The advantage of threading is the ability to create applications that use more than one thread of execution. For example, a process can have a user interface thread that manages interactions with the user and worker threads that perform other tasks while the user interface thread waits for user input.
This tutorial demonstrates various thread activities:
Creating and executing a thread
Synchronization of threads
Interaction between threads
Using a thread pool
Using a mutex object to protect a shared resource
See Threading Sample to download and build the sample files discussed in this tutorial.
Sample Files
Example 1: Creating, starting, and interacting between threads
This example demonstrates how to create and start a thread, and shows the interaction between two threads running simultaneously within the same process. Note that you don’t have to stop or free the thread. This is done automatically by the .NET Framework common language runtime.
The program begins by creating an object of type Alpha (oAlpha) and a thread (oThread) that references the Beta method of the Alpha class. The thread is then started. TheIsAlive property of the thread allows the program to wait until the thread is initialized (created, allocated, and so on). The main thread is accessed through Thread, and theSleep method tells the thread to give up its time slice and stop executing for a certain amount of milliseconds. The oThread is then stopped and joined. Joining a thread makes the main thread wait for it to die or for a specified time to expire (for more details, see Thread.Join Method). Finally, the program attempts to restart oThread, but fails because a thread cannot be restarted after it is stopped (aborted). For information on temporary cessation of execution, see Suspending Thread Execution.
1: // StopJoin.cs
2: using System;
3: using System.Threading;
4:
5: public class Alpha
6: {
7:
8: // This method that will be called when the thread is started
9: public void Beta()
10: {
11: while (true)
12: {
13: Console.WriteLine("Alpha.Beta is running in its own thread.");
14: }
15: }
16: };
17:
18: public class Simple
19: {
20: public static int Main()
21: {
22: Console.WriteLine("Thread Start/Stop/Join Sample");
23:
24: Alpha oAlpha = new Alpha();
25:
26: // Create the thread object, passing in the Alpha.Beta method
27: // via a ThreadStart delegate. This does not start the thread.
28: Thread oThread = new Thread(new ThreadStart(oAlpha.Beta));
29:
30: // Start the thread
31: oThread.Start();
32:
33: // Spin for a while waiting for the started thread to become
34: // alive:
35: while (!oThread.IsAlive);
36:
37: // Put the Main thread to sleep for 1 millisecond to allow oThread
38: // to do some work:
39: Thread.Sleep(1);
40:
41: // Request that oThread be stopped
42: oThread.Abort();
43:
44: // Wait until oThread finishes. Join also has overloads
45: // that take a millisecond interval or a TimeSpan object.
46: oThread.Join();
47:
48: Console.WriteLine();
49: Console.WriteLine("Alpha.Beta has finished");
50:
51: try
52: {
53: Console.WriteLine("Try to restart the Alpha.Beta thread");
54: oThread.Start();
55: }
56: catch (ThreadStateException)
57: {
58: Console.Write("ThreadStateException trying to restart Alpha.Beta. ");
59: Console.WriteLine("Expected since aborted threads cannot be restarted.");
60: }
61: return 0;
62: }
63: }
Example Output
Thread Start/Stop/Join Sample
Alpha.Beta is running in its own thread.
Alpha.Beta is running in its own thread.
Alpha.Beta is running in its own thread.
...
...
Alpha.Beta has finished
Try to restart the Alpha.Beta thread
ThreadStateException trying to restart Alpha.Beta. Expected since aborted threads cannot be restarted.
Example 2: Synchronizing two threads: a producer and a consumer
The following example shows how synchronization can be accomplished using the C# lock keyword and the Pulse method of the Monitor object. The Pulse method notifies a thread in the waiting queue of a change in the object’s state (for more details on pulses, see the Monitor.Pulse Method).
The example creates a Cell object that has two methods: ReadFromCell and WriteToCell. Two other objects are created from classes CellProd and CellCons; these objects both have a method ThreadRun whose job is to call ReadFromCell and WriteToCell. Synchronization is accomplished by waiting for “pulses” from the Monitor object, which come in order. That is, first an item is produced (the consumer at this point is waiting for a pulse), then a pulse occurs, then the consumer consumes the production (while the producer is waiting for a pulse), and so on.
