开发自定义的Provider

asp.net 2.0在很多地方都用到Provider Pattern，比如Ｐersonalization、ＭemberShip等都用到了该模式，其可插拔的特性给开发人员极大的便利，开发人员可以更据自己的需求来开发自己的需要

.Net　２.０中给我们已经定义了一个ProviderBase的抽象类，继承自它我们可以方便的开发自定义的Provider

首先我们写一个抽象类，用来定义好我们需要的抽象方法和接口，为了方便调用，我们为这个类定义了一个静态的构造器，用来实例化该抽象类调用的真正的Ｐrovider的实现，并通过Instance（）返回其实例化后的对象,我们就可以通过BlogDataProvider.Instance()来调来所用到的实例了

    public abstract  class BlogDataProvider:ProviderBase
    {
        private static BlogDataProvider _provider = null;
        private static CusProviderCollection<BlogDataProvider> _providers = null;
        private static object _lock = new object();

        static BlogDataProvider()
        {
            CreateDefaultCommonProvider();
        }
        public static BlogDataProvider Instance()
        {
            return _provider;
        }

        private static void CreateDefaultCommonProvider()
        {
            if (_provider ==null)
            {
                lock (_lock)
                {
                    if (_provider == null)
                    {
                        ProvidersConfigSection section = (ProvidersConfigSection)ConfigurationManager.GetSection("system.web/dataService");

                        //ProviderSettingsCollection Settings = (ProviderSettingsCollection)section["providers"];

                        _providers = new CusProviderCollection<BlogDataProvider>();

                        ProvidersHelper.InstantiateProviders(section.Providers, _providers, typeof(BlogDataProvider));

                        _provider = _providers["SqlBlogDataProvider"];
                    }
                }

            }
        }
        public abstract void AddBlog(DHBlog blog);
        public abstract void UpdateBlog(DHBlog blog);

        public abstract void DeleteBlog(int id);

    }

由于我们用的是ProviderHelper.InstatiateProviders来调用配置文件中定义的类，所以我们还要实现其两个参数定义的类，一个是继承自ConfigurationSection用来读
取配置节点的ProvidersConfigSection

    public class ProvidersConfigSection : ConfigurationSection
    {
        [ConfigurationProperty("providers")]
        public ProviderSettingsCollection Providers
        {
            get
            {
                return (ProviderSettingsCollection)base["providers"];
            }
        }
    }

还有一个就是实现自ProviderCollection的一个Provider的集合类，因为我们要定义多种不同的Provider所以用一个泛型来实现

    public class CusProviderCollection<T>:ProviderCollection
        where T:ProviderBase
    {
        
        public new T this[string name]
        {
            get
            {
                return (T)base[name];
            }
        }

        public override void Add(ProviderBase provider)
        {
            if (provider == null)
                throw new ArgumentNullException("provider");
            if (!(provider is T))
                throw new ArgumentException
                      ("Invalid provider type", "provider");
            base.Add(provider);
        }
    }

然后是我们要求的类的功能的具体实现

 public class SqlBlogDataProvider:BlogDataProvider
    {
        private string _applicationName;
        public string ConnectonString { get; set; }

        public override void Initialize(string name, NameValueCollection config)
        {
            if (config == null)
                throw new ArgumentException("config");

            if (String.IsNullOrEmpty(name))
                name = "SqlBlogDataProvider";

            if (string.IsNullOrEmpty(config["description"]))
            {
                config.Remove("description");
                config.Add("description", "SQL Blog provider");
            }

            base.Initialize(name, config);

            _applicationName = config["applicationName"];
            if (string.IsNullOrEmpty(_applicationName))
                _applicationName = "/";

            string strconn = config["connectionStringName"];

            if (string.IsNullOrEmpty(strconn))
                throw new ProviderException ("Empty or missing connectionStringName");


            config.Remove("connectionStringName");

            if (ConfigurationManager.ConnectionStrings[strconn] == null)
                throw new ProviderException("Missing connection string");
            this.ConnectonString = ConfigurationManager.ConnectionStrings[strconn].ConnectionString;

            if (String.IsNullOrEmpty(this.ConnectonString))
                throw new ProviderException("Empty connection string");

        }

        public override void AddBlog(DHBlog blog)
        {
            DataBaseHelper.Insert<DHBlog>(blog);
        }

        public override void UpdateBlog(DHBlog blog)
        {
            throw new Exception("The method or operation is not implemented.");
        }

        public override void DeleteBlog(int id)
        {
            throw new Exception("The method or operation is not implemented.");
        }
    }
}

Provider模式是能过配置文件来定义具体要实例化的类的，我们可以在我们的Web.config加入如下代码

<configuration>
  <configSections>
    <sectionGroup name="system.web">
      <section name="dataService"
               type="MyDHServer.Compontents.DataProvider.ProvidersConfigSection,MyDHServer.Compontents"
               allowDefinition="MachineToApplication" 
               restartOnExternalChanges="true"/>
    </sectionGroup>
</configSections>
</configuration>
.
<system.web>
<dataService>
      <providers>
        <add name="SqlBlogDataProvider"
             type="MyDHServer.DataProvider.SqlDataProvider.SqlBlogDataProvider,MyDHServer.DataProvider"
             connectionStringName="MyDHServerConn" />
      </providers>
    </dataService>
</system.web>