Pro Asynchronous Programming with.net. Richard Blewett Andrew Clymer

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1 Pro Asynchronous Programming with.net Richard Blewett Andrew Clymer

2 Pro Asynchronous Programming with.net Copyright 2013 by Richard Blewett and Andrew Clymer This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. ISBN-13 (pbk): ISBN-13 (electronic): Trademarked names, logos, and images may appear in this book. Rather than use a trademark symbol with every occurrence of a trademarked name, logo, or image we use the names, logos, and images only in an editorial fashion and to the benefit of the trademark owner, with no intention of infringement of the trademark. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. President and Publisher: Paul Manning Lead Editor: Gwenan Spearing Technical Reviewer: Liam Westley Editorial Board: Steve Anglin, Mark Beckner, Ewan Buckingham, Gary Cornell, Louise Corrigan, Morgan Ertel, Jonathan Gennick, Jonathan Hassell, Robert Hutchinson, Michelle Lowman, James Markham, Matthew Moodie, Jeff Olson, Jeffrey Pepper, Douglas Pundick, Ben Renow-Clarke, Dominic Shakeshaft, Gwenan Spearing, Matt Wade, Tom Welsh Coordinating Editor: Christine Ricketts Copy Editor: James Fraleigh Compositor: SPi Global Indexer: SPi Global Artist: SPi Global Cover Designer: Anna Ishchenko Distributed to the book trade worldwide by Springer Science+Business Media New York, 233 Spring Street, 6th Floor, New York, NY Phone SPRINGER, fax (201) , orders-ny@springer-sbm.com, or visit Apress Media, LLC is a California LLC and the sole member (owner) is Springer Science + Business Media Finance Inc (SSBM Finance Inc). SSBM Finance Inc is a Delaware corporation. For information on translations, please rights@apress.com, or visit Apress and friends of ED books may be purchased in bulk for academic, corporate, or promotional use. ebook versions and licenses are also available for most titles. For more information, reference our Special Bulk Sales ebook Licensing web page at Any source code or other supplementary materials referenced by the author in this text is available to readers at For detailed information about how to locate your book s source code, go to

3 For my mum and dad Rich For my loving wife For my mum and dad From tiny ZX81 mighty programs of today do grow. Andy

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5 Contents at a Glance About the Authors... xvii About the Technical Reviewer... xix Acknowledgments... xxi Chapter 1: An Introduction to Asynchronous Programming...1 Chapter 2: The Evolution of the.net Asynchronous API...7 Chapter 3: Tasks...31 Chapter 4: Basic Thread Safety...57 Chapter 5: Concurrent Data Structures...89 Chapter 6: Asynchronous UI Chapter 7: async and await Chapter 8: Everything a Task Chapter 9: Server-Side Async Chapter 10: TPL Dataflow Chapter 11: Parallel Programming Chapter 12: Task Scheduling Chapter 13: Debugging Async with Visual Studio Chapter 14: Debugging Async Beyond Visual Studio Index v

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7 Contents About the Authors... xvii About the Technical Reviewer... xix Acknowledgments... xxi Chapter 1: An Introduction to Asynchronous Programming...1 What Is Asynchronous Programming?...1 The Drive to Asynchrony...1 Mechanisms for Asynchrony...2 Multiple Machines... 2 Multiple Processes... 3 Multiple Threads... 3 Thread Scheduling...3 Threads and Resources...4 Thread-Specific Resources... 4 Resources Shared by Threads... 4 Summary...5 Chapter 2: The Evolution of the.net Asynchronous API...7 Asynchrony in the World of.net System.Threading.Thread... 7 Using the System Thread Pool Changes to Async in.net vii

