MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · ArgumentOutOfRangeException

Item level must not be less than the level item after it ({n

Error message

Item level must not be less than the level item after it ({nextItemLevel})

What it means

An item may not be shallower than the item that follows it, because deeper items in a flat ordered list depend on a preceding ancestor at level-1. InsertWithLevel throws ArgumentOutOfRangeException when level < nextItemLevel, which would otherwise orphan the following subtree.

Source

Thrown at src/MaterialDesignThemes.Wpf/Internal/TreeListViewItemsCollection.cs:78

    public void SetIsExpanded(int index, bool isExpanded)
        => ItemIsExpanded[index] = isExpanded;

    public void InsertWithLevel(int index, object? item, int level)
    {
        if (level < 0) throw new ArgumentOutOfRangeException(nameof(level), level, "Item level must not be negative");

        //Always allowed to request previous item parentLevel + 1 as this is inserting a "child"
        int previousItemLevel = index > 0 ? ItemLevels[index - 1] : -1;
        if (level > previousItemLevel + 1)
        {
            throw new ArgumentOutOfRangeException(nameof(level), level, $"Item level must not be more than one level greater the previous item ({previousItemLevel})");
        }

        int nextItemLevel = index < Count ? ItemLevels[index] : 0;
        if (level < nextItemLevel)
        {
            throw new ArgumentOutOfRangeException(nameof(level), level, $"Item level must not be less than the level item after it ({nextItemLevel})");
        }

        InternalInsertItem(index, item, level);
        if (previousItemLevel >= 0 && previousItemLevel == level - 1)
        {
            ItemIsExpanded[index - 1] = true;
        }
    }

    public object? GetParent(int index)
    {
        if (index < 0 || index >= Count) throw new ArgumentOutOfRangeException(nameof(index));
        int level = ItemLevels[index];
        if (level == 0) return null;
        for (int i = index - 1; i >= 0; i--)
        {
            if (ItemLevels[i] == level - 1)
            {

View on GitHub (pinned to 98edec3a0b)

Solutions

1. Ensure the inserted level is >= the level of the item currently at that index, or move/relocate the deeper items first.
2. Insert new roots at the end of the collection (or after the last item at the same level) rather than mid-subtree.
3. When wrapping a hierarchical source, let the built-in ItemsSource_CollectionChanged handler manage insertion rather than calling InsertWithLevel directly.

Example fix

// before: inserting a root (0) before a nested child (1)
collection.InsertWithLevel(2, newRoot, 0);
// after: insert at the end of the sibling group
int end = collection.Count;
collection.InsertWithLevel(end, newRoot, 0);

int next = index < collection.Count ? collection.GetLevel(index) : 0;
if (level < next) throw new InvalidOperationException("Cannot insert above a deeper subtree.");
collection.InsertWithLevel(index, item, level);

When it happens

Trigger: Inserting a shallow (e.g. root-level) item immediately before an existing nested item whose level is greater, without relocating that subtree.

Common situations: Reordering or inserting a new parent above an already-nested subtree; moving a root node into the middle of a child group.

Related errors

• Item level must not be negative
• Item level must not be more than one level greater the previ
• index
• Value must be positive
• Password cannot be empty

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/f567d6b20481afd8. Report an issue: GitHub.