1: // MonitorSample.cs
2: // This example shows use of the following methods of the C# lock keyword
3: // and the Monitor class
4: // in threads:
5: // Monitor.Pulse(Object)
6: // Monitor.Wait(Object)
7: using System;
8: using System.Threading;
9:
10: public class MonitorSample
11: {
12: public static void Main(String[] args)
13: {
14: int result = 0; // Result initialized to say there is no error
15: Cell cell = new Cell( );
16:
17: CellProd prod = new CellProd(cell, 20); // Use cell for storage,
18: // produce 20 items
19: CellCons cons = new CellCons(cell, 20); // Use cell for storage,
20: // consume 20 items
21:
22: Thread producer = new Thread(new ThreadStart(prod.ThreadRun));
23: Thread consumer = new Thread(new ThreadStart(cons.ThreadRun));
24: // Threads producer and consumer have been created,
25: // but not started at this point.
26:
27: try
28: {
29: producer.Start( );
30: consumer.Start( );
31:
32: producer.Join( ); // Join both threads with no timeout
33: // Run both until done.
34: consumer.Join( );
35: // threads producer and consumer have finished at this point.
36: }
37: catch (ThreadStateException e)
38: {
39: Console.WriteLine(e); // Display text of exception
40: result = 1; // Result says there was an error
41: }
42: catch (ThreadInterruptedException e)
43: {
44: Console.WriteLine(e); // This exception means that the thread
45: // was interrupted during a Wait
46: result = 1; // Result says there was an error
47: }
48: // Even though Main returns void, this provides a return code to
49: // the parent process.
50: Environment.ExitCode = result;
51: }
52: }
53:
54: public class CellProd
55: {
56: Cell cell; // Field to hold cell object to be used
57: int quantity = 1; // Field for how many items to produce in cell
58:
59: public CellProd(Cell box, int request)
60: {
61: cell = box; // Pass in what cell object to be used
62: quantity = request; // Pass in how many items to produce in cell
63: }
64: public void ThreadRun( )
65: {
66: for(int looper=1; looper<=quantity; looper++)
67: cell.WriteToCell(looper); // "producing"
68: }
69: }
70:
71: public class CellCons
72: {
73: Cell cell; // Field to hold cell object to be used
74: int quantity = 1; // Field for how many items to consume from cell
75:
76: public CellCons(Cell box, int request)
77: {
78: cell = box; // Pass in what cell object to be used
79: quantity = request; // Pass in how many items to consume from cell
80: }
81: public void ThreadRun( )
82: {
83: int valReturned;
84: for(int looper=1; looper<=quantity; looper++)
85: // Consume the result by placing it in valReturned.
86: valReturned=cell.ReadFromCell( );
87: }
88: }
89:
90: public class Cell
91: {
92: int cellContents; // Cell contents
93: bool readerFlag = false; // State flag
94: public int ReadFromCell( )
95: {
96: lock(this) // Enter synchronization block
97: {
98: if (!readerFlag)
99: { // Wait until Cell.WriteToCell is done producing
100: try
101: {
102: // Waits for the Monitor.Pulse in WriteToCell
103: Monitor.Wait(this);
104: }
105: catch (SynchronizationLockException e)
106: {
107: Console.WriteLine(e);
108: }
109: catch (ThreadInterruptedException e)
110: {
111: Console.WriteLine(e);
112: }
113: }
114: Console.WriteLine("Consume: {0}",cellContents);
115: readerFlag = false; // Reset the state flag to say consuming
116: // is done.
117: Monitor.Pulse(this); // Pulse tells Cell.WriteToCell that
118: // Cell.ReadFromCell is done.
119: } // Exit synchronization block
120: return cellContents;
121: }
122:
123: public void WriteToCell(int n)
124: {
125: lock(this) // Enter synchronization block
126: {
127: if (readerFlag)
128: { // Wait until Cell.ReadFromCell is done consuming.
