AvaloniaUI/Avalonia · error · ArgumentException

Disposables can't contain null

Error message

Disposables can't contain null

What it means

CompositeDisposable.ToList helper copies the source disposables and, per the doc contract, throws ArgumentException('Disposables can't contain null', nameof(disposables)) if any element is null. The copy and null-check are fused into one loop for efficiency. This enforces the invariant that the composite never stores null disposables.

Source

Thrown at src/Avalonia.Base/Reactive/CompositeDisposable.cs:88

    private static List<IDisposable?> ToList(IEnumerable<IDisposable> disposables)
    {
        var capacity = disposables switch
        {
            IDisposable[] a => a.Length,
            ICollection<IDisposable> c => c.Count,
            _ => 12
        };

        var list = new List<IDisposable?>(capacity);

        // do the copy and null-check in one step to avoid a
        // second loop for just checking for null items
        foreach (var d in disposables)
        {
            if (d == null)
            {
                throw new ArgumentException("Disposables can't contain null", nameof(disposables));
            }

            list.Add(d);
        }

        return list;
    }

    /// <summary>
    /// Gets the number of disposables contained in the <see cref="CompositeDisposable"/>.
    /// </summary>
    public int Count => Volatile.Read(ref _count);

    /// <summary>
    /// Adds a disposable to the <see cref="CompositeDisposable"/> or disposes the disposable if the <see cref="CompositeDisposable"/> is disposed.
    /// </summary>
    /// <param name="item">Disposable to add.</param>
    /// <exception cref="ArgumentNullException"><paramref name="item"/> is <c>null</c>.</exception>

View on GitHub (pinned to 11c5427268)

Solutions

1. Filter nulls before construction: disposables.Where(d => d is not null).
2. Fix the producing code so it never returns null disposables.
3. If an absent disposable is legitimate, substitute Disposable.Empty rather than null.

Example fix

// before
var cd = new CompositeDisposable(maybeWithNulls);

// after
var cd = new CompositeDisposable(maybeWithNulls.Where(d => d is not null).ToArray());

var clean = disposables.Where(d => d is not null).ToArray();
var cd = new CompositeDisposable(clean);

bool NoNulls(IEnumerable<IDisposable?> ds) => ds.All(d => d is not null);

When it happens

Trigger: Constructing CompositeDisposable with any collection (params array or IList) whose enumeration contains at least one null element, e.g. new CompositeDisposable(d1, null, d3).

Common situations: A disposable returned null from a factory (e.g. a subscribe that returned null), a LINQ Select producing nulls, or an array initializer with an accidental null slot.

Related errors

• Specified argument was out of the range of valid values. (Pa
• Value cannot be null. (Parameter 'disposables')
• Value cannot be null. (Parameter 'item')
• Value cannot be null. (Parameter 'array')
• Specified argument was out of the range of valid values. (Pa

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/f7b3d46e5ac2c6bb. Report an issue: GitHub.