Unity-Technologies/UnityCsReference · error · ArgumentException

Invalid argument

Error message

Invalid argument: {dropPosition}

What it means

Thrown by GetPreviousAndNextItemsIgnoringDraggedItems when the supplied DropPosition is anything other than Above or Below (i.e. Upon or an out-of-range value). The method computes neighboring rows for sibling insertion, an operation that is meaningless for a 'drop onto' position, so it rejects the input with an ArgumentException. The message concatenates the offending enum value so you can see exactly what was passed.

Solutions

1. Ensure the DropPosition passed into sibling-insertion code is strictly DropPosition.Above or DropPosition.Below before calling.
2. If your code can legitimately receive DropPosition.Upon, branch early and only call this method in the Above/Below branch.
3. When overriding DoDrag/DoDragInternal, map Upon to Above or Below (or skip the call) instead of forwarding it.
4. Validate the enum at the boundary of your own method and reject/map unexpected values before they reach TreeView internals.

Example fix

// before
GetPreviousAndNextItemsIgnoringDraggedItems(row, dropPosition, out prev, out next);

// after
if (dropPosition != DropPosition.Above && dropPosition != DropPosition.Below)
    return; // 'Upon' is handled by drop-onto-parent logic elsewhere
GetPreviousAndNextItemsIgnoringDraggedItems(row, dropPosition, out prev, out next);

static bool IsValidSiblingDropPosition(TreeViewDragging<int>.DropPosition p)
    => p == TreeViewDragging<int>.DropPosition.Above || p == TreeViewDragging<int>.DropPosition.Below;

When it happens

Trigger: Calling GetPreviousAndNextItemsIgnoringDraggedItems directly with DropPosition.Upon; passing a DropPosition value that the internal drag pipeline only expects to be Above/Below; a custom TreeViewDragging<T> subclass that overrides drag handling and forwards a non-Above/Below DropPosition into sibling-insertion logic; an uninit/default DropPosition (== Upon, value 0) leaking into the path.

Common situations: Implementing a custom editor TreeView drag-drop handler and forgetting that 'Upon' is only valid for drop-onto-parent semantics; upgrading Unity where DropPosition enum gained/changed members; copy-pasting a drop handler that works for Upon drops but reusing it for between-row insertion.

Related errors

• Duplicating Assets is unsupported by this function. Use…
• Invalid search: cannot be null or empty
• Invalid tree for finding descendants: Ensure a complete…
• The root is null
• The value ( ) must not be negative

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/ab1a2e7adf376749. Report an issue: GitHub.

Appendix: source

Thrown at Editor/Mono/GUI/TreeView/TreeViewDragging.cs:126

        // When hovering outside any items: target and parent is null, dropPos is invalid
        // If parentItem and targetItem is the same then insert as first child of parent, dropPos is invalid
        // If parentItem and targetItem is different then use dropPos to insert dragged items relative to targetItem
        // parentItem can be null when root is visible and hovering above or below the root

        // if targetItem is null then parent can be null if root is visible
        // if targetitem is null then parent might be valid if root is hidden
        public virtual DragAndDropVisualMode DoDrag(TreeViewItem<TIdentifier> parentItem, TreeViewItem<TIdentifier> targetItem, bool perform, DropPosition dropPosition) => DoDragInternal(parentItem, targetItem, perform, dropPosition);
        public virtual DragAndDropVisualMode DoDragInternal(TreeViewItem<TIdentifier> parentItem, TreeViewItem<TIdentifier> targetItem, bool perform, DropPosition dropPosition) => throw new NotImplementedException();

        protected float GetDropBetweenHalfHeight(TreeViewItem<TIdentifier> item, Rect itemRect)
        {
            return m_TreeView.data.CanBeParent(item) ? m_TreeView.gui.halfDropBetweenHeight : itemRect.height * 0.5f;
        }

        void GetPreviousAndNextItemsIgnoringDraggedItems(int targetRow, DropPosition dropPosition, out TreeViewItem<TIdentifier> previousItem, out TreeViewItem<TIdentifier> nextItem)
        {
            if (dropPosition != DropPosition.Above && dropPosition != DropPosition.Below)
                throw new ArgumentException("Invalid argument: " + dropPosition);

            previousItem = nextItem = null;
            int curPrevRow = (dropPosition == DropPosition.Above) ? targetRow - 1 : targetRow;
            int curNextRow = (dropPosition == DropPosition.Above) ? targetRow : targetRow + 1;

            while (curPrevRow >= 0)
            {
                var curPreviousItem = m_TreeView.data.GetItem(curPrevRow);
                if (!m_TreeView.IsDraggingItem(curPreviousItem))
                {
                    previousItem = curPreviousItem;
                    break;
                }
                curPrevRow--;
            }

            while (curNextRow < m_TreeView.data.rowCount)
            {

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