129: try
130: {
131: Monitor.Wait(this); // Wait for the Monitor.Pulse in
132: // ReadFromCell
133: }
134: catch (SynchronizationLockException e)
135: {
136: Console.WriteLine(e);
137: }
138: catch (ThreadInterruptedException e)
139: {
140: Console.WriteLine(e);
141: }
142: }
143: cellContents = n;
144: Console.WriteLine("Produce: {0}",cellContents);
145: readerFlag = true; // Reset the state flag to say producing
146: // is done
147: Monitor.Pulse(this); // Pulse tells Cell.ReadFromCell that
148: // Cell.WriteToCell is done.
149: } // Exit synchronization block
150: }
151: }
Example Output
Produce: 1
Consume: 1
Produce: 2
Consume: 2
Produce: 3
Consume: 3
...
...
Produce: 20
Consume: 20
Example 3: Using a thread pool
The following example shows how to use a thread pool. It first creates a ManualResetEvent object, which enables the program to know when the thread pool has finished running all of the work items. Next, it attempts to add one thread to the thread pool. If this succeeds, it adds the rest (four in this example). The thread pool will then put work items into available threads. The WaitOne method on eventX is called, which causes the rest of the program to wait until the event is triggered (with the eventX.Set method). Finally, the program prints out the load (the thread that actually executes a particular work item) on the threads.
1: // SimplePool.cs
2: // Simple thread pool example
3: using System;
4: using System.Collections;
5: using System.Threading;
6:
7: // Useful way to store info that can be passed as a state on a work item
8: public class SomeState
9: {
10: public int Cookie;
11: public SomeState(int iCookie)
12: {
13: Cookie = iCookie;
14: }
15: }
16:
17: public class Alpha
18: {
19: public Hashtable HashCount;
20: public ManualResetEvent eventX;
21: public static int iCount = 0;
22: public static int iMaxCount = 0;
23: public Alpha(int MaxCount)
24: {
25: HashCount = new Hashtable(MaxCount);
26: iMaxCount = MaxCount;
27: }
28:
29: // Beta is the method that will be called when the work item is
30: // serviced on the thread pool.
31: // That means this method will be called when the thread pool has
32: // an available thread for the work item.
33: public void Beta(Object state)
34: {
35: // Write out the hashcode and cookie for the current thread
36: Console.WriteLine(" {0} {1} :", Thread.CurrentThread.GetHashCode(),
37: ((SomeState)state).Cookie);
38: // The lock keyword allows thread-safe modification
39: // of variables accessible across multiple threads.
40: Console.WriteLine(
41: "HashCount.Count=={0}, Thread.CurrentThread.GetHashCode()=={1}",
42: HashCount.Count,
43: Thread.CurrentThread.GetHashCode());
44: lock (HashCount)
45: {
46: if (!HashCount.ContainsKey(Thread.CurrentThread.GetHashCode()))
47: HashCount.Add (Thread.CurrentThread.GetHashCode(), 0);
48: HashCount[Thread.CurrentThread.GetHashCode()] =
49: ((int)HashCount[Thread.CurrentThread.GetHashCode()])+1;
50: }
51:
52: // Do some busy work.
53: // Note: Depending on the speed of your machine, if you
54: // increase this number, the dispersement of the thread
55: // loads should be wider.
56: int iX = 2000;
57: Thread.Sleep(iX);
58: // The Interlocked.Increment method allows thread-safe modification
59: // of variables accessible across multiple threads.
60: Interlocked.Increment(ref iCount);
61: if (iCount == iMaxCount)
62: {
63: Console.WriteLine();
64: Console.WriteLine("Setting eventX ");
65: eventX.Set();
66: }
67: }
68: }
69:
70: public class SimplePool
71: {
72: public static int Main(string[] args)
73: {
74: Console.WriteLine("Thread Pool Sample:");
75: bool W2K = false;
76: int MaxCount = 10; // Allow a total of 10 threads in the pool
77: // Mark the event as unsignaled.
