C# Language Specification

Async Functions

A method (§10.6) or anonymous function (§7.15) with the async modifier is called an async function. In general, the term async is used to describe any kind of function that has the async modifier.

It is a compile-time error for the formal parameter list of an async function to specify any ref or out parameters.

The return-type of an async method must be either void or a task type. The task types are System.Threading.Tasks.Task and types constructed from System.Threading.Tasks.Task. For the sake of brevity, in this chapter these types are referenced as Task and Task, respectively. An async method returning a task type is said to be task-returning.

The exact definition of the task types is implementation defined, but from the language’s point of view a task type is in one of the states incomplete, succeeded or faulted. A faulted task records a pertinent exception. A succeeded Task<T> records a result of type T. Task types are awaitable, and can therefore be the operands of await expressions (§7.7.7).

An async function invocation has the ability to suspend evaluation by means of await expressions (§7.7.7) in its body. Evaluation may later be resumed at the point of the suspending await expression by means of a resumption delegate. The resumption delegate is of type System.Action, and when it is invoked, evaluation of the async function invocation will resume from the await expression where it left off. The current caller of an async function invocation is the original caller if the function invocation has never been suspended, or the most recent caller of the resumption delegate otherwise.

1.