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Async in C# 5.0

Cover of Async in C# 5.0 by Alex Davies Published by O'Reilly Media, Inc.
  1. Async in C# 5.0
  2. SPECIAL OFFER: Upgrade this ebook with O’Reilly
  3. Preface
    1. Intended Audience
    2. How to Read This Book
    3. Conventions Used in This Book
    4. Using Code Examples
    5. Safari® Books Online
    6. How to Contact Us
    7. Acknowledgments
  4. 1. Introduction
    1. Asynchronous Programming
    2. What’s So Great About Asynchronous Code?
    3. What Is Async?
    4. What Async Does
    5. Async Doesn’t Solve Everything
  5. 2. Why Programs Need to Be Asynchronous
    1. Desktop User Interface Applications
      1. An Analogy: The Cafe
    2. Web Application Server Code
      1. Another Analogy: The Restaurant Kitchen
    3. Silverlight, Windows Phone, and Windows 8
    4. Parallel Code
    5. An Example
  6. 3. Writing Asynchronous Code Manually
    1. Some Asynchronous Patterns Used in .NET
    2. The Simplest Asynchronous Pattern
    3. An Introduction to Task
    4. The Problem with Manual Asynchrony
    5. Converting the Example to Use Manual Asynchronous Code
  7. 4. Writing Async Methods
    1. Converting the Favicon Example to Async
    2. Task and await
    3. Async Method Return Types
    4. Async, Method Signatures, and Interfaces
    5. The return Statement in Async Methods
    6. Async Methods Are Contagious
    7. Async Anonymous Delegates and Lambdas
  8. 5. What await Actually Does
    1. Hibernating and Resuming a Method
    2. The State of the Method
    3. Context
    4. Where await Can’t Be Used
      1. catch and finally Blocks
      2. lock Blocks
      3. LINQ Query Expressions
      4. Unsafe Code
    5. Exception Capture
    6. Async Methods Are Synchronous Until Needed
  9. 6. The Task-Based Asynchronous Pattern
    1. What the TAP Specifies
    2. Using Task for Compute-Intensive Operations
    3. Creating a Puppet Task
    4. Interacting with Old Asynchronous Patterns
    5. Cold and Hot Tasks
    6. Up-Front Work
  10. 7. Utilities for Async Code
    1. Delaying for a Period of Time
    2. Waiting for a Collection of Tasks
    3. Waiting for Any One Task from a Collection
    4. Creating Your Own Combinators
    5. Cancelling Asynchronous Operations
    6. Returning Progress During an Asynchronous Operation
  11. 8. Which Thread Runs My Code?
    1. Before the First await
    2. During the Asynchronous Operation
    3. SynchronizationContext in Detail
    4. await and SynchronizationContext
    5. The Lifecycle of an Async Operation
    6. Choosing Not to Use SynchronizationContext
    7. Interacting with Synchronous Code
  12. 9. Exceptions in Async Code
    1. Exceptions in Async Task-Returning Methods
    2. Unobserved Exceptions
    3. Exceptions in Async void Methods
    4. Fire and Forget
    5. AggregateException and WhenAll
    6. Throwing Exceptions Synchronously
    7. finally in Async Methods
  13. 10. Parallelism Using Async
    1. await and locks
    2. Actors
    3. Using Actors in C#
    4. Task Parallel Library Dataflow
  14. 11. Unit Testing Async Code
    1. The Problem with Unit Testing in Async
    2. Writing Working Async Tests Manually
    3. Using Unit Test Framework Support
  15. 12. Async in ASP.NET Applications
    1. Advantages of Asynchronous Web Server Code
    2. Using Async in ASP.NET MVC 4
    3. Using Async in Older Versions of ASP.NET MVC
    4. Using Async in ASP.NET Web Forms
  16. 13. Async in WinRT Applications
    1. What Is WinRT?
    2. IAsyncAction and IAsyncOperation<T>
    3. Cancellation
    4. Progress
    5. Providing Asynchronous Methods in a WinRT Component
  17. 14. The Async Compiler Transform—in Depth
    1. The stub Method
    2. The State Machine Struct
    3. The MoveNext Method
      1. Your Code
      2. Transforming Returns to Completions
      3. Get to the Right Place in the Method
      4. Pausing the Method for the await
      5. Resuming after the Await
      6. Completing Synchronously
      7. Catching Exceptions
      8. More Complicated Code
    4. Writing Custom Awaitable Types
    5. Interacting with the Debugger
  18. 15. The Performance of Async Code
    1. Measuring Async Overhead
    2. Async Versus Blocking for a Long-Running Operation
    3. Optimizing Async Code for a Long-Running Operation
    4. Async Versus Manual Asynchronous Code
    5. Async Versus Blocking Without a Long-Running Operation
    6. Optimizing Async Code Without a Long-Running Operation
    7. Async Performance Summary
  19. About the Author
  20. SPECIAL OFFER: Upgrade this ebook with O’Reilly
  21. Copyright

Chapter 5. What await Actually Does

There are two ways to think about the async feature of C# 5.0, and in particular what happens at an await keyword:

  • As a language feature, which has a defined behavior that you can learn

  • As a compile-time transformation, that is syntactic sugar for a more complex piece of C# that doesn’t use async

Both are completely true; they are two sides of the same coin. In this chapter, we will concentrate on the first way of looking at async. In Chapter 14, we’ll look at it from the other point of view, which is more complex but provides some details that will make debugging and performance considerations more clear.

Hibernating and Resuming a Method

When the execution of your program reaches an await keyword, we want two things to happen:

  • The current thread executing your code should be released to make your code asynchronous. That means from a normal, synchronous, point of view, your method should return.

  • When the Task that you awaited is complete, your method should continue from where it used to be, as if it hadn’t returned earlier.

To achieve this behavior, your method must pause when it reaches an await, and then resume at a later point.

I think of this process as a small scale version of when you hibernate a computer (S4 sleep). The current state of the method is stored away, and the method exits completely. When a computer hibernates, the dynamic, running state of the computer is saved to disk, and it turns completely off. Just as you can unplug the power ...

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