取得随机字符串 数字和字母

public static string RndNum( int VcodeNum)
       {
           string Vchar = "1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,g,h,i,j,k,m,n,p,q,r,s,t,u,v,w,x,y,z" ;
           string [] VcArray = Vchar.Split( new Char[] { ',' }); //拆分成数组
           string VNum = "" ;
           int temp = -1; //记录上次随机数值，尽量避避免生产几个一样的随机数

           Random rand = new Random();
           //采用一个简单的算法以保证生成随机数的不同
           for ( int i = 1; i < VcodeNum + 1; i++)
           {
               if (temp != -1)
               {
                   rand = new Random(i * temp * unchecked (( int )DateTime.Now.Ticks));
               }

               int t = rand.Next(33);
               if (temp != -1 && temp == t)
               {
                   return RndNum(VcodeNum);

               }
               temp = t;
               VNum += VcArray[t];
           }
           return VNum;
       }