public string RandomNum(int n) //
    {
        string strchar = "0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z";
        string[] VcArray = strchar.Split(',');
        string VNum = "";                    //
        int temp = -1;                       //记录上次随机数值，尽量避免产生几个一样的随机数
                                             //采用一个简单的算法以保证生成随机数的不同
        Random rand = new Random();
        for (int i = 1; i < n + 1; i++)
        {
            if (temp != -1)
            {
                rand = new Random(i * temp * unchecked((int)DateTime.Now.Ticks));
            }
            int t = rand.Next(61);
            if (temp != -1 && temp == t)
            {
                return RandomNum(n);——————这个递归不怎么明白
            }
            temp = t;
            VNum += VcArray[t];
        }
        return VNum;//返回生成的随机数
    }