejects a positional identifier. Converting a valid name into an index through an obsolete cache can turn a name lookup problem into 1551, so preserve both values when the application performs that translation.

Example

A monitoring plug-in enumerates four internal tasks and caches index 3. A refresh removes the second task, leaving only three entries, but a delayed callback still uses index 3 and receives 1551. Replacing the cached position with a stable task ID and snapshot generation removes the race.

References

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