gistfile1.cs

// The code below would print overlapped A and B sequences like:
// ...
// A
// A
// B
// B
// ...
//
// Please add multi-threading protection code to make sure that
// As and Bs are printed in the order of:
// ...
// A
// B
// A
// B
// ...

static void PrintA()
{
    Console.WriteLine("A");
}

static void PrintB()
{
    Console.WriteLine("B");
}

// DO NOT touch the code below

for (var i = 0; i < 10; i++)
{
    new Thread(() => {
        while (true) {
            PrintA();
            PrintB();
        }
    }).Start();
}

static Mutex mut = new Mutex();

static void PrintA()
{
mut.WaitOne();
Console.WriteLine("A");
}

static void PrintB()
{
Console.WriteLine("B");
mut.ReleaseMutex();
}

// DO NOT touch the code below

for (var i = 0; i < 10; i++)
{
new Thread(() => {
while (true) {
PrintA();
PrintB();
}
}).Start();
}

    static object obj = new object();

        for (var i = 0; i < 10; i++)
        {
            new Thread(() =>
            {
                while (true)
                {
                    lock (obj)
                    {
                        PrintA();
                        PrintB();
                    };
                }
            }).Start();
        }
    }

这样行不行？
using System;
using System.Threading;

class Program
{
static void PrintA()
{
Console.WriteLine("A");
}

static void PrintB()
{
    Console.WriteLine("B");
}

public static void Main(string[] args)
{
    Object m_lock = new Object();
    for(var i=0; i<10; i++)
    {
        new Thread(() => {
            while(true) {
                Monitor.Enter(m_lock);
                PrintA();
                PrintB();
                Monitor.Exit(m_lock);
            }
        }).Start();
    }
}

}

private static readonly object LockObj = new object();

  static void PrintA()
  {
     Monitor.Enter(LockObj);
     Console.WriteLine("A");
  }

  static void PrintB()
  {
      Console.WriteLine("B");
      Monitor.Exit(LockObj);
  }

    static object locka = new object();
    static object lockb = new object();
    static bool result = true;

    static void PrintA()
    {
        if (result == true)
        {
            lock (locka)
            {
                if (result == true)
                {
                    Console.Write("A");
                    result = false;
                }
            }
        }
    }


    static void PrintB()
    {
        if (result == false)
        {
            lock (lockb)
            {
                if (result == false)
                {
                    Console.Write("B");
                    result = true;
                }
            }
        }
    }

class MainClass
{
static readonly object obj = new object();

    static void PrintA()
    {
        Monitor.Enter(obj);
        Console.WriteLine("A");
    }

    static void PrintB()
    {
        Console.WriteLine("B");
        Monitor.Exit(obj);
    }

    public static void Main (string[] args)
    {
        // DO NOT touch the code below

        for (var i = 0; i < 10; i++)
        {
            new Thread(() => {
                while (true) {
                    PrintA();
                    PrintB();
                }
            }).Start();
        }
    }
}

多打印几个，最后一个字符换行，比较容易看出效果
class Program
{
static object o = new object();
static void Main(string[] args)
{
// DO NOT touch the code below

        for (var i = 0; i < 10; i++)
        {
            new Thread(() =>
            {
                while (true)
                {
                    lock (o)
                    {
                        PrintA();
                        PrintB();
                        PrintC();
                        PrintD();
                        PrintE();
                    }
                }
            }).Start();
        }

        Console.Read();
    }

    static void PrintA()
    {
        Console.Write("A");
    }

    static void PrintB()
    {
        Console.Write("B");
    }

    static void PrintC()
    {
        Console.Write("C");
    }

    static void PrintD()
    {
        Console.Write("D");
    }

    static void PrintE()
    {
        Console.WriteLine("E");
    }
}

    private static Semaphore semA = new Semaphore(1,1);
    private static Semaphore semB = new Semaphore(0,1);

    static void PrintA()
    {
        semA.WaitOne();
        Console.WriteLine("A");
        semB.Release();
    }

    static void PrintB()
    {
        semB.WaitOne();
        Console.WriteLine("B");
        semA.Release();
    }

private readonly static object lockerA = new object();
private readonly static object lockerB = new object();
static void PrintA()
{
lock(lockerA)
{
Console.WriteLine("A");
}
}

static void PrintB()
{
lock(lockerB)
{
Console.WriteLine("B");
}
}

// DO NOT touch the code below

for (var i = 0; i < 10; i++)
{
new Thread(() => {
while (true) {
PrintA();
PrintB();
}
}).Start();
}

        //俩ThreadStart 把俩方法委托进去
        ThreadStart tsA = new ThreadStart(PrintA);
        ThreadStart tsB = new ThreadStart(PrintB);
        //俩委托调用. acB = null 是因为不赋值后面报未赋值错误.
        AsyncCallback acA, acB = null;

        //PrintA结束委托后,再调用tsB
        acA = new AsyncCallback(delegate(IAsyncResult r)
        {
            tsA.EndInvoke(r);
            Thread.Sleep(37);
            tsB.BeginInvoke(acB, null);
        });
        //PrintB结束委托后,再调用tsA,写成个套套.
        acB = new AsyncCallback(delegate(IAsyncResult r)
        {
            tsB.EndInvoke(r);
            Thread.Sleep(51);
            tsA.BeginInvoke(acA, null);
        });
        //然后就开始死磕
        tsA.BeginInvoke(acA, null);

        //以下是干扰代码 用于测试
        new Thread(delegate() {
            while (true)
            {
                Thread.Sleep(521);
                Console.Write("-");
            }
        }).Start();
        //主线程的干扰
        while (true)
        {
            Thread.Sleep(795);
            Console.Write("=");
        }