C#的两种常见的死锁情况

工作中遇到的两种线程锁死的情况，特此记录：

1.Thread1和Thread2抢占同一对象中的2个锁obj1和obj2,对象如下：

public class Test
{
    object _obj1;
    object _obj2;
    public Test()
    {
        _obj1 = new object();
        _obj2 = new object();
    }
    public void Test1(string name)//线程1调用
    {
        lock (_obj1)
        {
            Console.WriteLine(name + " Lock Obj1");
            lock (_obj2)
            {
                Console.WriteLine(name + " Lock Obj2");
            }
            Console.WriteLine(name + " Release Obj2");
        }
        Console.WriteLine(name + " Release Obj1");
    }
    public void Test2(string name)//线程2调用
    {
        lock (_obj2)
        {
            Console.WriteLine(name + " Lock Obj2");
            lock (_obj1)
            {
                Console.WriteLine(name + " Lock Obj1");
            }
            Console.WriteLine(name + " Release Obj1");
        }
        Console.WriteLine(name + " Release Obj2");
    }
}

调用方式如下：

static void Main(string[] args)
{
    Test p = new Test();
    Thread th = new Thread(() =>
    {
        while (true)
        {
            p.Test1(Thread.CurrentThread.ManagedThreadId.ToString());
        }
    });
    th.Start();
    th = new Thread(() =>
    {
        while (true)
        {
            p.Test2(Thread.CurrentThread.ManagedThreadId.ToString());
        }
    });
    th.Start();
}

此种情况导致Thread1占用obj1等待obj2，Thread2占用obj2等待obj1，两个线程均阻塞导致死锁情况发生。执行最终如下：

解决方案：1.编程中不要使用这种嵌套写法；2.使用Monitor.TryEnter()方法。

2.Winform主线程抢占对象中的锁obj1，对象子线程占用obj1使用invoke方法调用，对象类如下：

public class Test
{
    string str;
    Form1 form;
    object _obj1;
    public Test(Form1 f)
    {
        this.form = f;
        _obj1 = new object();
        Thread th = new Thread(Monitor);
        th.IsBackground = true;
        th.Start();
    }
    void Monitor()//将字符串显示在UI界面
    {
        while (true)
        {
            Thread.Sleep(50);
            lock (_obj1)
            {
                form.Show(str);
            }
        }
    }
    public void Test1()//UI调用,修改字符串的值
    {
        lock (_obj1)
        {
            str = DateTime.Now.ToString("HH:mm:ss:fff");
        }
    }
}

Winform代码如下：

Test t;
public Form1()
{
    InitializeComponent();
    t = new Test(this);
}
public void Show(string txt)//子线程占用锁时来操作控件
{
    if (this.InvokeRequired)
        this.Invoke(new Action<string>(Show), txt);
    else
        this.label1.Text = txt;
}
private void timer1_Tick(object sender, EventArgs e)
{
    t.Test1();//定时器不停地调用
}

启动后可看到程序会卡死，字符串不再刷新。

 解决方案：使用BeginInvoke方法替换Invoke;