lock 是什么？
public void UseMultiThread2()
{
Console.WriteLine("Child Thread 2 Started");
Console.WriteLine("Child Thread 2 - Counting from 11 to 20");
for (int i = 11; i <= 20; i++)
{
for (int a = 0; a < 20; a++)
{
Console.Write(".");
}
Console.WriteLine(i);
}
Console.WriteLine("Child Thread 2 finished");

Monitor.Enter(this);
Console.WriteLine("Child Thread 2 Started");
Console.WriteLine("Child Thread 2 - Counting from 11 to 20");
for (int i = 11; i <= 20; i++)
{
for (int a = 0; a < 20; a++)
{
Console.Write(".");
}
Console.WriteLine(i);
}
Console.WriteLine("Child Thread 2 finished");
Monitor.Exit(this);

lock (this)
{
Console.WriteLine("Child Thread 2 Started");
Console.WriteLine("Child Thread 2 - Counting from 11 to 20");
for (int i = 11; i <= 20; i++)
{
for (int a = 0; a < 20; a++)
{
Console.Write(".");
}
Console.WriteLine(i);
}
Console.WriteLine("Child Thread 2 finished");
}
}

以下转自MSDN
---------------------------------------
lock 确保当一个线程位于代码的临界区时，另一个线程不进入临界区。如果其他线程试图进入一个锁定代码，则它将在释放该对象前一直等待（块）。
下例使用线程和 lock。只要 lock 语句存在，语句块就是临界区并且 balance 永远不会是负数。
// statements_lock2.cs
using System;
using System.Threading;

class Account
{
int balance;

Random r = new Random();

public Account(int initial)
{
balance = initial;
}

int Withdraw(int amount)
{

// This condition will never be true unless the lock statement
// is commented out:
if (balance < 0)
{
throw new Exception("Negative Balance");
}

// Comment out the next line to see the effect of leaving out
// the lock keyword:
lock (this)
{
if (balance >= amount)
{
Console.WriteLine("Balance before Withdrawal : " + balance);
Console.WriteLine("Amount to Withdraw : -" + amount);
balance = balance - amount;
Console.WriteLine("Balance after Withdrawal : " + balance);
return amount;
}
else
{
return 0; // transaction rejected
}
}
}

public void DoTransactions()
{
for (int i = 0; i < 100; i++)
{
Withdraw(r.Next(1, 100));
}
}
}

class Test
{
public static void Main()
{
Thread[] threads = new Thread[10];
Account acc = new Account (1000);
for (int i = 0; i < 10; i++)
{
Thread t = new Thread(new ThreadStart(acc.DoTransactions));
threads[i] = t;
}
for (int i = 0; i < 10; i++)
{
threads[i].Start();
}
}
}