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:

 [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。