这篇文章讨论如何在c＃中实现3层架构，使用MS Access数据库存储数据。在此，我在3层架构中实现一个小型的可复用的组件保存客户数据。并提供添加，更新，查找客户数据的功能。
背景
首先，我介绍一些3层架构的理论知识。简单说明：什么是3层架构？3层架构的优点是什么？
什么是3层架构？
3层架构是一种“客户端－服务器”架构，在此架构中用户接口，商业逻辑，数据保存以及数据访问被设计为独立的模块。主要有3个层面，第一层（表现层，GUI层），第二层（商业对象，商业逻辑层），第三层（数据访问层）。这些层可以单独开发，单独测试。
为什么要把程序代码分为3层。把用户接口层，商业逻辑层，数据访问层分离有许多的优点。
在快速开发中重用商业逻辑组件，我们已经在系统中实现添加，更新，删除，查找客户数据的组件。这个组件已经开发并且测试通过，我们可以在其他要保存客户数据的项目中使用这个组件。
系统比较容易迁移，商业逻辑层与数据访问层是分离的，修改数据访问层不会影响到商业逻辑层。系统如果从用SQL Server存储数据迁移到用Oracle存储数据，并不需要修改商业逻辑层组件和GUI组件
系统容易修改，假如在商业层有一个小小的修改，我们不需要在用户的机器上重装整个系统。我们只需要更新商业逻辑组件就可以了。
应用程序开发人员可以并行，独立的开发单独的层。
代码
这个组件有3层，第一个层或者称为GUI层用form实现，叫做FrmGUI。第二层或者称为商业逻辑层，叫做BOCustomer，是Bussniess Object Customer的缩写。最后是第三层或者称为数据层，叫做DACustomer，是Data Access Customer的缩写。为了方便，我把三个层编译到一个项目中。
用户接口层
下面是用户接口成的一段代码，我只选取了调用商业逻辑层的一部分代码。
//This function get the details from the user via GUI
//tier and calls the Add method of business logic layer.
private void cmdAdd_Click(object sender, System.EventArgs e)
{
try
{
cus = new BOCustomer();
cus.cusID=txtID.Text.ToString();
cus.LName = txtLName.Text.ToString();
cus.FName = txtFName.Text.ToString();
cus.Tel= txtTel.Text.ToString();
cus.Address = txtAddress.Text.ToString();
cus.Add();
}
catch(Exception err)
{
MessageBox.Show(err.Message.ToString());
}
}
//This function gets the ID from the user and finds the
//customer details and return the details in the form of
//a dataset via busniss object layer. Then it loops through
//the content of the dataset and fills the controls.
private void cmdFind_Click(object sender, System.EventArgs e)
{
try
{
String cusID = txtID.Text.ToString();
BOCustomer thisCus = new BOCustomer();
DataSet ds = thisCus.Find(cusID);
DataRow row;
row = ds.Tables[0].Rows[0];
//via looping
foreach(DataRow rows in ds.Tables[0].Rows )
{
txtFName.Text = rows["CUS_F_NAME"].ToString();
txtLName.Text = rows["CUS_L_NAME"].ToString();
txtAddress.Text = rows["CUS_ADDRESS"].ToString();
txtTel.Text = rows["CUS_TEL"].ToString();
}
}
catch (Exception err)
{
MessageBox.Show(err.Message.ToString());
}
}
//this function used to update the customer details.
private void cmdUpdate_Click(object sender, System.EventArgs e)
{
try
{
cus = new BOCustomer();
cus.cusID=txtID.Text.ToString();
cus.LName = txtLName.Text.ToString();
cus.FName = txtFName.Text.ToString();
cus.Tel= txtTel.Text.ToString();
cus.Address = txtAddress.Text.ToString();
cus.Update();
}
catch(Exception err)
{
MessageBox.Show(err.Message.ToString());
}
}
商业逻辑层
下面是商业逻辑层的所有代码，主要包括定义customer对象的属性。但这仅仅是个虚构的customer对象，如果需要可以加入其他的属性。商业逻辑层还包括添加，更新，查找，等方法。
商业逻辑层是一个中间层，处于GUI层和数据访问层中间。他有一个指向数据访问层的引用cusData = new DACustomer().而且还引用了System.Data名字空间。商业逻辑层使用DataSet返回数据给GUI层。
using System;
using System.Data;
namespace _3tierarchitecture
{
/// Summary description for BOCustomer.
public class BOCustomer
{
//Customer properties
private String fName;
private String lName;
private String cusId;
private String address;
private String tel;
private DACustomer cusData;
public BOCustomer()
{
//An instance of the Data access layer!
