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Here is complete from MSDN.. The Async feature provides an easy and intuitive way to write asynchronous code. This feature makes asynchronous programming almost as straightforward as synchronous programming. Previously, writing asynchronous code has required you to define callbacks (also referred to as continuations) to capture what occurs after asynchronous operations finish. This complicates your code, and makes routine tasks, such as exception handling, awkward and difficult. By using the Async feature, you can call asynchronous methods without writing continuations, and without splitting your code across multiple methods or lambda expressions. The compiler does the hard work for you. The feature introduces two new keywords, async and await. The async modifier specifies that a method is an async method. When called, an async method returns a Task or Task(Of TResult), which represents the ongoing work of the method. The task contains information that the caller of the asynchronous method can use, such as the status of the task and its unique ID. Typically, the await operator is applied to the returned task. The await operator suspends execution of the method that called the asynchronous method until the task is completed. In the meantime, control is returned to the caller of the suspended method. For more information, see Asynchronous Programming with Async and Await (C# )