public static  string selectuser()
    {
        string s = "";
        string sql = "select yonghuming from zhuce";
        SqlConnection cn = new SqlConnection("data source=.;database=youke");
        SqlDataAdapter da = new SqlDataAdapter(sql,cn);
        DataSet ds = new DataSet();
        da.Fill(ds);
       int ret = 0;
        for (int i = 0; i <=  zhuceq . count() - 1; i++)
        {
            string s1 = ds.Tables[0].Rows[i]["yonghuming"].ToString();
            if (s1.Trim() == yonghuming )
            {
                ret = 1;
            }
        }
        if (ret == 1)
        {
            s = "用户名已经被注册";
        }
        else
        {
            zhuceq.insertone();
            s = "注册成功";
        }
        return s;
    }