在SQL Server中编写通用数据访问方法

using System;
using System.Data;
using System.Data.Common;
using System.Data.OleDb;  
using System.Configuration;    
using Common;

namespace DAL
{
   public class CustomersData : IDbCustomers
   {
      public DataTable GetCustomers()
      {
         string ConnectionString = 
            ConfigurationSettings.AppSettings
            ["ConnectionString"];

         using (OleDbConnection cnn = new OleDbConnection
                  (ConnectionString))
         {
            string cmdString = "SELECT CustomerID," +
               "CompanyName,ContactName " +
               "FROM Customers";

            OleDbCommand cmd = 
               new OleDbCommand (cmdString, cnn);

            OleDbDataAdapter da = new 
               OleDbDataAdapter(cmd); 

            DataTable dt = new DataTable("Customers");

            da.Fill(dt); 

            return dt;
         }
      }
      public DataTable GetCustomerOrders(string CustomerID)
      {
         // 待定
         return null;
      }
      public DataTable GetCustomersByCountry
         (string CountryCode)
      {
         // 待定
         return null;
      }
      public bool InsertCustomer()
      {
         // 待定
         return false;
      }
   }
}

using System;
using System.Data; 

namespace Common
{
    public interface IDbCustomers
    {
        DataTable GetCustomers();
        DataTable GetCustomerOrders(string CustomerID);
        DataTable GetCustomersByCountry(string CountryCode);
        bool InsertCustomer();
    }
}