三层架构最初目的是不把数据库操作语句放到界面里，虽然可以写个公共类操作数据库，但是SQL语句还是免不了。这样做小项目没有什么问题，但是项目做大了就会难以维护。三层架构把数据访问、业务逻辑、界面分离，方便大项目维护。
为了减少难度，这里举个简单的三层架构雏形:只用DAL（Data Access Layer）层，把数据访问封装到DAL层中，UI调用DAL，原则：UI不能出现SQL。
比如我们有个表：
第0列 | 第一列 | 第二列 | 第三列 |
Id | Name | Age | Hobby |
1 | 泰迪熊 | 18 | 勾搭妹子 |
2 | 王旭 | 30 | 勾搭妹子 |
图 1 1 数据库T_Student表
我们添加一个类StudentDAL，SqlHelper是我们上次讲过的。
class StudentDAL
{
public static int GetCount()
{
return (int)SqlHelper.ExecuteScalar(“select count(*) from T_Student”);
}
}
然后在UI层上就可以代入
label1.Text= （StudentDAL.GetCount()）.ToString();
但是当列非常多，参数就会非常多，这时候就要把参数封装到Model中。这里我们建一个类Student。
class Student
{
public long Id{get;set;}
public string Name{set;get;}
public int Age{set;get;}
public string Hobby{set;get;}//可空列注意int？问题
}
然后在class StudentDAL类中添加方法
public static void Insert（Student student）
{
SqlHelper.ExecuteNonQuery（@”Insert into T_Student(Name,Age,Hobby)Values(@Name,@Age,@Hobby)”,new SqlParameter(“@Name”,student.Name), new SqlParameter(“@Age”,student.Age), new SqlParameter(“@Hobby”,student.Hobby)）;
}
UI层就可以直接跟student打交道了。注意：DAL尽量不要返回DataTable和DataRow之类的ADO.NET类。
比如把界面上的数据插入到数据库，这时候先把界面上的数据写入一个Student对象，然后把Student对象作为参数调用StudentDAL类。
总结下基本步骤：
(1)写实体类，实体类对应数据库中的列。比如：
public class Customer
{
public long Id { get; set; }
public string Name { get; set; }
public DateTime? BirthDay { get; set; }
public string Address { get; set; }
public string TelNum { get; set; }
public int CustLevel { get; set; }
}
（2）写DAL类，DAL类要实现把数据库读取到类中、把类实例化的数据放到数据库里的多种方法。比如：
public class CustomerDAL
{
//把DataRow中数据放入实例化的类并返回
private Customer ToCustomer(DataRow row)
{
Customer cust = new Customer();
cust.Id = (long)row["Id"];
cust.Name = (string)row["Name"];
cust.BirthDay = (DateTime?)SqlHelper.FromDbValue(row["BirthDay"]);
cust.Address = (string)row["Address"];
cust.CustLevel = (int)row["CustLevel"];
cust.TelNum = (string)row["TelNum"];
return cust;
}
//通过id查找数据库，返回给实例化的实体类。
public Customer GetById(long id)
{
DataTable dt = SqlHelper.ExecuteDataTable("select * from T_Customer where Id=@Id",
new SqlParameter("@Id", id));
if (dt.Rows.Count <= 0)
{
return null;
}
else if (dt.Rows.Count > 1)
{
throw new Exception("严重错误，查出多条数据！");
}
else
{
DataRow row = dt.Rows[0];
return ToCustomer(row);
}
}
//根据id删除数据库中的元素
public void DeleteById(long id)
{
SqlHelper.ExecuteNonQuery("delete from T_Customer where Id=@Id",
new SqlParameter("@Id", id));
}
//把实体类中的数据插入数据库（新增数据）
public void Insert(Customer customer)
{
SqlHelper.ExecuteNonQuery(@"INSERT INTO [T_Customer]
([Name]
,[BirthDay]
,[Address]
,[TelNum]
,[CustLevel])
VALUES
(@Name, @BirthDay,@Address,@TelNum,@CustLevel)",
new SqlParameter("@Name",customer.Name),
new SqlParameter("@BirthDay", SqlHelper.ToDbValue(customer.BirthDay)),
new SqlParameter("@Address", customer.Address),
