艾伟_转载：在C#中实现3层架构

　　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;          
                    }
        }
}