dotnet/reactive · error · InvalidOperationException

Element no longer available in the buffer.

Error message

Element no longer available in the buffer.

What it means

RefCountList<T> is an internal ref-counted circular buffer used by buffering operators such as Memoize/Buffer with reference semantics. Its indexer decrements the reference count for index i and removes entries whose count reaches zero. If the entry is already gone, the buffer's contract (Debug.Assert(i < Count)) was violated by the caller, and the list throws InvalidOperationException("Element no longer available in the buffer.") instead of returning the value.

Solutions

1. Increase the buffer capacity/size passed to the buffering operator so elements live long enough.
2. Ensure each consumer enumerates its own enumerator rather than sharing one enumerator across loops.
3. Avoid re-enumerating a shared buffered sequence more times than supported; memoize materialized results (ToList) when reuse is needed.
4. This is an internal invariant breach — if reproducible with normal usage, file a bug against System.Interactive with a minimal repro.
5. Serialize access if multiple threads enumerate the shared buffer concurrently.

try { var item = buffer[i]; }
catch (InvalidOperationException ex) when (ex.Message.Contains("no longer available")) {
    // re-enumerate from the original source or re-memoize
}

When it happens

Trigger: Enumerating a buffered/shared sequence more times or in more overlapping passes than the buffer capacity/ref-counting allows; reading an index after its references were exhausted (item evicted from the circular buffer); concurrent enumeration of the shared buffer from multiple consumers.

Common situations: Multiple foreach loops over one Memoize'd/buffered sequence with too small a buffer, re-enumerating an enumerator past eviction, or consumers of different speeds over a shared buffer sequence.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• Specified argument was out of the range of valid values. (Pa
• Cannot access a disposed object. Object name: ''.
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'source')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/436119c6304d8c80. Report an issue: GitHub.

Appendix: source

Thrown at Ix.NET/Source/System.Interactive/System/Linq/RefCountList.cs:27

{
    internal sealed class RefCountList<T>(int readerCount) : IRefCountList<T>
    {
        private readonly IDictionary<int, RefCount> _list = new Dictionary<int, RefCount>();

        public int ReaderCount { get; set; } = readerCount;

        public void Clear() => _list.Clear();

        public int Count { get; private set; }

        public T this[int i]
        {
            get
            {
                Debug.Assert(i < Count);

                if (!_list.TryGetValue(i, out var res))
                    throw new InvalidOperationException("Element no longer available in the buffer.");

                var val = res.Value;

                if (--res.Count == 0)
                {
                    _list.Remove(i);
                }

                return val;
            }
        }

        public void Add(T item)
        {
            _list[Count] = new RefCount(item, ReaderCount);

            Count++;
        }

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