線程處理的優(yōu)點是可以創(chuàng)建使用多個執(zhí)行線程的應(yīng)用程序。例如，某一進(jìn)程可以具有管理與用戶交互的用戶界面線程，以及在用戶界面線程等待用戶輸入時執(zhí)行其他任務(wù)的輔助線程。
該教程說明各種線程活動：
- 創(chuàng)建和執(zhí)行線程
- 線程同步
- 線程間交互
- 使用線程池
- 使用 mutex 對象保護(hù)共享資源
教程
該教程包含下列示例：
示例 1：創(chuàng)建線程、啟動線程和線程間交互
本示例說明如何創(chuàng)建和啟動線程，并顯示了同時在同一進(jìn)程內(nèi)運行的兩個線程間的交互。請注意，不必停止或釋放線程。這由 .NET Framework 公共語言運行庫自動完成。
程序從創(chuàng)建 Alpha 類型的對象 (oAlpha) 和引用 Alpha 類的 Beta 方法的線程 (oThread) 開始。然后啟動該線程。線程的 IsAlive 屬性允許程序等待，直到線程被初始化（被創(chuàng)建、被分配等）為止。主線程通過 Thread 訪問，而 Sleep 方法通知線程放棄其時間片并在一定毫秒數(shù)期間停止執(zhí)行。然后 oThread 被停止和聯(lián)接。聯(lián)接一個線程將使主線程等待它死亡或等待它在指定的時間后過期。最后，程序嘗試重新啟動 oThread，但由于線程無法在停止（中止）后重新啟動而告失敗。有關(guān)臨時停止執(zhí)行的信息，請參見掛起線程執(zhí)行。
// StopJoin.cs
using System;
using System.Threading;
public class Alpha
{
// This method that will be called when the thread is started
public void Beta()
{
while (true)
{
Console.WriteLine(“Alpha.Beta is running in its own thread.“);
}
}
};
public class Simple
{
public static int Main()
{
Console.WriteLine(“Thread Start/Stop/Join Sample“);
Alpha oAlpha = new Alpha();
// Create the thread object, passing in the Alpha.Beta method
// via a ThreadStart delegate. This does not start the thread.
Thread oThread = new Thread(new ThreadStart(oAlpha.Beta));
// Start the thread
oThread.Start();
// Spin for a while waiting for the started thread to become
// alive:
while (!oThread.IsAlive);
// Put the Main thread to sleep for 1 millisecond to allow oThread
// to do some work:
Thread.Sleep(1);
// Request that oThread be stopped
oThread.Abort();
// Wait until oThread finishes. Join also has overloads
// that take a millisecond interval or a TimeSpan object.
oThread.Join();
Console.WriteLine();
Console.WriteLine(“Alpha.Beta has finished“);
try
{
Console.WriteLine(“Try to restart the Alpha.Beta thread“);
oThread.Start();
}
catch (ThreadStateException)
{
Console.Write(“ThreadStateException trying to restart Alpha.Beta. “);
Console.WriteLine(“Expected since aborted threads cannot be restarted.“);
}
return 0;
}
}
輸出示例
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.
示例 2：同步兩個線程：制造者和使用者
下面的示例顯示如何使用 C# lock 關(guān)鍵字和 Monitor 對象的 Pulse 方法完成同步。Pulse 方法通知等待隊列中的線程對象的狀態(tài)已更改。（有關(guān)脈沖的更多詳細(xì)信息，請參見 Monitor.Pulse 方法）。
本示例創(chuàng)建一個 Cell 對象，它具有兩個方法：ReadFromCell 和 WriteToCell。從 CellProd 和 CellCons 類創(chuàng)建另外兩個對象；這兩個對象均具有調(diào)用 ReadFromCell 和 WriteToCell 的 ThreadRun 方法。通過等待依次到達(dá)的來自 Monitor 對象的“脈沖”即可完成同步。也就是說，首先產(chǎn)生一個項（此時使用者等待脈沖），然后發(fā)生一個脈沖，接著使用者使用所產(chǎn)生的項（此時制造者等待脈沖），依此類推。
// MonitorSample.cs
// This example shows use of the following methods of the C# lock keyword
// and the Monitor class
// in threads:
// Monitor.Pulse(Object)
// Monitor.Wait(Object)
using System;
using System.Threading;
public class MonitorSample
{
public static void Main(String[] args)
{
int result = 0; // Result initialized to say there is no error
Cell cell = new Cell( );
CellProd prod = new CellProd(cell, 20); // Use cell for storage,
// produce 20 items
CellCons cons = new CellCons(cell, 20); // Use cell for storage,
// consume 20 items
Thread producer = new Thread(new ThreadStart(prod.ThreadRun));
Thread consumer = new Thread(new ThreadStart(cons.ThreadRun));
// Threads producer and consumer have been created,
// but not started at this point.
try
{
producer.Start( );
consumer.Start( );
producer.Join( ); // Join both threads with no timeout
// Run both until done.
consumer.Join( );
// threads producer and consumer have finished at this point.
