dotnetcore/CAP · error · ArgumentException

arrayIndex

Error message

arrayIndex

What it means

CircularBuffer<T>.CopyTo throws ArgumentException("arrayIndex") when Count > array.Length - arrayIndex, i.e. the destination array is too small to hold all buffered items starting at the given offset. This is the capacity guard for the copy loop array[arrayIndex++] = item.

Solutions

1. Allocate the destination with room for the offset: new T[buffer.Count + arrayIndex].
2. Use array.Length - arrayIndex check at the call site before copying, or call ToArray()/ToArray(segment) style helpers.
3. Re-derive Count from the buffer at copy time rather than caching a stale size.

Example fix

// before
var dest = new T[buffer.Count];
buffer.CopyTo(dest, 5); // needs Count+5 slots
// after
var dest = new T[buffer.Count + 5];
buffer.CopyTo(dest, 5);

if (array.Length - offset < buffer.Count)
    array = new T[buffer.Count + offset];
buffer.CopyTo(array, offset);

try { buffer.CopyTo(array, offset); }
catch (ArgumentException) { array = new T[buffer.Count + offset]; buffer.CopyTo(array, offset); }

When it happens

Trigger: Calling CopyTo with an array whose available space (Length - arrayIndex) is less than the buffer's Count, e.g. copying into new T[n] with n < Count or using a nonzero offset in an array sized exactly to Count.

Common situations: Reusing a stale array allocated for an older smaller Count while the ring buffer wrapped and grew, or applying a non-zero arrayIndex without enlarging the array.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

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

AI-assisted analysis of dotnetcore/CAP@e52b8508e5 (2026-09-14). Data as JSON: /api/errors/df426627e66beaba. Report an issue: GitHub.

Appendix: source

Thrown at src/DotNetCore.CAP.Dashboard/CircularBuffer.cs:164

    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)
    {
        throw new NotImplementedException();
    }

    #endregion
}

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