78: ManualResetEvent eventX = new ManualResetEvent(false);
79: Console.WriteLine("Queuing {0} items to Thread Pool", MaxCount);
80: Alpha oAlpha = new Alpha(MaxCount); // Create the work items.
81: // Make sure the work items have a reference to the signaling event.
82: oAlpha.eventX = eventX;
83: Console.WriteLine("Queue to Thread Pool 0");
84: try
85: {
86: // Queue the work items, which has the added effect of checking
87: // which OS is running.
88: ThreadPool.QueueUserWorkItem(new WaitCallback(oAlpha.Beta),
89: new SomeState(0));
90: W2K = true;
91: }
92: catch (NotSupportedException)
93: {
94: Console.WriteLine("These API's may fail when called on a non-Windows 2000 system.");
95: W2K = false;
96: }
97: if (W2K) // If running on an OS which supports the ThreadPool methods.
98: {
99: for (int iItem=1;iItem < MaxCount;iItem++)
100: {
101: // Queue the work items:
102: Console.WriteLine("Queue to Thread Pool {0}", iItem);
103: ThreadPool.QueueUserWorkItem(new WaitCallback(oAlpha.Beta),new SomeState(iItem));
104: }
105: Console.WriteLine("Waiting for Thread Pool to drain");
106: // The call to exventX.WaitOne sets the event to wait until
107: // eventX.Set() occurs.
108: // (See oAlpha.Beta).
109: // Wait until event is fired, meaning eventX.Set() was called:
110: eventX.WaitOne(Timeout.Infinite,true);
111: // The WaitOne won't return until the event has been signaled.
112: Console.WriteLine("Thread Pool has been drained (Event fired)");
113: Console.WriteLine();
114: Console.WriteLine("Load across threads");
115: foreach(object o in oAlpha.HashCount.Keys)
116: Console.WriteLine("{0} {1}", o, oAlpha.HashCount[o]);
117: }
118: return 0;
119: }
120: }
Example Output
Note The following output will vary from one computer to another.
Thread Pool Sample:
Queuing 10 items to Thread Pool
Queue to Thread Pool 0
Queue to Thread Pool 1
...
...
Queue to Thread Pool 9
Waiting for Thread Pool to drain
98 0 :
HashCount.Count==0, Thread.CurrentThread.GetHashCode()==98
100 1 :
HashCount.Count==1, Thread.CurrentThread.GetHashCode()==100
98 2 :
...
...
Setting eventX
Thread Pool has been drained (Event fired)
Load across threads
101 2
100 3
98 4
102 1
Example 4: Using the Mutex object
You can use a mutex object to protect a shared resource from simultaneous access by multiple threads or processes. The state of a mutex object is either set to signaled, when it is not owned by any thread, or nonsignaled, when it is owned. Only one thread at a time can own a mutex object. For example, to prevent two threads from writing to shared memory at the same time, each thread waits for ownership of a mutex object before executing the code that accesses the memory. After writing to the shared memory, the thread releases the mutex object.
This example demonstrates how to use the classes Mutex, AutoResetEvent, and WaitHandle in processing threads. It also demonstrates the methods used in processing the mutex object.
1: // Mutex.cs
2: // Mutex object example
3: using System;
4: using System.Threading;
5:
6: public class MutexSample
7: {
8: static Mutex gM1;
9: static Mutex gM2;
10: const int ITERS = 100;
11: static AutoResetEvent Event1 = new AutoResetEvent(false);
12: static AutoResetEvent Event2 = new AutoResetEvent(false);
13: static AutoResetEvent Event3 = new AutoResetEvent(false);
14: static AutoResetEvent Event4 = new AutoResetEvent(false);
15:
16: public static void Main(String[] args)
17: {
18: Console.WriteLine("Mutex Sample ...");
19: // Create Mutex initialOwned, with name of "MyMutex".
20: gM1 = new Mutex(true,"MyMutex");
21: // Create Mutex initialOwned, with no name.
