整数不是'；t由线程c#升级

我在C#中声明了一个静态公共int，然后这个int被它的构造函数中的线程所支持，线程的工作很简单，可以增加它，但它不会发生在这里我声明静态值：

class Global
{
    static public  int hardcap = 100;
    public int speed;
    static public Semaphore myhitpoints = new Semaphore(1, 1);
    static public Semaphore oponenthitpoints = new Semaphore(1, 1);
   static  public int mhp = 100;
    static public int ohp = 100;
    static public int mmana = 0;
    static public int omana = 0;
  public static Charm dragonblade = new   Charm(10, 30, 3, myhitpoints, oponenthitpoints, mhp, ohp, "dragon blade", mmana);
  public  static  Charm dragonshield = new Charm(30, 10, 5, myhitpoints, oponenthitpoints, mhp, ohp, "dragon shield", mmana);
    public static Charm b1charm;
    public static  Charm b2charm;
    public static Opponent enemy;
}
class ManaWell
    {
        int mana_regen;
        int cap = 1000;
        int target;
        public ManaWell(int x, int y)
        {
           mana_regen = x;
            target = y;
        }
        public void Refill()
        {
            while (true)
            {
               // if (this.target + mana_regen <= cap)
                if (target+mana_regen<cap)
                {
                    Thread.Sleep(3000);
                    target += mana_regen;
                    MessageBox.Show(target.ToString());
                }
            }
        }
    }
 ManaWell mw1 = new ManaWell(20,Global.mmana);
        ManaWell mw2 = new ManaWell(20,Global.omana);
        Thread tmw1 = new Thread(new ThreadStart(mw1.Refill));
        Thread tmw2 = new Thread(new ThreadStart(mw2.Refill));
        tmw1.Start();
        tmw2.Start();

所以target很好，但y不会增加。