8 Contents Asynchrony in.net Logical and Physical Separation Passing Data into a Thread Closures SynchronizationContext Event-Based Asynchronous Pattern Minor Changes in.net Lambda Expressions Thread Pool Heuristics in.net Big Changes in.net Remodeling the Thread Pool Queue Work-Stealing Queues Thread Pool Heuristics in.net Summary...30 Chapter 3: Tasks...31 What Is a Task?...31 Creating a Compute-Based Task...31 Passing Data into a Task Dangers of Closures Returning Data from a Task...37 Creating I/O-Based Tasks Error Handling...41 Ignoring Errors Designing Task-Based APIs Cancellation Progress Task Relationships...50 Chaining Tasks (Continuations) Nested and Child Tasks Conclusion...55 viii

9 Contents Chapter 4: Basic Thread Safety...57 Asynchrony and Data...57 It s Not Always Good to Share Immutable State Atomic State Transition Nonatomic State Transition Correctness Is Not the Only Problem Thread Safety The Interlocked Class...61 Basic Operations Richer Functions Monitor: The Workhorse of.net Synchronization...64 The lock Keyword Timing Out of Monitor Acquisition Signaling with Monitors Signaling As a Building Block Optimizing for Read...73 ReaderWriterLock ReaderWriterLockSlim A Semaphore Out of the Box...75 Raising the Starting Gate: ManualResetEventSlim...78 CountdownEvent: Simplifying Fork and Join...80 Barrier: Rendezvous-Based Synchronization...81 Crossing the AppDomain Boundary with WaitHandle...83 Mutex Semaphore Events WaitHandle The Kernel Synchronization Abstraction Working with Multiple WaitHandles Integrating Standard Primitives and Kernel Objects ix

10 Contents x Synchronization Is Not the Only Answer...87 Conclusion...88 Chapter 5: Concurrent Data Structures...89 Simplifying Thread Safety...89 Lazy<T>...94 Concurrent Collections...98 ConcurrentDictionary<K,V>...99 Locking Mechanics ConcurrentQueue<T> and ConcurrentStack<T> ConcurrentBag<T> Blocking Collections Graceful Shutdown Consuming Enumerable BlockingCollection of X Summary Chapter 6: Asynchronous UI UI Mechanics UI Threading Model Synchronization Context Send and Post Task Continuations Event-Based Asynchronous Pattern (EAP) Background Worker Data Binding Windows Forms Windows Presentation Foundation (WPF) WinRT WPF Dispatcher Obtaining the Dispatcher Executing Work Through the Dispatcher

11 Contents WinRT Dispatcher Obtaining the Dispatcher Executing Work Through the Dispatcher UI Timers Windows Forms Timer WinRT and WPF Dispatch Timers WPF Freezable Components Too Much of a Good Thing Summary Chapter 7: async and await Making Asynchronous Programming Simpler What Do async and await Actually Do? Returning Values from async Methods Should You Always Continue on the UI Thread? Task.Delay Task.WhenAll Task.WhenAll, Error Handling Task.WhenAny async/await Mechanics Summary Chapter 8: Everything a Task TaskCompletionSource<T> Worked Example: Creating a Foreground Task Unit Testing and Stubbing Asynchronous Methods Building Task-Based Combinators Improved WhenAny Alternative WhenAll, WhenAllOrFail Summary xi

12 Contents Chapter 9: Server-Side Async Natural Parallelism The Problem of I/O ASP.NET WebForms A Synchronous WebForms Implementation Asynchronous Pages in WebForms Asynchronous Pages in WebForms ASP.NET MVC Asynchronous MVC Processing in.net Asynchronous MVC Processing in.net ASP.NET Web API Asynchronous Web API Operations in.net Asynchronous Web API Operations in.net Windows Communication Foundation Asynchronous WCF Services in.net Asynchronous WCF Services in.net Summary Chapter 10: TPL Dataflow The Building Blocks Producer and Consumer Revisited Linking Blocks Transform Block Transform Many Block Linking to Multiple Targets Shutting Down Gracefully Propagating Completion Error Handling Cancellation xii