22: gM2 = new Mutex(true);
23: Console.WriteLine(" - Main Owns gM1 and gM2");
24:
25: AutoResetEvent[] evs = new AutoResetEvent[4];
26: evs[0] = Event1; // Event for t1
27: evs[1] = Event2; // Event for t2
28: evs[2] = Event3; // Event for t3
29: evs[3] = Event4; // Event for t4
30:
31: MutexSample tm = new MutexSample( );
32: Thread t1 = new Thread(new ThreadStart(tm.t1Start));
33: Thread t2 = new Thread(new ThreadStart(tm.t2Start));
34: Thread t3 = new Thread(new ThreadStart(tm.t3Start));
35: Thread t4 = new Thread(new ThreadStart(tm.t4Start));
36: t1.Start( ); // Does Mutex.WaitAll(Mutex[] of gM1 and gM2)
37: t2.Start( ); // Does Mutex.WaitOne(Mutex gM1)
38: t3.Start( ); // Does Mutex.WaitAny(Mutex[] of gM1 and gM2)
39: t4.Start( ); // Does Mutex.WaitOne(Mutex gM2)
40:
41: Thread.Sleep(2000);
42: Console.WriteLine(" - Main releases gM1");
43: gM1.ReleaseMutex( ); // t2 and t3 will end and signal
44:
45: Thread.Sleep(1000);
46: Console.WriteLine(" - Main releases gM2");
47: gM2.ReleaseMutex( ); // t1 and t4 will end and signal
48:
49: // Waiting until all four threads signal that they are done.
50: WaitHandle.WaitAll(evs);
51: Console.WriteLine("... Mutex Sample");
52: }
53:
54: public void t1Start( )
55: {
56: Console.WriteLine("t1Start started, Mutex.WaitAll(Mutex[])");
57: Mutex[] gMs = new Mutex[2];
58: gMs[0] = gM1; // Create and load an array of Mutex for WaitAll call
59: gMs[1] = gM2;
60: Mutex.WaitAll(gMs); // Waits until both gM1 and gM2 are released
61: Thread.Sleep(2000);
62: Console.WriteLine("t1Start finished, Mutex.WaitAll(Mutex[]) satisfied");
63: Event1.Set( ); // AutoResetEvent.Set() flagging method is done
64: }
65:
66: public void t2Start( )
67: {
68: Console.WriteLine("t2Start started, gM1.WaitOne( )");
69: gM1.WaitOne( ); // Waits until Mutex gM1 is released
70: Console.WriteLine("t2Start finished, gM1.WaitOne( ) satisfied");
71: Event2.Set( ); // AutoResetEvent.Set() flagging method is done
72: }
73:
74: public void t3Start( )
75: {
76: Console.WriteLine("t3Start started, Mutex.WaitAny(Mutex[])");
77: Mutex[] gMs = new Mutex[2];
78: gMs[0] = gM1; // Create and load an array of Mutex for WaitAny call
79: gMs[1] = gM2;
80: Mutex.WaitAny(gMs); // Waits until either Mutex is released
81: Console.WriteLine("t3Start finished, Mutex.WaitAny(Mutex[])");
82: Event3.Set( ); // AutoResetEvent.Set() flagging method is done
83: }
84:
85: public void t4Start( )
86: {
87: Console.WriteLine("t4Start started, gM2.WaitOne( )");
88: gM2.WaitOne( ); // Waits until Mutex gM2 is released
89: Console.WriteLine("t4Start finished, gM2.WaitOne( )");
90: Event4.Set( ); // AutoResetEvent.Set() flagging method is done
91: }
92: }
Sample Output
Mutex Sample ...
- Main Owns gM1 and gM2
t1Start started, Mutex.WaitAll(Mutex[])
t2Start started, gM1.WaitOne( )
t3Start started, Mutex.WaitAny(Mutex[])
t4Start started, gM2.WaitOne( )
- Main releases gM1
t2Start finished, gM1.WaitOne( ) satisfied
t3Start finished, Mutex.WaitAny(Mutex[])
- Main releases gM2
t1Start finished, Mutex.WaitAll(Mutex[]) satisfied
t4Start finished, gM2.WaitOne( )
... Mutex Sample
