【IT168 技术文档】

　　最基础的面向对象的任务是使用继承的实现来创建抽象基类，该基类可以包含所有数据访问类可以通过继承使用的服务。如果这些服务足够通用，则可以通过在整个组织中分发基类来对它们进行重用。例如，在最简单的情况下，基类可以为派生类完成连接对象的创建，如下：

Imports System.Data.SqlClient
Namespace ACME.Data
Public MustInherit Class DALBase : Implements IDisposable
　Private _connection As SqlConnection
　Protected Sub New(ByVal connect As String)
　　_connection = New SqlConnection(connect)
　End Sub
　Protected ReadOnly Property Connection() As SqlConnection
　Get
　　Return _connection
　End Get
End Property
Public Sub Dispose() Implements IDisposable.Dispose
　_connection.Dispose()
End Sub
End Class
End Namespace

Public Class WebData : Inherits DALBase
Public Sub New()
　MyBase.New(ConfigurationSettings.AppSettings("ConnectString"))
End Sub
Public Function GetOrders() As DataSet
　Dim da As New SqlDataAdapter("usp_GetOrders", Me.Connection)
　da.SelectCommand.CommandType = CommandType.StoredProcedure
　Dim ds As New DataSet()
　da.Fill(ds)
　Return ds
End Function
End Class

<ConstructionEnabled(True), _
Transaction(TransactionOption.Supported), _
EventTrackingEnabled(True)> _
Public MustInherit Class DALServicedBase : Inherits ServicedComponent
Private _connection As SqlConnection
　Protected Overrides Sub Construct(ByVal s As String)
　　_connection = New SqlConnection(s)
　End Sub
　Protected ReadOnly Property Connection() As SqlConnection
　Get
　　Return _connection
　End Get
End Property
End Class

Public Overloads Function GetOrders() As DataSet
Public Overloads Function GetOrders_
(ByVal customerId As Integer) As DataSet

规则2：遵守设计准则

　　.NET Framework包含一些可以帮助处理与基础结构相关的一般性任务（例如，检测和异常处理）的类和结构，通过基类将这些概念与继承相结合可能十分有用。例如，请考虑在System.Diagnostics命名空间中公开的跟踪功能。除了Trace和Debug类，该命名空间还包括从Switch和TraceListener派生的类。Switch类—BooleanSwitch和TraceSwitch，可以通过编程方式以及通过应用程序的配置文件而被配置为打开和关闭。就TraceSwitch而言，可以公开多个级别的跟踪。TraceListener类—TextWriterTraceListener和EventLogTraceListener将Trace和Debug方法的输出分别定向到文本文件和事件日志。因此，您可以将跟踪功能添加到基类中，以便派生类可以轻松地记录消息。继而，应用程序可以使用应用程序配置文件来控制是否启用跟踪。您可以通过包含一个BooleanSwitch类型的私有变量并在构造函数中将其实例化，以将该功能添加到DALBase类中：

Public Sub New(ByVal connect As String)
　_connection = New SqlConnection(connect)
　_dalSwitch = New BooleanSwitch("DAL", "Data Access Code")
End Sub

Protected Property TracingEnabled() As Boolean
　Get
　　Return _dalSwitch.Enabled
　End Get
　Set(ByVal Value As Boolean)
　　_dalSwitch.Enabled = Value
　End Set
End Property
Protected Sub WriteTrace(ByVal message As String)
　Trace.WriteLineIf(Me.TracingEnabled, Now & ": " & message)
End Sub

<?XML version="1.0" encoding="utf-8" ?>
<configuration>
<system.diagnostics>
<switches>
<add name="DAL" value="1" />
</switches>
<trace autoflush="true" indentsize="4">
<listeners>
<add name="myListener" type="System.Diagnostics.
TextWriterTraceListener" initializeData="DALLog.txt"/>
</listeners>
</trace>
</system.diagnostics>
</configuration>

Public Class DALException : Inherits ApplicationException
　Public Sub New()
　　MyBase.New()
　End Sub
　Public Sub New(ByVal message As String)
　　MyBase.New(message)
　End Sub
　Public Sub New(ByVal message As String,_
ByVal innerException As Exception)
　　MyBase.New(message, innerException)
　End Sub
　' Add custom members here
　Public ConnectString As String
End Class

Protected Sub ThrowDALException(ByVal message As String, _
ByVal innerException As Exception)
Dim newMine As New DALException(message, innerException)
newMine.ConnectString = Me.Connection.ConnectionString
Me.WriteTrace(message & "{" & innerException.Message & "}")
Throw newMine
End Sub

规则4：仔细选择外部接口

　　最后，您还可以决定用公共属性返回自定义类。这些类可以用Serialization属性标记，以便能够跨应用程序域进行复制。另外，如果您要从方法中返回多个对象，则可能需要强类型集合类。

Imports System.Xml.Serialization

<Serializable()> _
Public Class BookClass Book : Implements IComparable

<XmlAttributeAttribute()> Public ProductID As Integer
Public ISBN As String
Public Title As String
Public Author As String
Public UnitCost As Decimal
Public Description As String
Public PubDate As Date

Public Function CompareTo()Function CompareTo(ByVal o As Object) As Integer _
Implements IComparable.CompareTo

　Dim b As Book = CType(o, Book)
　Return Me.Title.CompareTo(b.Title)
End Function

End Class

Public NotInheritable Class BookCollectionClass BookCollection : Inherits ArrayList

