AvaloniaUI/Avalonia · error · ArgumentNullException

Value cannot be null. (Parameter 'item')

Error message

Value cannot be null. (Parameter 'item')

What it means

CompositeDisposable.Add(item) appends a disposable to the composite (or disposes it immediately if the composite is already disposed). It throws ArgumentNullException(nameof(item)) when item is null, preserving the no-null-disposables invariant at the mutation boundary.

Source

Thrown at src/Avalonia.Base/Reactive/CompositeDisposable.cs:111

        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>
    public void Add(IDisposable item)
    {
        if (item == null)
        {
            throw new ArgumentNullException(nameof(item));
        }

        lock (_gate)
        {
            if (!_disposed)
            {
                _disposables.Add(item);

                // If read atomically outside the lock, it should be written atomically inside
                // the plain read on _count is fine here because manipulation always happens
                // from inside a lock.
                Volatile.Write(ref _count, _count + 1);
                return;
            }
        }

        item.Dispose();
    }

View on GitHub (pinned to 11c5427268)

Solutions

1. Null-check before Add: if (d is not null) composite.Add(d);.
2. Ensure subscription helpers always return a non-null IDisposable (use Disposable.Empty for no-ops).
3. Prefer the params/IList constructor for bulk adds so nulls surface at one validated point.

Example fix

// before
composite.Add(maybeNullDisposable);

// after
if (maybeNullDisposable is not null) composite.Add(maybeNullDisposable);

if (item is not null) composite.Add(item);

bool IsDisposable(IDisposable? d) => d is not null;

When it happens

Trigger: Calling composite.Add(null), or Add-ing a disposable obtained from a subscription/factory that returned null.

Common situations: Adding a subscription token whose creation method returned null, or a conditional add where the disposable was not created.

Related errors

• Value cannot be null. (Parameter 'disposables')
• Value cannot be null. (Parameter 'array')
• Value cannot be null. (Parameter 'tcs')
• Value cannot be null. (Parameter 'onNext')
• Value cannot be null. (Parameter 'onError')

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/8b994fe5289b82b4. Report an issue: GitHub.