}
catch (ThreadStateException e)
{
Console.WriteLine(e); // Display text of exception
result = 1; // Result says there was an error
}
catch (ThreadInterruptedException e)
{
Console.WriteLine(e); // This exception means that the thread
// was interrupted during a Wait
result = 1; // Result says there was an error
}
// Even though Main returns void, this provides a return code to
// the parent process.
Environment.ExitCode = result;
}
}
public class CellProd
{
Cell cell; // Field to hold cell object to be used
int quantity = 1; // Field for how many items to produce in cell
public CellProd(Cell box, int request)
{
cell = box; // Pass in what cell object to be used
quantity = request; // Pass in how many items to produce in cell
}
public void ThreadRun( )
{
for(int looper=1; looper<=quantity; looper++)
cell.WriteToCell(looper); // “producing“
}
}
public class CellCons
{
Cell cell; // Field to hold cell object to be used
int quantity = 1; // Field for how many items to consume from cell
public CellCons(Cell box, int request)
{
cell = box; // Pass in what cell object to be used
quantity = request; // Pass in how many items to consume from cell
}
public void ThreadRun( )
{
int valReturned;
for(int looper=1; looper<=quantity; looper++)
// Consume the result by placing it in valReturned.
valReturned=cell.ReadFromCell( );
}
}
public class Cell
{
int cellContents; // Cell contents
bool readerFlag = false; // State flag
public int ReadFromCell( )
{
lock(this) // Enter synchronization block
{
if (!readerFlag)
{ // Wait until Cell.WriteToCell is done producing
try
{
// Waits for the Monitor.Pulse in WriteToCell
Monitor.Wait(this);
}
catch (SynchronizationLockException e)
{
Console.WriteLine(e);
}
catch (ThreadInterruptedException e)
{
Console.WriteLine(e);
}
}
Console.WriteLine(“Consume: {0}“,cellContents);
readerFlag = false; // Reset the state flag to say consuming
// is done.
Monitor.Pulse(this); // Pulse tells Cell.WriteToCell that
// Cell.ReadFromCell is done.
} // Exit synchronization block
return cellContents;
}
public void WriteToCell(int n)
{
lock(this) // Enter synchronization block
{
if (readerFlag)
{ // Wait until Cell.ReadFromCell is done consuming.
try
{
Monitor.Wait(this); // Wait for the Monitor.Pulse in
// ReadFromCell
}
catch (SynchronizationLockException e)
{
Console.WriteLine(e);
}
catch (ThreadInterruptedException e)
{
Console.WriteLine(e);
}
}
cellContents = n;
Console.WriteLine(“Produce: {0}“,cellContents);
readerFlag = true; // Reset the state flag to say producing
// is done
Monitor.Pulse(this); // Pulse tells Cell.ReadFromCell that
// Cell.WriteToCell is done.
} // Exit synchronization block
}
}
輸出示例
Produce: 1
Consume: 1
Produce: 2
Consume: 2
Produce: 3
Consume: 3
...
...
Produce: 20
Consume: 20
示例 3：使用線程池
以下示例顯示如何使用線程池。首先創(chuàng)建 ManualResetEvent 對象，此對象使程序能夠知道線程池何時運行完所有的工作項。接著，嘗試向線程池添加一個線程。如果添加成功，則添加其余的線程（本例中為 4 個）。然后線程池將工作項放入可用線程中。調(diào)用 eventX 上的 WaitOne 方法，這會使程序的其余部分等待，直到用 eventX.Set 方法觸發(fā)事件為止。最后，程序打印出線程上的負(fù)載（實際執(zhí)行某一特定工作項的線程）。
// SimplePool.cs
// Simple thread pool example
using System;
using System.Collections;
using System.Threading;
// Useful way to store info that can be passed as a state on a work item
public class SomeState
{
public int Cookie;
public SomeState(int iCookie)
{
Cookie = iCookie;
}
}
public class Alpha
{
public Hashtable HashCount;
public ManualResetEvent eventX;
public static int iCount = 0;
public static int iMaxCount = 0;
public Alpha(int MaxCount)
{
HashCount = new Hashtable(MaxCount);
iMaxCount = MaxCount;
}
// Beta is the method that will be called when the work item is
// serviced on the thread pool.