13 Contents Glue Blocks Buffer Block Batch Block Broadcast Block Joining Asynchronous Blocks Summary Chapter 11: Parallel Programming What Is Driving the Need for Parallelism? Coarse- and Fine-Grained Parallelism Task and Data-Based Parallelism Is It Worth Trying to Parallelize Everything? Before You Parallelize Parallel Class Parallel.Invoke Parallel Loops PLINQ Moving from Sequential LINQ to PLINQ Influencing and Configuring the Query ForAll Aggregating Results Summary Chapter 12: Task Scheduling ConcurrentExclusiveSchedulerPair Why Write a Task Scheduler? The TaskScheduler Abstraction Implementing QueueTask Implementing GetScheduledTasks Implementing TryExecuteTaskInline Executing Tasks xiii

14 Contents Implementing a Custom Scheduler Creating a Basic Implementation Adding Threads on Demand Removing Idle Threads Unit Testing Custom Schedulers Controlling Execution Order with Synchronization Primitives Adding Members to the Scheduler to Provide Insight Deriving a Testable Class from the Scheduler Summary Chapter 13: Debugging Async with Visual Studio Types of Multithreading Bugs Data Corruption Race Conditions Deadlocks Runaway Threads The Limitations of Using Visual Studio for Debugging The Interactive Debugger It Works on My Machine Multithreaded Visual Studio Debugging Basics Breakpoints and Threads Locals, Autos, and Watch Windows The Call Stack Window The Threads Window Debugging Tasks The Parallel Tasks / Tasks Window The Parallel Stacks Window The Concurrency Visualizer Summary xiv

15 Contents Chapter 14: Debugging Async Beyond Visual Studio Memory Dumps Generating a Memory Dump Task Manager DebugDiag ADPLUS Analyzing Memory Dumps WinDbg SOS SOSEX PSSCOR Summary Index xv

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17 About the Authors Richard Blewett is a professional software developer and trainer living in Bristol in the UK. He has been writing software for over 20 years and has spent most of that time building distributed systems of one form or another. He first started writing multithreaded code in C on OS/2 and continued when moving to Windows, COM, and.net. Over the years he has built software for banks, insurance companies, travel companies, emergency services, health companies, and more. He is a cofounder of Rock Solid Knowledge, a development, training and consultancy company specializing in.net development, and a Microsoft Integration MVP. Andrew Clymer is a professional software developer and educator living in Chippenham, Wiltshire (UK). Andy cut his teeth working in various startups and programming on a host of platforms, finally working for a small startup that was acquired by Cisco in Having worked for Cisco for a few years, the lure of the startup world was too much to resist. He is a cofounder of Rock Solid Knowledge. xvii

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19 About the Technical Reviewer Liam Westley has been using Microsoft technologies since the days of Visual Basic 1.0 and is currently a Microsoft MVP in C#. He s a prolific speaker in the UK developer community, talking at user groups and community conferences. After 15 years in the television industry, including working with BSkyB, Reuters Television, chellomedia, GMTV, and the QVC UK shopping channel, Liam moved into the world of tech startups in London. He is currently one of a team of application architects at Huddle, where he works with some of the best.net developers and UX designers to deliver world-class collaboration software. He quite likes working just off Old Street as there is some fantastic food and coffee to be had within a few minutes walk. Photo courtesy of Ian Battersby/DDDNorth. xix

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21 Acknowledgments We d like to thank all the folks at Apress who have helped us ship this book: Gwenan Spearing for getting involved in the first place; Christine Ricketts for her patience and perseverance in keeping us on track; Tom Welsh and Matthew Moodie for their great editing; and James Fraleigh for his copyediting. We d also like to thank Liam Westley for his technical reviews of the chapters. Thank you and apologies to Michele, Carmel, Tegen, and Ewan for bearing with me as I paced around the house with writer s block. I couldn t have gotten here without your understanding and support. Richard Blewett Thanks to Jules, Emily, and Joseph for putting up with me juggling so many things over the last 12 months and for picking up the things I dropped. Thanks to my Mum and Dad, who through the purchase of many home computers set me off on the most incredible of journeys. Andrew Clymer xxi