Threads c sharp

Threads c sharp

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  C# Threads Jim Fawcett CSE681 – Software Modeling and Analysis Fall 2005
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  Thread Class • Every Win32 thread is passed a function to run when created. – When the thread returns from the function it terminates. • In C#, Threads are managed by the System.Threading.Thread class. – C# threads are passed a static or instance function of some C# class using a standard delegate of type ThreadStart.
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  Starting C# Threads • Thread thread = new Thread(new ThreadStart(ThreadFunc)); • thread.Start(); • ThreadFunc can be: – Static or instance member of the class instance that created the thread – Static or instance member of some other class, e.g.: ThreadStart(SomeClass.aStaticFunction); ThreadStart(someClassInstance.aNonStaticFunction);
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  Thread States • A thread that has been started, but not yet terminated can be in one of the following states: – Running – Waiting to run – Suspended – Blocked
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  Thread Properties • IsBackground – get, set – Process does not end until all Foreground threads have ended. – Background threads are terminated when application ends. • CurrentThread – get, static – Returns thread reference to calling thread • IsAlive – get – Has thread started but not terminated? • Priority – get, set – Highest, AboveNormal, Normal, BelowNormal, Lowest • ThreadState – get – Unstarted, Running, Suspended, Stopped, WaitSleepJoin, ..
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  Sharing Resources • A child thread often needs to communciate with its parent thread. It does this via some shared resource, like a queue. Parent Thread Sending Message to Child Child Thread Shared Queue Receiving Message from Parent
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  Synchronization • When two or more threads share a common resource access needs to be serialized - a process called synchronization. – Consider the shared queue on the previous slide. Should the parent start to enqueue an element in an empty queue, but have its time-slice expire before finishing, the queues links are in an undefined state. – Now, if the child thread wakes up, and attempts to dequeue an element the result is undefined.
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  Synchronization with C#Lock // send messages to child thread string msg = ""; for(int i=0; i<50; ++i) { msg = "message #" + i.ToString(); Console.Write("n Sending {0},",msg); // Enqueuing changes links so must lock lock(demo.threadQ) { demo.threadQ.Enqueue(msg); } // control writer speed - twice as fast as reader Thread.Sleep(50); } lock(demo.threadQ) { demo.threadQ.Enqueue("end"); } child.Join(); Console.Write( "nn child thread state = {0}nn",child.ThreadState.ToString() );
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  Demonstration Program • QueuedMessages folder – Illustrates communication between parent and child threads using a queue. – Also illustrates use of C# lock operation.
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  Other Locking Mechanisms • The .Net Threading Library also provides: – Monitor • Locks an object, like C# lock, but provides more control. – Interlocked • Provides atomic operations on 32 bit and 64 bit data types, e.g., ints, longs, pointers. – Mutex • Guards a region of code. • Can synchronize across process boundaries. – AutoResetEvent and WaitOne • Allows fine-grained control of the sequencing of thread operations. – ReaderWriterLock • Locks only when writing, allowing free reads.
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  Locking Certain Collections • ArrayList, Hashtable, Queue, Stack, and other collections provide Synchronized() function, supporting high performance locking. ArrayList unsync = new ArrayList(); ArrayList sync = ArrayList.Synchronized(unsynch); Your code needs no lock constructs with sync.
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  Method Decoration • Methods can be decorated with a MethodImpl attribute, synchronizing access much like a Win32 critical section. [MethodImpl (MethodImplOptions.Synchronized)] string myMethod(string input) { … } Note that this synchronizes a region of code, while lock and Monitor synchronize objects.
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  WinForms and WorkerThreads • A UI thread is a thread that creates a window. A worker thread is a thread spawned by a UI thread to do work in the background while the UI thread services UI messages. • A worker thread must never access UI functions directly. It accesses them through Form’s Invoke, BeginInvoke, and EndInvoke functions, passing a delegate as an argument.
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  BeginInvoke Example for (i= 1; i <= 25; i++) { s = "Step number " + i.ToString() + " executed"; Thread.Sleep(400); // Make asynchronous call to main form. // MainForm.AddString function runs in main thread // because we activated the delegate through form's // Invoke (synchronous) or BeginInvoke (asynchronous) functions. // To make synchronous call use Invoke. m_form.BeginInvoke(m_form.m_DelegateAddString, new Object[] {s}); // check if thread is cancelled if ( m_EventStop.WaitOne(0, true) ) { // clean-up operations may be placed here // ... Delegate arguments passed as an array of // inform main thread that this thread stopped m_EventStopped.Set(); objects return; } }
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  Demonstration Programs • ProcessDemo and ProcessDemoWin32 – Illustrates creating a child process • QueuedMessages – Illustrates communication between threads using queues and the C# lock operation. • FormInvokeDemo folder – A more interesting demonstration of the above. • WorkerThread folder – Simple Demonstration of UI and Worker thread communication using Form.Invoke(…) • ThreadPoolDemo folder – Illustrates how to use the ThreadPool to run functions
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  End of Presentation