cusData = new DACustomer();
}
///
/// Property FirstName (String)
///
public String FName
{
get
{
return this.fName;
}
set
{
try
{
this.fName = value;
if (this.fName == "")
{
throw new Exception(
"Please provide first name ...");
}
}
catch(Exception e)
{
throw new Exception(e.Message.ToString());
}
}
}
/// Property LastName (String)
public String LName
{
get
{
return this.lName;
}
set
{
//could be more checkings here eg revmove ' chars
//change to proper case
//blah blah
this.lName = value;
if (this.LName == "")
{
throw new Exception("Please provide name ...");
}
}
}
/// Property Customer ID (String)
public String cusID
{
get
{
return this.cusId;
}
set
{
this.cusId = value;
if (this.cusID == "")
{
throw new Exception("Please provide ID ...");
}
}
}
/// Property Address (String)
public String Address
{
get
{
return this.address;
}
set
{
this.address = value;
if (this.Address == "")
{
throw new Exception("Please provide address ");
}
}
}
///
/// Property Telephone (String)
///
public String Tel
{
get
{
return this.tel;
}
set
{
this.tel = value;
if (this.Tel == "")
{
throw new Exception("Please provide Tel ...");
}
}
}
///
/// Function Add new customer. Calls
/// the function in Data layer.
///
public void Add()
{
cusData.Add(this);
}
///
/// Function Update customer details.
/// Calls the function in Data layer.
///
public void Update()
{
cusData.Update(this);
}
///
/// Function Find customer. Calls the
/// function in Data layer.
/// It returns the details of the customer using
/// customer ID via a Dataset to GUI tier.
public DataSet Find(String str)
{
if (str == "")
throw new Exception("Please provide ID to search");
DataSet data = null;
data = cusData.Find(str);
return data;
}
}
}
数据访问层
数据层包括处理MS Access数据库的细节。所有这些细节都是透明的，不会影响到商业逻辑层。数据访问层有个指向商业逻辑层的引用BOCustomer cus。为了应用方便并且支持其他数据库。
using System;
using System.Data.OleDb;
using System.Data;
namespace _3tierarchitecture
{
/// Summary description for DACustomer.
public class DACustomer
{
private OleDbConnection cnn;
//change connection string as per the
//folder you unzip the files
private const string CnnStr =
"Provider=Microsoft.Jet.OLEDB.4.0;Data " +
"Source= D:\\Rahman_Backup\\Programming\\" +
"Csharp\\3tierarchitecture\\customer.mdb;";
//local variables
private String strTable="";
private String strFields="";
private String strValues="";
private String insertStr="";
//this needs to be changed based on customer
//table fields' Name of the database!
private const String thisTable = "tblCustomer";
private const String cus_ID = "CUS_ID";
private const String cus_LName = "CUS_L_NAME";
private const String cus_FName = "CUS_F_NAME";
private const String cus_Tel = "CUS_TEL";
private const String cus_Address = "CUS_ADDRESS";
public DACustomer()
{
}
public DACustomer(BOCustomer cus)
{
// A reference of the business object class
}
//standard dataset function that adds a new customer
public void Add(BOCustomer cus)
{
String str = BuildAddString(cus);
OpenCnn();
//Open command option - cnn parameter is imporant
OleDbCommand cmd = new OleDbCommand(str,cnn);
//execute connection
cmd.ExecuteNonQuery();
// close connection
CloseCnn();
}
//standard dataset function that updates
//details of a customer based on ID
public void Update(BOCustomer cus)
{
OpenCnn();
String selectStr = "UPDATE " + thisTable +
" set " + cus_LName + " = '" + cus.LName + "'" +
", " + cus_FName + " = '" + cus.FName + "'" +
", " + cus_Address + " = '" + cus.Address + "'" +
", " + cus_Tel + " = '" + cus.Tel + "'" +
" where cus_ID = '" + cus.cusID + "'";
OleDbCommand cmd = new OleDbCommand(selectStr,cnn);
cmd.ExecuteNonQuery();
CloseCnn();
}
//standard dataset function that finds and
//return the detail of a customer in a dataset
public DataSet Find(String argStr)
{
DataSet ds=null;
try
{
OpenCnn();
String selectStr = "select * from " + thisTable +
" where cus_ID = '" + argStr + "'";
OleDbDataAdapter da =
new OleDbDataAdapter(selectStr,cnn);
ds = new DataSet();
da.Fill(ds,thisTable);
CloseCnn();
}
catch(Exception e)
{
String Str = e.Message;
}
return ds;
}
private void OpenCnn()
{
// initialise connection
String cnnStr = CnnStr;
cnn = new OleDbConnection(cnnStr);
// open connection
cnn.Open();
}
private void CloseCnn()
{
// 5- step five
cnn.Close();
}
// just a supporting function that builds
// and return the insert string for dataset.
private String BuildAddString(BOCustomer cus)
{
// these are the constants as
// set in the top of this module.
strTable="Insert into " + thisTable;
strFields=" (" + cus_ID + "," + cus_LName +
"," + cus_FName + "," + cus_Address + "," + cus_Tel + ")";
//these are the attributes of the
//customer business object.
strValues= " Values ( '" + cus.cusID +
"' , '" + cus.LName + "' , '" + cus.FName +
"' , '" + cus.Address + "' , '" + cus.Tel + "' )";
insertStr = strTable + strFields + strValues;
return insertStr;
}
}
}