new SqlParameter("@TelNum", customer.TelNum),
new SqlParameter("@CustLevel", customer.CustLevel));
}
//把实体类中的数据插入数据库（修改数据）
public void Update(Customer customer)
{
SqlHelper.ExecuteNonQuery(@"UPDATE [T_Customer]
SET [Name] = @Name
,[BirthDay] = @BirthDay
,[Address] = @Address
,[TelNum] = @TelNum
,[CustLevel] = @CustLevel
WHERE Id=@Id",
new SqlParameter("@Name",customer.Name),
new SqlParameter("@BirthDay", SqlHelper.ToDbValue(customer.BirthDay)),
new SqlParameter("@Address", customer.Address),
new SqlParameter("@TelNum", customer.TelNum),
new SqlParameter("@CustLevel", customer.CustLevel)
,new SqlParameter("@Id",customer.Id));
}
//获得全部数据并返回实体类的一个数组
public Customer[] GetAll()
{
DataTable table = SqlHelper.ExecuteDataTable("select * from T_Customer");
Customer[] customers = new Customer[table.Rows.Count];
for (int i = 0; i < table.Rows.Count; i++)
{
DataRow row = table.Rows[i];
customers[i] = ToCustomer(row);
}
return customers;
}
}
接着做个实验举例：
首先设计WPF窗体，命名CustomerList，注意最上面三个图标本质上是button控件，XAML代码如下:
<Button Name="btnEdit" Click="btnEdit_Click">
<Image Source="Images/edit.ico"></Image>
</Button>
下面是DataGrid数据表格控件，用来显示数据，同时绑定实体类的属性，XAML代码如下：
<DataGrid AutoGenerateColumns="False" Name="gridCustomers" IsReadOnly="True" DockPanel.Dock="Top">
<DataGrid.Columns>
<DataGridTextColumn Header="姓名" Width="100" Binding="{Binding Name}"></DataGridTextColumn>
<DataGridTextColumn Header="生日" Width="100" Binding="{Binding BirthDay}"></DataGridTextColumn>
<DataGridTextColumn Header="电话" Width="100" Binding="{Binding TelNum}"></DataGridTextColumn>
<DataGridTextColumn Header="地址" Width="150" Binding="{Binding Address}"></DataGridTextColumn>
<DataGridTextColumn Header="等级" Width="50" Binding="{Binding CustLevel}"></DataGridTextColumn>
</DataGrid.Columns>
</DataGrid>
编辑CustomerEdit窗体
进行数据绑定，XAML代码如下:
<TextBox Text="{Binding Name}" Height="23" HorizontalAlignment="Left" Margin="92,9,0,0" Name="txtName" VerticalAlignment="Top" Width="120" TabIndex="1" />
<TextBox Text="{Binding TelNum}" Height="23" HorizontalAlignment="Left" Margin="368,8,0,0" Name="txtTelNum" VerticalAlignment="Top" Width="120" TabIndex="4" />
<DatePicker SelectedDate="{Binding BirthDay}" Height="25" HorizontalAlignment="Left" Margin="92,53,0,0" Name="dpBirthDay" VerticalAlignment="Top" Width="120" TabIndex="2" />
<TextBox Text="{Binding CustLevel}" Height="23" HorizontalAlignment="Left" Margin="370,50,0,0" Name="txtCustLevel" VerticalAlignment="Top" Width="120" TabIndex="5" />
<TextBox Text="{Binding Address}" Height="23" HorizontalAlignment="Left" Margin="92,106,0,0" Name="txtAddress" VerticalAlignment="Top" Width="474" TabIndex="3" />
在CustomerEdit窗体类中添加如下代码：
//设置表示字段：是新增数据还是修改数据，由CustomerList赋值
public bool IsInsert { get; set; }
//设置标识字段，如果是编辑的话获得被编辑行的Id，，由CustomerList赋值
