自己写的SqlHelper,提示在调用"Fill"前,SelectCommand 属性尚未初始化.错误

namespace 操作数据
{
    class SqlHelper
    {
        public DataSet SqlTODs(string cmdstring)
        {
            DataSet ds = new DataSet();
            try
            {
                string sqlstring = System.Configuration.ConfigurationManager.ConnectionStrings["dbStr"].ConnectionString;
                using (SqlConnection conn = new SqlConnection(sqlstring))
                {
                    conn.Open();
                    //string cmd = cmdstring;
                    SqlCommand cmd = conn.CreateCommand();
                    cmd.CommandText = cmdstring;
                    SqlDataAdapter da = new SqlDataAdapter();
                    da.Fill(ds);
                    return ds;
                }
            }
            catch(Exception ex)
            {
                MessageBox.Show("错误信息为:  " + ex.Message);
                return ds;
            }
          
        }
    }
}

原因是：没有给SqlDataAdapter传递参数  ，