Cysharp/UniTask · error · ArgumentException

The tasks argument contains no tasks.

Error message

The tasks argument contains no tasks.

What it means

Thrown by WhenAnyPromise<T> constructor (the generic WhenAny for UniTask<T>) when tasksLength is zero. WhenAny semantically requires at least one task to race; with zero tasks there is nothing to await and the result is undefined. UniTask rejects this at construction rather than hanging forever.

Solutions

1. Check that the task collection is non-empty before calling WhenAny
2. Fall back to a default task or UniTask.Never when the list is empty
3. Guard with: if (tasks.Count == 0) return; before WhenAny

Example fix

// before
var (winIndex, _) = await UniTask.WhenAny(tasks.ToArray());

// after
if (tasks.Count == 0) return;
var (winIndex, _) = await UniTask.WhenAny(tasks.ToArray());

if (tasks == null || tasks.Length == 0)
{
    // handle empty case: return default, await Never, or skip
    return default;
}
var (winIndex, result) = await UniTask.WhenAny(tasks);

static bool HasAnyTasks<T>(UniTask<T>[] tasks) => tasks != null && tasks.Length > 0;

When it happens

Trigger: Calling UniTask.WhenAny(emptyArray) or WhenAny(list.ToArray()) on an empty collection. Passing a tasks array whose effective length (tasksLength) is zero.

Common situations: Dynamically building a task list that ends up empty due to filtering or all items being skipped. A race between populating a list and calling WhenAny. Editor or test scenarios with no registered tasks.

Related errors

• Delay does not allow minus delayFrameCount. delayFrameCount:
• Delay does not allow minus delayTimeSpan. delayTimeSpan:
• Already continuation registered, can not await twice or get…
• Can not trigger itself in iterating.
• continuation

AI-assisted analysis of Cysharp/UniTask@ceac8d6946 (2026-08-13). Data as JSON: /api/errors/8d0b6520c2406eac. Report an issue: GitHub.

Appendix: source

Thrown at src/UniTask/Assets/Plugins/UniTask/Runtime/UniTask.WhenAny.cs:186

            }

            void IUniTaskSource.GetResult(short token)
            {
                GetResult(token);
            }
        }


        sealed class WhenAnyPromise<T> : IUniTaskSource<(int, T)>
        {
            int completedCount;
            UniTaskCompletionSourceCore<(int, T)> core;

            public WhenAnyPromise(UniTask<T>[] tasks, int tasksLength)
            {
                if (tasksLength == 0)
                {
                    throw new ArgumentException("The tasks argument contains no tasks.");
                }

                TaskTracker.TrackActiveTask(this, 3);

                for (int i = 0; i < tasksLength; i++)
                {
                    UniTask<T>.Awaiter awaiter;
                    try
                    {
                        awaiter = tasks[i].GetAwaiter();
                    }
                    catch (Exception ex)
                    {
                        core.TrySetException(ex);
                        continue; // consume others.
                    }

                    if (awaiter.IsCompleted)

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