// That means this method will be called when the thread pool has
// an available thread for the work item.
public void Beta(Object state)
{
// Write out the hashcode and cookie for the current thread
Console.WriteLine(“ {0} {1} :“, Thread.CurrentThread.GetHashCode(),
((SomeState)state).Cookie);
// The lock keyword allows thread-safe modification
// of variables accessible across multiple threads.
Console.WriteLine(
“HashCount.Count=={0}, Thread.CurrentThread.GetHashCode()=={1}“,
HashCount.Count,
Thread.CurrentThread.GetHashCode());
lock (HashCount)
{
if (!HashCount.ContainsKey(Thread.CurrentThread.GetHashCode()))
HashCount.Add (Thread.CurrentThread.GetHashCode(), 0);
HashCount[Thread.CurrentThread.GetHashCode()] =
((int)HashCount[Thread.CurrentThread.GetHashCode()])+1;
}
// Do some busy work.
// Note: Depending on the speed of your machine, if you
// increase this number, the dispersement of the thread
// loads should be wider.
int iX = 2000;
Thread.Sleep(iX);
// The Interlocked.Increment method allows thread-safe modification
// of variables accessible across multiple threads.
Interlocked.Increment(ref iCount);
if (iCount == iMaxCount)
{
Console.WriteLine();
Console.WriteLine(“Setting eventX “);
eventX.Set();
}
}
}
public class SimplePool
{
public static int Main(string[] args)
{
Console.WriteLine(“Thread Pool Sample:“);
bool W2K = false;
int MaxCount = 10; // Allow a total of 10 threads in the pool
// Mark the event as unsignaled.
ManualResetEvent eventX = new ManualResetEvent(false);
Console.WriteLine(“Queuing {0} items to Thread Pool“, MaxCount);
Alpha oAlpha = new Alpha(MaxCount); // Create the work items.
// Make sure the work items have a reference to the signaling event.
oAlpha.eventX = eventX;
Console.WriteLine(“Queue to Thread Pool 0“);
try
{
// Queue the work items, which has the added effect of checking
// which OS is running.
ThreadPool.QueueUserWorkItem(new WaitCallback(oAlpha.Beta),
new SomeState(0));
W2K = true;
}
catch (NotSupportedException)
{
Console.WriteLine(“These API‘s may fail when called on a non-Windows 2000 system.“);
W2K = false;
}
if (W2K) // If running on an OS which supports the ThreadPool methods.
{
for (int iItem=1;iItem < MaxCount;iItem++)
{
// Queue the work items:
Console.WriteLine(“Queue to Thread Pool {0}“, iItem);
ThreadPool.QueueUserWorkItem(new WaitCallback(oAlpha.Beta),new SomeState(iItem));
}
Console.WriteLine(“Waiting for Thread Pool to drain“);
// The call to exventX.WaitOne sets the event to wait until
// eventX.Set() occurs.
// (See oAlpha.Beta).
// Wait until event is fired, meaning eventX.Set() was called:
eventX.WaitOne(Timeout.Infinite,true);
// The WaitOne won‘t return until the event has been signaled.
Console.WriteLine(“Thread Pool has been drained (Event fired)“);
Console.WriteLine();
Console.WriteLine(“Load across threads“);
foreach(object o in oAlpha.HashCount.Keys)
Console.WriteLine(“{0} {1}“, o, oAlpha.HashCount[o]);
}
return 0;
}
}
輸出示例
注意???下列輸出隨計算機的不同而不同。
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
示例 4：使用 Mutex 對象
可以使用 mutex 對象保護(hù)共享資源不被多個線程或進(jìn)程同時訪問。mutex 對象的狀態(tài)或者設(shè)置為終止（當(dāng)它不屬于任何線程時），或者設(shè)置為非終止（當(dāng)它屬于某個線程時）。同時只能有一個線程擁有一個 mutex 對象。例如，為了防止兩個線程同時寫入共享內(nèi)存，每個線程在執(zhí)行訪問該共享內(nèi)存的代碼之前等待 mutex 對象的所屬權(quán)。寫入共享內(nèi)存后，線程將釋放該 mutex 對象。
此示例闡釋如何在處理線程過程中使用 Mutex 類、AutoResetEvent 類和 WaitHandle 類。它還闡釋在處理 mutex 對象過程中所用的方法。
// Mutex.cs
// Mutex object example
using System;
using System.Threading;
public class MutexSample
{
static Mutex gM1;
static Mutex gM2;
const int ITERS = 100;
static AutoResetEvent Event1 = new AutoResetEvent(false);
static AutoResetEvent Event2 = new AutoResetEvent(false);
static AutoResetEvent Event3 = new AutoResetEvent(false);
static AutoResetEvent Event4 = new AutoResetEvent(false);
public static void Main(String[] args)
{
Console.WriteLine(“Mutex Sample ...“);
// Create Mutex initialOwned, with name of “MyMutex“.