Default Public Shadows Property Item_()Property Item_
(ByVal productId As Integer)As Book

Get
　Return Me(IndexOf(productId))
End Get
Set(ByVal Value As Book)
　Me(IndexOf(productId)) = Value
End Set
End Property

Public Overloads Function Contains()Function Contains(ByVal_
 productId As Integer) As Boolean
　Return (-1 <> IndexOf(productId))
End Function

Public Overloads Function IndexOf()Function IndexOf(ByVal_
 productId As Integer) As Integer
　Dim index As Integer = 0
　Dim item As Book

　For Each item In Me
　　If item.ProductID = productId Then
　　　Return index
　　End If
　　index = index + 1
　Next
　Return -1
End Function

Public Overloads Sub RemoveAt()Sub RemoveAt(ByVal productId As Integer)
　RemoveAt(IndexOf(productId))
End Sub

Public Shadows Function Add()Function Add(ByVal value As Book) As Integer
　Return MyBase.Add(value)
End Function

End Class

规则5：抽象化.NET Framework data provider

　　最后一个规则指定,为什么应该对在DAL内部使用的.NET Framework data provider抽象化，以及应该如何进行抽象。正如我已经提到的那样，ADO.NET编程模型公开了独特的.NET Framework data provider，包括SqlClient、OleDb和其他可从 MSDN 在线Web站点上获得的data provider。尽管该设计能够提高性能，并且使provider能够公开特定于数据源的功能（例如SqlCommand对象的ExecuteXMLReader方法），但它会迫使开发人员决定针对哪个provider进行编码。换句话说，开发人员通常选择使用SqlClient或OleDb，然后直接针对各个命名空间中的类编写代码。

public enum ProviderType : int {SqlClient = 0, OLEDB = 1}
public class ProviderFactory {
　public ProviderFactory(ProviderType provider) {
　　_pType = provider;
　　_initClass();
　}
　public ProviderFactory() {
　　_initClass();
　}
　private ProviderType _pType = ProviderType.SqlClient;
　private bool _pTypeSet = false;
　private Type[] _conType, _comType, _parmType, _daType;
　private void _initClass() {
　　_conType = new Type[2];
　　_comType = new Type[2];
　　_parmType = new Type[2];
　　_daType = new Type[2];
　　// Initialize the types for the providers
　　_conType[(int)ProviderType.SqlClient] = typeof(SqlConnection);
　　_conType[(int)ProviderType.OLEDB] = typeof(OleDbConnection);
　　_comType[(int)ProviderType.SqlClient] = typeof(SqlCommand);
　　_comType[(int)ProviderType.OLEDB] = typeof(OleDbCommand);
　　_parmType[(int)ProviderType.SqlClient] = typeof(SqlParameter);
　　_parmType[(int)ProviderType.OLEDB] = typeof(OleDbParameter);
　　_daType[(int)ProviderType.SqlClient] = typeof(SqlDataAdapter);
　　_daType[(int)ProviderType.OLEDB] = typeof(OleDbDataAdapter);
　}
　public ProviderType Provider {
　　get {
　　　return _pType;
　　}
　　set {
　　　if (_pTypeSet) {
　　　　throw new ReadOnlyException_
("Provider already set to " + _pType.ToString());
　　　}
　　　else {
　　　　_pType = value;
　　　　_pTypeSet = true;
　　　}
　　}
　}
　public IDataAdapter CreateDataAdapter_
(string commandText,IDbConnection connection) {
　　IDataAdapter d;
　　IDbDataAdapter da;
　　d = (IDataAdapter)Activator.CreateInstance_
(_daType[(int)_pType], false);
　　da = (IDbDataAdapter)d;
　　da.SelectCommand = this.CreateCommand(commandText, connection);
　　return d;
　}
　public IDataParameter CreateParameter_
(string paramName, DbType paramType) {
　　IDataParameter p;
　　p = (IDataParameter)Activator.CreateInstance_
(_parmType[(int)_pType], false);
　　p.ParameterName = paramName;
　　p.DbType = paramType;
　　return p;
　}
　public IDataParameter CreateParameter_
(string paramName, DbType paramType, Object value) {
　　IDataParameter p;
　　p = (IDataParameter)Activator.CreateInstance_
(_parmType[(int)_pType], false);
　　p.ParameterName = paramName;
　　p.DbType = paramType;
　　p.Value = value;
　　return p;
　}
　public IDbConnection CreateConnection(string connect) {
　　IDbConnection c;
　　c = (IDbConnection)Activator.CreateInstance_
(_conType[(int)_pType], false);
　　c.ConnectionString = connect;
　　return c;
　}
　public IDbCommand CreateCommand(string cmdText,_
IDbConnection connection) {
　　IDbCommand c;
　　c = (IDbCommand)Activator.CreateInstance_
(_comType[(int)_pType], false);
　　c.CommandText = cmdText;
　　c.Connection = connection;
　　return c;
　}
}

Dim _pf As New ProviderFactory(ProviderType.SqlClient)
Dim cn As IDbConnection = _pf.CreateConnection(_connect)
Dim da As IDataAdapter = _pf.CreateDataAdapter("usp_GetBook", cn)
Dim db As IDbDataAdapter = CType(da, IDbDataAdapter)
db.SelectCommand.CommandType = CommandType.StoredProcedure
db.SelectCommand.Parameters.Add(_pf.CreateParameter("@productId", _
DbType.Int32, id))
Dim ds As New DataSet("Books")
da.Fill(ds)