static void Main(string[] args)
{
    Counter counter = new Counter();
    var t1 = new Thread(() => TestCounter(counter));
    var t2 = new Thread(() => TestCounter(counter));
    t1.Start();
    t2.Start();

    Thread.Sleep(TimeSpan.FromSeconds(3));//睡眠3秒，保证t1、t2两个线程都运行完毕
    Console.WriteLine(counter.count);
    Console.Read();
}

static void Main(string[] args)
{
    CounterWithLock counterlock = new CounterWithLock();
    var t1 = new Thread(() => TestCounter(counterlock));
    var t2 = new Thread(() => TestCounter(counterlock));
    t1.Start();
    t2.Start();  

    Thread.Sleep(TimeSpan.FromSeconds(3));//睡眠3秒，保证t1、t2两个线程都运行完毕
    Console.WriteLine(counterlock.count);
    Console.Read();
}

/// <summary>
/// 对同一个变量进行1000次加减操作
/// </summary>
/// <param name="c"></param>
static void TestCounter(CounterBase c)
{
    Console.WriteLine("TestCounter start");
    for (int i = 0; i < 1000; i++)
    {
        c.Increment();
        c.Decrement();
    }
    Console.WriteLine("TestCounter end");
}

abstract class CounterBase
{
    public abstract void Increment();
    public abstract void Decrement();
}

/// <summary>
/// 没有锁
/// </summary>
class Counter : CounterBase
{
    public int count { get; private set; }

    public override void Increment()
    {
        count ++;
    }

    public override void Decrement()
    {
        count --;
    }
}

/// <summary>
/// 加锁
/// </summary>
class CounterWithLock : CounterBase 
{
    private readonly object locker = new object();
    public int count { get; private set; }

    public override void Increment()
    {
        lock (locker)
        {
            count++;
        }
    }

    public override void Decrement()
    {    
        lock (locker)
        {
            count--;
        }
    }
}