The async modifier indicates that a method or lambda expression contains at least one await expression. An async method executes its body in the same thread as the calling method. (Throughout the remainder of this discussion, we’ll use the term “method” to mean “method or lambda expression.”)

An await expression, which can appear only in an async method, consists of the await operator followed by an expression that returns an awaitable entity—typically a Task object (as you’ll see in Section 23.3), though it is possible to create your own awaitable entities. Creating awaitable entities is beyond the scope of our discussion. For more information, see

http://blogs.msdn.com/b/pfxteam/archive/2011/01/13/10115642.aspx

When an async method encounters an await expression:

• If the asynchronous task has already completed, the async method simply continues executing.

• Otherwise, program control returns to the async method’s caller until the asynchronous task completes execution. This allows the caller to perform other work that does not depend on the results of the asynchronous task.

When the asynchronous task completes, control returns to the async method and continues with the next statement after the await expression.

The async and await mechanism does not create new threads. If any threads are required, the method that you call to start an asynchronous task on which you await the results is responsible for creating the threads that are used to perform the asynchronous task. For example, we’ll show how to use class Task’s Run method in several examples to start new threads of execution for executing tasks asynchronously. Task method Run returns a Task on which a method can await the result.