Fawcett C#Threads
Jim CSE681 Software Modeling & Analysis Fall 2002
ThreadClass
Every Win32 thread is passed a function to
run when created.
terminates.
When the thread returns from the function it
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.
StartingC#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
It must have the signature: void myThreadFunc(); You can supply parameters for thread operation as member data of the ThreadFuncs class.
ExamplesofThreadStart
ThreadFunc is a static member of TFClass:
Thread thread = new ThreadStart(TFClass.ThreadFunc)
ThreadFunc is a non-static member of TFClass:
TFClass tfc; Thread thread = new ThreadStart(tfc.ThreadFunc);
ThreadStates
A thread that has been started, but not yet
terminated can be in one of the following states:
Running Waiting to run Suspended Blocked
ThreadProperties
IsBackground get, set
Process does not end until all Foreground threads have ended. Background threads are terminated when application ends. Returns thread reference to calling thread Has thread started but not terminated? Highest, AboveNormal, Normal, BelowNormal, Lowest Unstarted, Running, Suspended, Stopped, WaitSleepJoin, ..
CurrentThread get, static
IsAlive get
Priority get, set
ThreadState get
SharingResources
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
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.
SynchronizationwithC#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( child "\n\n thread state = {0}\n\n",child.ThreadState.ToString() );
DemonstrationProgram
QueuedMessages folder
Illustrates communication between parent and
child threads using a queue. Also illustrates use of C# lock operation.
OtherLockingMechanisms
The .Net Threading Library also provides: Monitor
Locks an object, like C# lock, but provides more control. Provides atomic operations on 32 bit and 64 bit data types, e.g., ints, longs, pointers. Guards a region of code. Can synchronize across process boundaries. Allows fine-grained control of the sequencing of thread operations. Locks only when writing, allowing free reads.
Interlocked Mutex
AutoResetEvent and WaitOne ReaderWriterLock
LockingCertainCollections
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.
MethodDecoration
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.
WinFormsandWorkerThreads
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. functions directly. It accesses them through Forms Invoke, BeginInvoke, and EndInvoke functions, passing a delegate as an argument.
A worker thread must never access UI
BeginInvokeExample
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 BeginInvoke function. To make synchronous call use Invoke.
Delegate arguments passed as an array of objects
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 // ... // inform main thread that this thread stopped m_EventStopped.Set(); return; }
}
DemonstrationPrograms
ProcessDemo
Illustrates creating a child process
Worker Thread folder
Simple Demonstration of UI and Worker thread
communication using Form.Invoke()
FormInvokeDemo folder
A more interesting demonstration of the above.
QueuedMessages
Illustrates communication between threads using
queues and the C# lock operation.
EndofPresentation