TaskWhenAny

Task.WhenAny is a static method in the .NET framework (System.Threading.Tasks) that implements a race among multiple tasks. It returns a task that completes as soon as any one of the supplied tasks completes. The result is the completed task itself, not its value.

Overloads and return types include non-generic and generic forms. For non-generic tasks, the method returns Task<Task>, with overloads such as Task.WhenAny(params Task[] tasks) and Task.WhenAny(IEnumerable<Task> tasks). For generic tasks, there are overloads like Task.WhenAny<TResult>(IEnumerable<Task<TResult>> tasks) which return Task<Task<TResult>>. In all cases, the outer task completes when any input task completes, and the inner task is the actual completed task.

Behavior and semantics: WhenAny completes, it does not guarantee that the first completed task is successful; it may be ran into a faulted or canceled state. The outer task completes as soon as the first input task finishes, regardless of outcome. To observe the result or exception, you typically await the inner task (for example, after awaiting the outer task you await the resulting completed task). It does not cancel or otherwise terminate the remaining tasks automatically; cancellation must be handled separately through cancellation tokens or manual logic.

Usage patterns include racing multiple asynchronous operations to proceed with the quickest result, implementing timeouts, or coordinating asynchronous work where the earliest completion dictates subsequent steps. It is often used in conjunction with Task.WhenAll or with explicit cancellation to manage remaining tasks after the first completion.

See also: Task.WhenAll, cancellation and timeouts in asynchronous programming, and patterns for coordinating multiple tasks in the .NET Task Parallel Library.