填充：SelectCommand.Connection属性尚未初始化

Question

我使用以下代码访问MS Access数据库。但我收到一条错误消息Fill：SelectCommand.Connection属性尚未初始化。如何解决此问题。

common.cs
=========
public static bool DBConnectionStatus()
        {
            try
            {
                string conString = @"Provider=Microsoft.Jet.OLEDB.4.0; Data Source=|DataDirectory|db_admin.mdb; Jet OLEDB:Database Password=admin";
                using (OleDbConnection conn = new OleDbConnection(conString))
                {
                    conn.Open();
                    return (conn.State == ConnectionState.Open);
                }
            }
            catch (OleDbException)
            {
                return false;
            }


protected void btn_general_Click(object sender, EventArgs e)
        {
            try
            {
                bool state = common.DBConnectionStatus();
                if (state == true)
                {
                    cmd = new OleDbCommand("select * from tbl_admin");
                    da = new OleDbDataAdapter(cmd);
                    DataSet ds = new DataSet();

                    da.Fill(ds); // Error Here 
                    if (ds.Tables[0].Rows.Count > 0)
                    {

                    }

                }
            }
            catch (Exception e1)
            {
            }
        }