dotnetcore/CAP · error · NotImplementedException

Specified method is not supported.

Error message

Specified method is not supported.

What it means

CircularBuffer<T>.Contains(T item) is a stub that unconditionally throws NotImplementedException, surfaced as NotSupportedException ("Specified method is not supported.") via the ICollection<T> contract. The Dashboard's in-memory ring buffer simply never implemented membership testing. Calling it will always fail regardless of arguments.

Solutions

1. Do not call Contains on CircularBuffer; implement the membership check in caller code by iterating the buffer manually.
2. Implement Contains in CircularBuffer by iterating items and using EqualityComparer<T>.Default.Equals.
3. If reached via LINQ, force enumeration with AsEnumerable() or ToList() first so a different Contains path is used.
4. Upgrade DotNetCore.CAP in case a newer version implemented the method.

Example fix

// before
if (buffer.Contains(messageId)) { ... }
// after
bool contains = false;
foreach (var item in buffer)
{
    if (EqualityComparer<string>.Default.Equals(item, messageId)) { contains = true; break; }
}
if (contains) { ... }

if (buffer is CircularBuffer<T> cb) { /* Contains unsupported */ }
// do membership check manually:
bool contains = false;
foreach (var item in buffer) if (EqualityComparer<T>.Default.Equals(item, target)) { contains = true; break; }

static bool SupportsContains<T>(ICollection<T> c) => !(c is DotNetCore.CAP.Dashboard.CircularBuffer<T>);

try { buffer.Contains(item); }
catch (NotImplementedException) { /* fall back to manual scan */ }

When it happens

Trigger: Calling Contains(item) on any CircularBuffer<T> instance, e.g. code that treats the buffer as an ICollection<T>/IReadOnlyCollection<T> and invokes Contains for membership checks.

Common situations: LINQ chains that resolve to ICollection<T>.Contains (e.g. some Contains optimizations), custom dashboard code checking whether a message/id is already buffered, or generic collection algorithms that probe for membership before insert.

Related errors

• Value cannot be null. (Parameter 'array')
• Specified argument was out of the range of valid values…
• arrayIndex
• Specified method is not supported.
• Value cannot be null. (Parameter 'topicNames')

AI-assisted analysis of dotnetcore/CAP@e52b8508e5 (2026-09-14). Data as JSON: /api/errors/aa0cad35e2ada4a2. Report an issue: GitHub.

Appendix: source

Thrown at src/DotNetCore.CAP.Dashboard/CircularBuffer.cs:155

        }
        else
        {
            itemIndex = _firstIndex + Count;
            Count++;
        }

        _items[itemIndex] = item;
    }

    public void Clear()
    {
        _firstIndex = 0;
        Count = 0;
    }

    public bool Contains(T item)
    {
        throw new NotImplementedException();
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        if (array == null) throw new ArgumentNullException(nameof(array));

        if (arrayIndex < 0) throw new ArgumentOutOfRangeException(nameof(arrayIndex));

        if (Count > array.Length - arrayIndex) throw new ArgumentException("arrayIndex");

        // Iterate through the buffer in correct order.
        foreach (var item in this)
        {
            array[arrayIndex++] = item;
        }
    }

    public bool Remove(T item)

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