dotnet/reactive · error · NotSupportedException

Specified method is not supported.

Error message

Specified method is not supported.

What it means

Yielder<T> is a compiler-less iterator helper implementing IEnumerator<T>. IEnumerator.Reset is not supported by iterator-style enumerators, so Yielder.Reset unconditionally throws NotSupportedException ("Specified method is not supported.") — mirroring the behavior of C# compiler-generated iterators.

Solutions

1. Get a fresh enumerator via GetEnumerator() (or re-invoke the operator) instead of calling Reset.
2. Materialize the sequence (ToList/ToArray) if you need multiple passes.
3. Restructure the consumer to a single forward-only pass.
4. If you own the consuming framework, remove Reset-based reuse logic.

Example fix

// before
e.Reset();
while (e.MoveNext()) Process(e.Current);
// after
foreach (var item in sequence) Process(item); // or sequence.GetEnumerator() again

try { enumerator.Reset(); }
catch (NotSupportedException) { enumerator = sequence.GetEnumerator(); // fresh enumerator instead
}

When it happens

Trigger: Calling `Reset()` on an enumerator produced by a Yielder-backed operator, e.g. legacy code that assumed enumerators are restartable, or code doing `enumerator.Reset(); enumerator.MoveNext()` to replay a sequence.

Common situations: Porting .NET 1.x-style enumeration code that relied on Reset, reusing an enumerator for a second pass over a LINQ/Ix sequence, or framework code that defensively calls Reset before reuse.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Cannot access a disposed object. Object name: ''.
• Element no longer available in the buffer.
• Specified argument was out of the range of valid values…
• Specified argument was out of the range of valid values…
• Specified method is not supported.

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/d3b26b9ce8302920. Report an issue: GitHub.

Appendix: source

Thrown at Ix.NET/Source/System.Interactive/System/Linq/Yielder.cs:75

        public Yielder<T> GetEnumerator() => this;

        public bool MoveNext()
        {
            if (!_running)
            {
                _running = true;
                _create(this);
            }
            else
            {
                _hasValue = false;
                _continuation!();
            }

            return !_stopped && _hasValue;
        }

        public void Reset() => throw new NotSupportedException();
    }
}

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