在c#中实现3层架构

原版英文文章地址：http://www.codeproject.com/csharp/three_tier_architecture.asp
介绍

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>

      /// Summary description for BOCustomer.

      /// </SUMMARY>

      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();

         }  

            /// <SUMMARY>

            /// Property FirstName (String)

            /// </SUMMARY>

            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());

                        }

                  }

            }

            /// <SUMMARY>

            /// Property LastName (String)

            /// </SUMMARY>

            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 ...");

                        }

                  }

            }

            /// <SUMMARY>

            /// Property Customer ID (String)

            /// </SUMMARY>

            public String cusID

            {

                  get

                  {

                        return this.cusId;

                  }

                  set

                  {

                        this.cusId = value;

                        if (this.cusID == "")

                        {

                              throw new Exception("Please provide ID ...");

                        }

                  }

            }

            /// <SUMMARY>

            /// Property Address (String)

            /// </SUMMARY>

            public String Address

            {

                  get

                  {

                        return this.address;

                  }

                  set

                  {

                        this.address = value;

                        if (this.Address == "")

                        {

                              throw new Exception("Please provide address ...");

                        }

                  }

            }

            /// <SUMMARY>

            /// Property Telephone (String)

            /// </SUMMARY>

            public String Tel

            {

                  get

                  {

                        return this.tel;

                  }

                  set

                  {

                        this.tel = value;

                        if (this.Tel == "")

                        {

                              throw new Exception("Please provide Tel ...");

                        }

                  }

            }

            /// <SUMMARY>

            /// Function Add new customer. Calls

            /// the function in Data layer.

            /// </SUMMARY>

            public void Add()

            {

                  cusData.Add(this);

            }

            /// <SUMMARY>

            /// Function Update customer details.

            /// Calls the function in Data layer.

            /// </SUMMARY>

            public void Update()

            {

                  cusData.Update(this);

            }

            /// <SUMMARY>

            /// 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.

            /// </SUMMARY>

            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>

    /// Summary description for DACustomer.

    /// </SUMMARY>

    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;

       }

    }

}