How to synchronize access to a shared resource in a multithreading environment by using Visual C#

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来自：

http://support.microsoft.com/default.aspx?scid=kb;en-us;816161

You can perform multiple tasks in Microsoft Visual C# applications at the same time by using multithreading. Multithreading can start different threads to complete different tasks at the same time. Multithreading also improves the performance and responsiveness of your applications.

Because multiple threads can access a resource at the same time, you may want to synchronize individual threads with other parts of your program. This article describes some common scenarios with multithreading programming, and explains how to synchronize the access to a shared resource among the multiple threads.

Help to Protect Your Global Data in Modules in a Multithreaded Environment

The public fields in methods are accessible to all the threads in your application. To synchronize the access to the public fields, you can use property instead of field, and use a ReaderWriterLock object to control the access. To do this, follow these steps:

Start Microsoft Visual Studio.
On the File menu, point to New, and then click Project.
Click Visual C# Projects under Project Types, and then click Console Application under Templates.

Note In Visual Studio 2005 or in Visual Studio 2008, click Visual C# under Project Types, and then click Console Application under Templates.
In the Name text box, type MultiThreadApplication, and then click OK.

Replace the existing code with the following code in Class1.cs.

Note In Visual Studio 2005 and in Visual Studio 2008, the default file is Program.cs.

using System;
using System.Threading;

namespace MultiThreadApplication
{
	class Class1
	{
		private ReaderWriterLock rwl = new ReaderWriterLock();
		private long myNumber;
		public long Number   // the Number property
		{
			get
			{
				//Acquire a read lock on the resource.
				rwl.AcquireReaderLock(Timeout.Infinite);                
				try
				{
					Console.WriteLine("Thread:{0} starts getting the Number", Thread.CurrentThread.GetHashCode());
					Thread.Sleep(50);
					Console.WriteLine("Thread:{0} got the Number", Thread.CurrentThread.GetHashCode());

				}
				finally
				{
					//Release the lock.
					rwl.ReleaseReaderLock();
				}
				return myNumber;
			}
			set
			{
				//Acquire a write lock on the resource.
				rwl.AcquireWriterLock(Timeout.Infinite);
				try
				{
					Console.WriteLine("Thread: {0} start writing the Number", Thread.CurrentThread.GetHashCode());
					Thread.Sleep(50);
					myNumber = value;
					Console.WriteLine("Thread: {0} written the Number", Thread.CurrentThread.GetHashCode());
				}
				finally
				{
					//Release the lock.
					rwl.ReleaseWriterLock();
				}
			}
		}
		
		[STAThread]
		static void Main(string[] args)
		{
			Thread []threadArray = new Thread[20]; 
			int threadNum;


			Class1 Myclass = new Class1();
			ThreadStart myThreadStart = new ThreadStart(Myclass.AccessGlobalResource);

			//Create 20 threads.
			for( threadNum = 0; threadNum < 20; threadNum++)
			{
				threadArray[threadNum] = new Thread(myThreadStart);
			}

			//Start the threads.
			for( threadNum = 0; threadNum < 20; threadNum++)
			{   
				threadArray[threadNum].Start();
			}

			//Wait until all the thread spawn out finish.
			for( threadNum = 0; threadNum < 20; threadNum++)
				threadArray[threadNum].Join();

			Console.WriteLine("All operations have completed. Press enter to exit");
			Console.ReadLine();
		}

		public void AccessGlobalResource()
		{
			Random rnd = new Random();
			long theNumber;
			
			if (rnd.Next() % 2 != 0)
				theNumber = Number;
			else
			{
				theNumber = rnd.Next();
				Number = theNumber;
			}

		}
	}
}

Build the project, and then run the application.

Help to Make Your Class Thread-Safe

Multiple threads may try to access an object at the same time. When more than one thread competes for access to an object at the same time, some threads may receive an invalid state if another thread modifies the resource at the same time. For example, if a thread reads the field of the object while another thread is modifying the field, the first thread may receive an invalid state of the field. This situation is named a race condition.

To avoid this situation, you can help to protect critical sections of your code from race conditions by using locks. A lock, represented by the Visual C# keyword lock Statement, allows a single thread of execution to obtain exclusive execution rights on an object. The following example steps demonstrate locks:

Open Visual Studio.
On the File menu, point to New, and then click Project.
Click Visual C# Projects under Project Types, and then click Console Application under Templates.

