private static void ReadOrderData(string connectionString)
{
    string queryString =
        "SELECT OrderID, CustomerID FROM dbo.Orders;";

    using (SqlConnection connection =
               new SqlConnection(connectionString))
    {
        SqlCommand command =
            new SqlCommand(queryString, connection);
        connection.Open();

        SqlDataReader reader = command.ExecuteReader();

        // Call Read before accessing data.
        while (reader.Read())
        {
            Console.WriteLine(String.Format("{0}, {1}",
                reader[0], reader[1]));
        }

        // Call Close when done reading.
        reader.Close();
    }
}

------解决方案--------------------
SqlDataReader reader = new SqlDataReader(sql, conn);
改成
OleDbCommand cmd=new OleDbCommand(sql.conn);
OleDbDataReader reader=cmd.ExecuteReader();

------解决方案--------------------
string strconn = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\\db1.mdb;";
  OleDbConnection conn = new OleDbConnection(strconn);
  conn.Open();
  string sql = "select [Time] from [project] where [Name]='" + DropDownList1.SelectedValue + "'";
 OleDbCommand cmd=new OleDbCommand(sql.conn);
OleDbDataReader reader=cmd.ExecuteReader();
  if (reader.Read())
  {
  TextBox3.Text = reader[0].ToString();
  }
conn.Close();