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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
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Chapter 8. Which Thread Runs My Code?

As I’ve said before, asynchronous programming is all about threads. In C#, that means we need to understand which .NET thread is running our code at what points in the program, and what happens to the threads while long-running operations take place.

Before the First await

In each async method you write, some code will be before the first occurrence of the await keyword. Equally, some code is in the expression that gets awaited.

This code always runs in the calling thread. Nothing interesting happens before the first await.

Note

This is one of the most common misconceptions about async. Async never schedules your method to run on a background thread. The only way to do that is using something like Task.Run, which is explicitly for that purpose.

In the case of a UI application, that means the code before the first await runs in the UI thread. Likewise, in an ASP.NET web application, it runs in an ASP.NET worker thread.

Typically, you might run another async method as the expression being awaited on the line containing the first await. Because this expression is executed before the first await, it must also get run in the calling thread. That means the calling thread will continue executing code deep into your application, all the way until some method actually returns a Task. The method that does that might be a framework method, or it might be a method using TaskCompletionSource to construct a puppet Task. That method is the source of the asynchrony ...

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