public long EditingId { get; set; }
public CustomerEdit()
{
InitializeComponent();
}
private void btnSave_Click(object sender, RoutedEventArgs e)
{
//如果是标识字段是新增，则把控件中的数据插入数据库
if (IsInsert)
{
Customer customer = new Customer();
customer.Address = txtAddress.Text;
customer.BirthDay = dpBirthDay.SelectedDate;
customer.CustLevel = Convert.ToInt32(txtCustLevel.Text);
customer.Name = txtName.Text;
customer.TelNum = txtTelNum.Text;
new CustomerDAL().Insert(customer);//插入数据库
//有Bug，没有判断是否是空
}
else
{
//如果不是新增，是编辑，先从数据库中查询旧的值，然后把界面中的值设置到旧对象上，然后Update更新
CustomerDAL dal = new CustomerDAL();
Customer customer = dal.GetById(EditingId);
customer.Address = txtAddress.Text;
customer.BirthDay = dpBirthDay.SelectedDate;
customer.CustLevel = Convert.ToInt32(txtCustLevel.Text);
customer.Name = txtName.Text;
customer.TelNum = txtTelNum.Text;
dal.Update(customer);
}
DialogResult = true;
}
private void btnCancel_Click(object sender, RoutedEventArgs e)
{
DialogResult = false;
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
//窗口打开的时候让第一个输入控件获得焦点
txtName.Focus();
if (IsInsert)
{//给控件赋初始值
txtName.Text = ““；
txtTelNum.Text = “”;
txtAddress.Text =“”;
txtCustLevel.Text = “”;
dpBirthDay.SelectedDate =“”;
}
else//修改
{
////把要编辑的数据从数据库中取出来
////可以把Customer直接在ListUI传进来，这样还省得查一次数据库
Customer customer = new CustomerDAL().GetById(EditingId);
////但是，一个原则：窗口传值、容器中存储值尽量放简单数据类型
////填充到界面上，显示编辑前的值
txtName.Text = customer.Name;
txtTelNum.Text = customer.TelNum;
txtAddress.Text = customer.Address;
txtCustLevel.Text = customer.CustLevel.ToString();
dpBirthDay.SelectedDate = customer.BirthDay;
}
}
最后是完成CustomerList，代码如下：
//加载时加载数据
private void Window_Loaded(object sender, RoutedEventArgs e)
{
LoadData();
}
private void LoadData()
{//获得所有的数据
CustomerDAL dal = new CustomerDAL();
gridCustomers.ItemsSource = dal.GetAll();
}
private void btnAdd_Click(object sender, RoutedEventArgs e)
{
CustomerEdit editUI = new CustomerEdit();
edit.IsInsert = true;//插入，如果
if (edit.ShowDialog() == true)
//如果显示Edit对话框，刷新所有数据
{
LoadData();
}
}
private void btnEdit_Click(object sender, RoutedEventArgs e)
{//把选择的对象赋给实体类
Customer customer = (Customer)gridCustomers.SelectedItem;
if(customer==null)
{
MessageBox.Show("请选择要编辑的行！");
return;
}
CustomerEdit editUI = new CustomerEdit();
editUI.IsInsert = false;
editUI.EditingId = customer.Id;
if (editUI.ShowDialog() == true)
{
LoadData();
}
}
//删除
private void btnDelete_Click(object sender, RoutedEventArgs e)
{
Customer customer = (Customer)gridCustomers.SelectedItem;
if (customer == null)
{
MessageBox.Show("请选择要删除的行！");
return;
}
if (MessageBox.Show("确认删除此条数据吗？", "提醒",
MessageBoxButton.YesNo) == MessageBoxResult.Yes)
{
new CustomerDAL().DeleteById(customer.Id);
LoadData();//刷新数据
}
}
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