How to Build Multi-threaded Applications in .NET Mazen S. Alzogbi Technology Specialist Microsoft Corporation

1. How to Build Multi-threaded Applications in .NET Mazen S. Alzogbi Technology SpecialistMicrosoft Corporation

2. What we will cover • The purpose of building multi-threading applications • When multi-threading should be used, the pros and cons for multi-threaded applications • The various multi-threading implementations, the pros and cons for each. • How .NET simplifies building multi-threaded applications

3. Session Prerequisites • Component-Based or Object-Oriented Development • At least one of: • Microsoft® C# • VB.NET • Visual C++® • Overview knowledge of the .NET Framework • MTS, COM, DCOM, COM+ Level 300

4. Agenda • Multi-threading in .NET • Synchronization • Inter-thread communication • Improving Performance with Thread Pools

5. Simplify coding Perceived performance Performance Multi-threading in .NETWhy use threads?

6. Multi-threading in .NETWhen Should You Use Threads? • Take advantage of multi-processor systems for intensive computation • Separate threads for different tasks • Keeping the user interface "live" • Waiting for I/O • Split out complex test conditionsand loops • Creating servers • Local and network

7. Multi-threading in .NETThreading Issues • If threads are so good, why not use them everywhere? • Contention for shared resources • Need to co-operate to avoid corruption • Writing is more of a problem than reading

8. Multi-threading in .NETThreading on the Windows® Platform • Win32 threads available from C/C++ • COM threads and apartments • Single Threaded Apartment (STA) • Developer does not worry about threads • Limitation on performance • Multi-Threaded Apartment (MTA) • Create thread-aware components • Marshal interface pointers across apartments

9. Multi-threading in .NET.NET Simplifies Threading Choices • Common access from all language variants using System.Threading; ... ThreadStart ts = new ThreadStart(myMethod); Thread p1 = new Thread(ts); p1.Name = "MyThread"; P1.Start(); public void myMethod() { // Do some work ... }

10. Multi-threading in .NETUsing Threads • Threads started by application • Pass in parameters as properties on target object • Threads stop when method is exited • Thread object cannot be re-used • Can stop thread with Abort method • Distinguish between thread and object

11. Consumer object and thread Producer object and thread Work queue Completed queue Multi-threading in .NETSimple Sample Application in C# Work units are arrays of integers Completed queue holds sorted workunits

12. Demonstration 1Multi-threaded application in C#Tour of ApplicationRunning the Application

13. Agenda • Multi-threading in .NET • Synchronization • Inter-thread communication • Improving Performance with Thread Pools

14. SynchronizationFrom STA to MTA • Developer must consider threading issues • Multiple threads contend for resources • Instance variables • Database connections • GUI controls • Writing is more of a problem than reading • Identify critical regions that access resources • Some more critical than others

15. SynchronizationManaging Contention • Protect critical regions with synchronization • Objects have a lock that can be obtained • Only one thread can own the lock at one time • Other threads can block and wait… • … or test the lock and take alternative action • Very important to release locks • Deadlock and "liveness problems" can ensue

16. SynchronizationUsing a lock Block SyncLock In VB.NET • Example: lock block in C# Object whose lock is obtained lock (this) { workQueue[nextFreeElement] = unit; nextFreeElement++; } } Criticalcode Lock released at end of block

17. Demonstration 2Adding SynchronizationAdding Basic Synchronization

18. SynchronizationObtaining Locks with Monitors • Example: Monitor usage in C# Monitor.Enter(this); workQueue[nextFreeElement] = unit; nextFreeElement++; Monitor.Exit(this); } Criticalcode Must remember to explicitly release the lock

19. Demonstration 3Using MonitorsUsing Monitors

20. SynchronizationChoosing a Synchronization Method • Things to consider • Complexity • Ability to split locks • Specialized mechanisms, e.g. ReaderWriterLock

21. Agenda • Multi-threading in .NET • Synchronization • Inter-thread Communication • Improving Performance with Thread Pools

22. Inter-thread CommunicationWhen Threads Need to Talk • Waiting for a shared resource to become free • Notification of the arrival of some work • Producer and consumer model • Waiting for clients • Network server model • Generally important application events • For example: A long-running task has finished

23. Inter-thread CommunicationThread lifecycle • Waiting and ready queues • States Suspended Running Unstarted Aborted WaitSleepJoin

24. Inter-thread CommunicationSimple communication with Wait/Pulse • Monitor.Wait() • Supply object to wait on • Enter WaitSleepJoin • Timed waits • Monitor.Pulse() and Monitor.PulseAll() • Choice between them • Can only call them when you have the lock • Wait() call gives up the lock until it resumes • What happens when Pulse() is called before Wait()?

25. Inter-thread CommunicationNotification Alternatives • Pausing/sleeping • Events (incl. ManualResetEvent) • Timers (server and windows) • Interlocked increment and decrement

26. Consumer object and thread Pulse Add workunit Add workunit Queue full- Wait Carry on Removework unit Inter-thread Communication Wait/Pulse in Sample Application Producer object and thread Work queue

27. Demonstration 4Notification Between ThreadsWaiting for an Event

28. Inter-thread CommunicationManipulating other threads • Finding threads • Thread information • ThreadState • IsAlive • Joining another thread • Thread.Join()

29. Inter-thread CommunicationTerminating threads • Stopping a thread • Interrupt + InterruptedException • Abort • Abort and inconsistent state • As locks are released, who will clean up objects? • Alternative approaches using flags • Use join() to check if thread has exited yet

30. Agenda • Multi-threading in .NET • Synchronization • Inter-thread communication • Improving Performance with Thread Pools

31. Thread PoolsWhat are thread pools? • Set of "worker" threads • Thread controller assigns them to tasks • Submit task to thread controller • Number of threads in pool can grow or shrink • Thread controller can use common metrics • Developer does not have to worry about starting and terminating threads • Useful for optimizing thread usage where there are many clients or units of work

32. Thread Pools.NET thread pools • One thread pool per-process • Pool is created when first work item is added • Developer has no control over number of threads created • Do NOT Abort pooled threads

33. Thread PoolsQueuing work items • ThreadPool class • QueueUserWorkItem static method • Work item consists of • Method to run is defined by WaitCallback delegate (public void (object state)) • State to be passed as object • Still pass other state by setting variables on the method's object

34. Thread PoolsCoding to work with thread pools • Still need all synchronization as before • Detection of when job is finished • Events work well in this environment • Use a .NET Event • Define a callback method • Register with event producer • Event producer then notifies all listeners

35. Thread PoolsDelaying execution • Timed execution of delegates • Useful for timer events etc. • Use RegisterWaitForSingleObject

36. Assign threadto delegate Queuework item Thread PoolsSample Application with thread pool Producer Consumer Thread pool

37. Demonstration 5Using a Thread PoolChanging the Consumer Changing the ProducerEvaluating the Changes

38. Session Summary • .NET has powerful, cross-language threading capabilities • .NET makes it easy to use multiple threads • Comprehensive library of classes and delegates • Benefits of multi-threading include • Simpler code • Faster execution • Improved user perception of performance • Developer must think more about thread interaction

39. Thank You!