gM1 = new Mutex(true,“MyMutex“);
// Create Mutex initialOwned, with no name.
gM2 = new Mutex(true);
Console.WriteLine(“ - Main Owns gM1 and gM2“);
AutoResetEvent[] evs = new AutoResetEvent[4];
evs[0] = Event1; // Event for t1
evs[1] = Event2; // Event for t2
evs[2] = Event3; // Event for t3
evs[3] = Event4; // Event for t4
MutexSample tm = new MutexSample( );
Thread t1 = new Thread(new ThreadStart(tm.t1Start));
Thread t2 = new Thread(new ThreadStart(tm.t2Start));
Thread t3 = new Thread(new ThreadStart(tm.t3Start));
Thread t4 = new Thread(new ThreadStart(tm.t4Start));
t1.Start( ); // Does Mutex.WaitAll(Mutex[] of gM1 and gM2)
t2.Start( ); // Does Mutex.WaitOne(Mutex gM1)
t3.Start( ); // Does Mutex.WaitAny(Mutex[] of gM1 and gM2)
t4.Start( ); // Does Mutex.WaitOne(Mutex gM2)
Thread.Sleep(2000);
Console.WriteLine(“ - Main releases gM1“);
gM1.ReleaseMutex( ); // t2 and t3 will end and signal
Thread.Sleep(1000);
Console.WriteLine(“ - Main releases gM2“);
gM2.ReleaseMutex( ); // t1 and t4 will end and signal
// Waiting until all four threads signal that they are done.
WaitHandle.WaitAll(evs);
Console.WriteLine(“... Mutex Sample“);
}
public void t1Start( )
{
Console.WriteLine(“t1Start started, Mutex.WaitAll(Mutex[])“);
Mutex[] gMs = new Mutex[2];
gMs[0] = gM1; // Create and load an array of Mutex for WaitAll call
gMs[1] = gM2;
Mutex.WaitAll(gMs); // Waits until both gM1 and gM2 are released
Thread.Sleep(2000);
Console.WriteLine(“t1Start finished, Mutex.WaitAll(Mutex[]) satisfied“);
Event1.Set( ); // AutoResetEvent.Set() flagging method is done
}
public void t2Start( )
{
Console.WriteLine(“t2Start started, gM1.WaitOne( )“);
gM1.WaitOne( ); // Waits until Mutex gM1 is released
Console.WriteLine(“t2Start finished, gM1.WaitOne( ) satisfied“);
Event2.Set( ); // AutoResetEvent.Set() flagging method is done
}
public void t3Start( )
{
Console.WriteLine(“t3Start started, Mutex.WaitAny(Mutex[])“);
Mutex[] gMs = new Mutex[2];
gMs[0] = gM1; // Create and load an array of Mutex for WaitAny call
gMs[1] = gM2;
Mutex.WaitAny(gMs); // Waits until either Mutex is released
Console.WriteLine(“t3Start finished, Mutex.WaitAny(Mutex[])“);
Event3.Set( ); // AutoResetEvent.Set() flagging method is done
}
public void t4Start( )
{
Console.WriteLine(“t4Start started, gM2.WaitOne( )“);
gM2.WaitOne( ); // Waits until Mutex gM2 is released
Console.WriteLine(“t4Start finished, gM2.WaitOne( )“);
Event4.Set( ); // AutoResetEvent.Set() flagging method is done
}
}
示例輸出
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
注意???此示例的輸出可能在每臺計算機上以及每次運行時均各不相同。運行此示例的計算機的速度及其操作系統(tǒng)都能影響輸出的順序。在多線程環(huán)境中，事件可能并不按預(yù)期的順序發(fā)生。