Note In Visual Studio 2005 or in Visual Studio 2008, click Visual C# under Project Types, and then click Console Application under Templates.
In the Name text box, type MultiThreadLockApplication, and then click OK.

Replace the existing code with the following code in Class1.cs:

using System;
using System.Threading;

namespace MultiThreadLockApplication
{
	class Student
	{
		private static string myTeacherName = "Bill";
		private string myName = "Grace";
		private static object somePrivateStaticObject = new Object();

		public static string TeacherName
		{
			get
			{
				string theName;

				// Synchronize access to the shared member.
				lock(somePrivateStaticObject)
				{
					Console.WriteLine("Thread {0} starts to get the teacher's name",Thread.CurrentThread.GetHashCode());
					theName = myTeacherName;

					// Wait for 0.3 second.
					Thread.Sleep(300);
					Console.WriteLine("Thread {0} finished to get the teacher's name:{1}.", Thread.CurrentThread.GetHashCode(), theName);
				}
				return theName;
			}

			set
			{
				lock(somePrivateStaticObject)
				{
					Console.WriteLine("Thread {0} starts to set the teacher's name.", Thread.CurrentThread.GetHashCode());
					myTeacherName = value;

					// Wait for 0.3 second.
					Thread.Sleep(300);
					Console.WriteLine("Thread {0} finished to set the teacher's name:{1}.", Thread.CurrentThread.GetHashCode(), value);
				}
			}
		}

		public string GetName()
		{
			string theName;
			lock(this)
			{
				Console.WriteLine("Thread {0} starts to get the student's name.", Thread.CurrentThread.GetHashCode());
				theName = myName;

				// Wait for 0.3 second.
				Thread.Sleep(300);
				Console.WriteLine("Thread {0} finished to get the student's name:{1}", Thread.CurrentThread.GetHashCode(), theName);
				return theName;
			}
		}

		public string SetName(string NewName)
		{
			string theOldName;
			lock(this)
			{
				Console.WriteLine("Thread {0} starts to set the student's name.", Thread.CurrentThread.GetHashCode());
				theOldName = myName;
				myName = NewName;

				// Wait for 0.3 second.
				Thread.Sleep(300);
				Console.WriteLine("Thread {0} finished to set the student's name:{1}", Thread.CurrentThread.GetHashCode(), NewName);
			}
			return theOldName;
		}
	}

	class Class1
	{
		public static int WorkItemNum = 20;
		public static AutoResetEvent Done = new AutoResetEvent(false);

		public static void AccessClassResource(object state)
		{
			Random rnd = new Random();
			string theName;
			Student AStudent = (Student) state;

			if( (rnd.Next() %2) != 0)
			{
				if( (rnd.Next() %2) != 0)
				{
					switch (rnd.Next() %3 )
					{
						case 0:
							Student.TeacherName = "Tom";
							break;
						case 1:
							Student.TeacherName = "Mike";
							break;
						case 2:
							Student.TeacherName = "John";
							break;
					}
				}
				else
				{
					theName = Student.TeacherName;
				}
			}
			else
			{
				if( (rnd.Next() %2) != 0)
				{
					switch (rnd.Next() %3 )
					{
						case 0:
							AStudent.SetName("Janet");
							break;
						case 1:
							AStudent.SetName("David");
							break;
						case 2:
							AStudent.SetName("Ben");
							break;
					}
				}
				else
				{
					theName = AStudent.GetName();
				}
			}

			if(Interlocked.Decrement( ref WorkItemNum) == 0)
			{
				Done.Set();
			}
		}

		[STAThread]
		static void Main(string[] args)
		{
			int threadNum;
			Student AStudent = new Student();

			// Queue up 20 work items in the ThreadPool.
			for (threadNum = 0 ; threadNum <= WorkItemNum -1 ; threadNum++) 
			{
				ThreadPool.QueueUserWorkItem(new WaitCallback(AccessClassResource),AStudent);
			}

			Done.WaitOne();
			Console.WriteLine("All operations have completed. Press enter to exit");
			Console.ReadLine();
		}
	}
}