microsoft/autogen · error · ArgumentNullException

Value cannot be null. (Parameter 'array')

Error message

Value cannot be null. (Parameter 'array')

What it means

Standard ICollection<AIContent>.CopyTo contract enforcement in MultiModalMessage/MultiModalData content collection. CopyTo throws ArgumentNullException(nameof(array)) when the destination array is null, before any bounds checks run.

Source

Thrown at dotnet/src/Microsoft.AutoGen/AgentChat/Abstractions/Messages.cs:321

    /// <inheritdoc cref="ICollection{AIContent}.Clear" />
    public void Clear()
    {
        this.Content.Clear();
    }

    /// <inheritdoc cref="ICollection{AIContent}.Contains" />
    public bool Contains(AIContent item)
    {
        return this.Content.Any(x => x.AIContent == item);
    }

    /// <inheritdoc cref="ICollection{AIContent}.CopyTo" />
    public void CopyTo(AIContent[] array, int arrayIndex)
    {
        if (array == null)
        {
            throw new ArgumentNullException(nameof(array));
        }

        if (arrayIndex < 0 || arrayIndex >= array.Length)
        {
            throw new ArgumentOutOfRangeException(nameof(arrayIndex));
        }

        if (array.Length - arrayIndex < this.Content.Count)
        {
            throw new ArgumentException("The number of elements in the source is greater than the available space from arrayIndex to the end of the destination array.");
        }

        for (var i = 0; i < this.Content.Count; i++)
        {
            array[arrayIndex + i] = this.Content[i].AIContent;
        }
    }

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Allocate the destination array with at least Content.Count elements before calling CopyTo
2. Null-check the array at the call site and fail with your own context-specific error
3. Prefer ToArray()/ToArray-then-Array.Copy or LINQ CopyTo alternatives that handle allocation for you

Example fix

// before
AIContent[] dest = GetBuffer(); // may return null
msg.Content.CopyTo(dest, 0);
// after
AIContent[] dest = new AIContent[msg.Content.Count];
msg.Content.CopyTo(dest, 0);

ArgumentNullException.ThrowIfNull(array);
msg.Content.CopyTo(array, index);

When it happens

Trigger: Calling CopyTo(null, index) on the Content collection of a MultiModalMessage, or a generic helper/BCL API (e.g. LINQ buffer copies, collection initializers) invoking CopyTo with a null array.

Common situations: Hand-written CopyTo calls during refactors; generic collection utilities that forward a caller-supplied array without null checks; interop layers that allocate arrays conditionally and pass null on a failed allocation path.

Related errors

• Specified argument was out of the range of valid values. (Pa
• The number of elements in the source is greater than the ava
• modelName is a required property for LMStudioConfig and cann
• The recent update is not a TextMessage
• The content has more than one choice. Please try another inp

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/89fe321f7187a30a. Report an issue: GitHub.