dotnet/wpf · error · ArgumentOutOfRangeException

throw new ArgumentOutOfRangeException(nameof(index));

Error message

throw new ArgumentOutOfRangeException(nameof(index));

What it means

SingleItemList.GetKeyValuePair(int index, ref int key, ref object value) is FrugalMap's one-entry store; it only serves index 0 when the lone entry exists. Any other index — including index > 0 on a map that holds exactly one entry — throws ArgumentOutOfRangeException(nameof(index)). The map's indexer/API contract is that ordinal access must stay below Count.

Solutions

1. Bound enumeration with the map's Count, not a fixed size
2. Use map.Iterate(list, callback) (or GetSortedArray) instead of manual index-based access
3. Check that index == 0 before calling GetKeyValuePair on a single-entry map
4. For general access, use the map's Search(key)/indexer API rather than ordinal positions

Example fix

// before
for (int i = 0; i < map.Count + 1; i++) map.GetKeyValuePair(i, ref key, ref value); // throws at last pass
// after
for (int i = 0; i < map.Count; i++) map.GetKeyValuePair(i, ref key, ref value);

if (index >= 0 && index < map.Count)
    map.GetKeyValuePair(index, ref key, ref value);

static bool TryGetPair(FrugalMap map, int index, ref int key, ref object value)
    => index >= 0 && index < map.Count; // single-entry maps only serve index 0

try { map.GetKeyValuePair(index, ref key, ref value); }
catch (ArgumentOutOfRangeException)
{
    // ran past the end; stop enumeration
}

When it happens

Trigger: Calling GetKeyValuePair with index != 0 on a FrugalMap whose store is a SingleItemList (i.e. the map contains exactly one entry with a valid key); iterating with a hand-rolled loop that doesn't stop at Count == 1.

Common situations: Custom code enumerating a FrugalMap (or the DependencyProperty/DependencyObject value store it backs) with a for-loop bounded by Capacity instead of Count, or continuing iteration after the single entry.

Related errors

• args
• args
• ArgumentOutOfRangeException(authenticationType)
• ArgumentOutOfRangeException
• ArgumentOutOfRangeException(nameof(characterHit))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/667056abe2f25700. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/Shared/MS/Utility/FrugalMap.cs:169

        }

        public override void Sort()
        {
            // Single items are already sorted.
        }

        public override void GetKeyValuePair(int index, out int key, out Object value)
        {
            if (0 == index)
            {
                value = _loneEntry.Value;
                key = _loneEntry.Key;
            }
            else
            {
                value = DependencyProperty.UnsetValue;
                key = INVALIDKEY;
                throw new ArgumentOutOfRangeException(nameof(index));
            }
        }
        
        public override void Iterate(ArrayList list, FrugalMapIterationCallback callback)
        {
            if (Count == 1)
            {
                callback(list, _loneEntry.Key, _loneEntry.Value);
            }
        }

        public override void Promote(FrugalMapBase newMap)
        {
            if (FrugalMapStoreState.Success == newMap.InsertEntry(_loneEntry.Key, _loneEntry.Value))
            {
            }
            else
            {

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