C#SQL操作问题：必须声明标量变量错误

       public int Login(Models.user m_user)
        {
            ConnectionStringSettings cnSetting =
               ConfigurationManager.ConnectionStrings["ConnectionString"];

            int res = -1;
            using (SqlConnection conn = new SqlConnection())
            {
                //conn.ConnectionString = "server=localhost;database=login;Trusted_Connection=true;";
                //conn.Open();

                string uid, pwd;
                uid = m_user.Id;
                pwd = m_user.Pwd;

                conn.ConnectionString = cnSetting.ConnectionString;

                SqlCommand cmd = conn.CreateCommand();
                string sql = "select * from Adm_Info where uid=@uid and pwd=@pwd";

                cmd.Parameters.AddWithValue("@uid", uid);//这里出错，@uid必须声明标量变量
                cmd.Parameters.AddWithValue("@pwd", pwd);
                conn.Open();

怎么办，在网上查了半天也没发现对我这个问题有帮助的。

你到底是什么数据库，sqlserver还是oracle，不同数据库里变量符号是不同的