在SQL Server数据库编写通用数据访问方法

using System;
using System.Data; 

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

<?xml version="1.0" encoding="utf-8" ?>
    <configuration>
    <appSettings>
  <add key="ConnectionString" 
value="Server=(local);Database=Northwind;
User ID=UserDemo;Pwd=UserDemo" />
  <add key="DALAssembly" value="DALAccess, 
version=1.0.0.0, PublicKeyToken=F5CD5666253D6082" />
<!--   <add key="ConnectionString" 
value="Provider=Microsoft.Jet.OLEDB.4.0; 
Data Source=..\..\..\Northwind.mdb" />
-->    
    </appSettings>
</configuration>

using System;
using System.Data; 
using System.Configuration;
using System.Reflection;
using Common;

namespace BLL
{
   public class Customers
   {
public DataTable GetAllCustomers()
{
   string AssemblyName = 
ConfigurationSettings.AppSettings
["DALAssembly"];
   string TypeName = "DAL.CustomersData";

   IDbCustomers cd = 
//(IDbCustomers)= 
Assembly.Load(AssemblyName).
CreateInstance(mytype); 

   DataTable dt = cd.GetCustomers();
   return dt;
}
public DataSet GetCustomerOrders()
{
   // 待定
   return null;
}
   }
}