C＃中实现三层架构

这篇文章讨论如何在C＃中实现三层架构，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？

什么是三层架构？
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。

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Windows.Forms;using System.Data;namespace _3tierarchitecture{    class Class1    {        BOCustomer cus;        //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;        }    }}