What causes "Index is not a valid index in the ActionCollection" in peass-ng/PEASS-ng?

Use Actions.Add(newAction) instead of the indexer to append new actions

peass-ng/PEASS-ng · error · System.ArgumentOutOfRangeException

Index is not a valid index in the ActionCollection

Error message

Index is not a valid index in the ActionCollection

What it means

The ActionCollection indexer setter validates that the index refers to an existing action; if index is negative or >= Count it throws ArgumentOutOfRangeException with this message. This protects the underlying COM collection (v2) and the internal v1 action list from out-of-range writes.

Source

Thrown at winPEAS/winPEASexe/winPEAS/TaskScheduler/ActionCollection.cs:249

            {
                if (v2Coll != null)
                    return Action.CreateAction(v2Coll[++index]);
                if (v1Task != null)
                {
                    if (SupportV1Conversion)
                        return v1Actions[index];
                    else
                    {
                        if (index == 0)
                            return v1Actions[0];
                    }
                }
                throw new ArgumentOutOfRangeException();
            }
            set
            {
                if (index < 0 || Count <= index)
                    throw new ArgumentOutOfRangeException(nameof(index), index, "Index is not a valid index in the ActionCollection");
                var orig = this[index].Clone();
                if (v2Coll != null)
                {
                    inV2set = true;
                    try
                    {
                        Insert(index, value);
                        RemoveAt(index + 1);
                    }
                    finally
                    {
                        inV2set = false;
                    }
                }
                else
                {
                    v1Actions[index] = value;
                    SaveV1Actions();

View on GitHub (pinned to 53fb989abc)

Solutions

Use Actions.Add(newAction) instead of the indexer to append new actions
Re-check Count immediately before each indexed write; iterate with for (int i = 0; i < actions.Count; i++)
If replacing, capture a fresh index from IndexOf rather than a cached value
Catch ArgumentOutOfRangeException and treat as a logic bug — log collection state for diagnosis

Example fix

// before
int saved = actions.Count;
// ... other code removes actions ...
actions[saved] = new ExecAction("cmd.exe"); // out of range
// after
actions.Add(new ExecAction("cmd.exe")); // append via Add
// or, for replace:
int idx = actions.IndexOf(existing);
if (idx >= 0 && idx < actions.Count) actions[idx] = new ExecAction("cmd.exe");

Defensive patterns

Strategy: validation

Validate before calling

static bool IsValidActionIndex(ActionCollection actions, int i) => i >= 0 && i < actions.Count;

Try / catch

try { actions[index] = newAction; }
catch (ArgumentOutOfRangeException ex)
{
    log.Error($"Action index {index} invalid (Count={actions.Count}); use Add to append.");
}

Prevention

Use Add for appending — the indexer only replaces existing items
Re-read Count right before indexed writes; never cache indices across modifications
Iterate with for loops bounded by the live Count
Fetch indices via IndexOf instead of remembering old positions

When it happens

Trigger: Assigning actions[Count] to 'append' (the setter does not append — use Add); looping with an upper bound from a stale count captured before removals; passing a saved index after the collection was modified.

Common situations: Code that assumes list-style append-by-index semantics; UI list updates after actions were deleted elsewhere; deserialization logic writing items at their original indices into a shorter collection.

Related errors

AI-assisted analysis of peass-ng/PEASS-ng@53fb989abc (2026-09-02). Data as JSON: /api/errors/0ed3b78003b057e0. Report an issue: GitHub.