ASP.NET中的Provider模式

      Provider Pattern 实现接口与具体实现的分离，通过配制文件灵活配制需要的具体实现。
沿用上次那个示例接口不变。

1.实现类必须继承System.Configuration.Provider.ProviderBase，这次我们多增加一个实现，看代码：

using  System;
using  SampleProviderPattern.Interface;
using  System.Configuration.Provider;

namespace  Machine
{
     public   class  MachineT50 : ProviderBase,IAction
    {
         public   void  Burn()
        {  
            Console.WriteLine( " MachineT50 is burning now. " );
        }
    }

     public   class  MachineT800 : ProviderBase, IAction
    {
         public   void  Burn()
        {
            Console.WriteLine( " MachineT800 is burning now. " );
        }
    }
}

2.自定义配制节ActionProviderSection继承自ConfigurationSection，看代码：

using  System.Configuration;

namespace  Interface
{
     public   class  ActionProviderSection : ConfigurationSection
    {
         ///   <summary>
         ///  A collection of registered providers.
         ///   </summary>
        [ConfigurationProperty( " providers " )]
         public  ProviderSettingsCollection Providers
        {
             get  {  return  (ProviderSettingsCollection) base [ " providers " ]; }
        }

         ///   <summary>
         ///  The name of the default provider
         ///   </summary>
        [StringValidator(MinLength  =   1 )]
        [ConfigurationProperty( " defaultProvider " , DefaultValue  =   " MachineT50Provider " )]
         public   string  DefaultProvider
        {
             get  {  return  ( string ) base [ " defaultProvider " ]; }
             set  {  base [ " defaultProvider " ]  =  value; }
        }
    }
}

3.ActionProviderCollection继承自ProviderCollection,注意引用namespace,System.Configuration.Provider

using  System;
using  System.Configuration.Provider;
using  SampleProviderPattern.Interface;

namespace  Interface
{
     public   class  ActionProviderCollection : ProviderCollection
    {
         ///   <summary>
         ///  Gets a provider by its name.
         ///   </summary>
         public   new  IAction  this [ string  name]
        {
             get  {  return  (IAction) base [name]; }
        }

         ///   <summary>
         ///  Add a provider to the collection.
         ///   </summary> s
         public   override   void  Add(ProviderBase provider)
        {
             if  (provider  ==   null )
                 throw   new  ArgumentNullException( " provider " );

             if  ( ! (provider  is  IAction))
                 throw   new  ArgumentException
                    ( " Invalid provider type " ,  " provider " );

             base .Add(provider);
        }
    }
}

4.让写一个Service实现取对象，引用System.web，使用ProvidersHelper,代码如下:

using  System.Configuration;
using  System.Configuration.Provider;
using  System.Web.Configuration;
using  SampleProviderPattern.Interface;

namespace  Interface
{
     ///   <summary>
     ///  GeneralProviderService
     ///   </summary>
     ///   <remarks> author PetterLiu  http://wintersun.cnblogs.com   </remarks>
     public   class  GeneralProviderService
    { 
         private   static  IAction _generalprovider;
         private   static  ActionProviderCollection _providers;
         private   static   object  _lock  =   new   object ();

         public   static  IAction CreateActionProvider
        {
             get
            {
                LoadProvider();
                 return  _generalprovider;
            }
        }

         ///   <summary>
         ///  Loads the provider.
         ///   </summary>
         private   static   void  LoadProvider()
        {
             if  (_generalprovider  ==   null )
            {
                 lock  (_lock)
                {
                     if  (_generalprovider  ==   null )
                    {
                        ActionProviderSection section  =  (ActionProviderSection)ConfigurationManager.GetSection( " ActionProvider " );
                         if  (section  !=   null )
                        {
                            _providers  =   new  ActionProviderCollection();
                            ProvidersHelper.InstantiateProviders(section.Providers, _providers,  typeof (IAction));
                            _generalprovider  =  _providers[section.DefaultProvider];
                             if  (_generalprovider  ==   null )
                                 throw   new  ProviderException( " Unable to load default Provider " );
                        }
                         else
                        {
                         
                             throw   new  ConfigurationErrorsException();
                        }
                    }
                }
            }
        }

         ///   <summary>
         ///  GetProvider using name
         ///   </summary>
         ///   <param name="name"> providername </param>
         ///   <returns> IAction </returns>
         public   static  IAction GetProvider( string  name)
        {
            LoadProvider();
             return  _generalprovider  =  _providers[name];      
        }

    }
}

5.让我们看一下配制文件是如何配制的：

< configuration >
   < configSections >
     < section  name ="ActionProvider"  type ="Interface.ActionProviderSection, Interface"  requirePermission ="false"   allowDefinition ="MachineToApplication"  restartOnExternalChanges ="true" />
   </ configSections >
   < ActionProvider  defaultProvider ="MachineT50Provider" >
     < providers >
       < add  name ="MachineT50Provider"  type ="Machine.MachineT50, Machine" />
       < add  name ="MachineT800Provider"  type ="Machine.MachineT800, Machine" />
     </ providers >
   </ ActionProvider >
</ configuration >

6.最后的调用与测试:

Test
 [TestFixture]
    public class TestProvider
    {
        [Test]
        public void Execute()
        {
            IAction action = GeneralProviderService.CreateActionProvider;
            Assert.IsNotNull(action);
            //output:MachineT50 is burning now.
            action.Burn();
        }

        [Test]
        public void TestGetInstanceByName()
        {
            IAction action = GeneralProviderService.GetProvider("MachineT800Provider");
            Assert.IsNotNull(action);
            //output:MachineT800 is burning now.
            action.Burn();
        }
    }

ps:这是ProvidersHelper来自asp.net,也可以自己写，但其实没必要，引用就可以了，实际还可以用容器来实现，后期
将介绍容器实现。