【IT168技术文档】

    在使用 Membership 的时候可以为同一种操作方法定义多种行为，而具体使用哪种行为只需要在 Web.Config 中定义即可。

    这样可以极大的促进了系统的灵活性，可是 Membership 这种 Provider 服务是怎么设计的呢？查了一些资料，也查看了 .Framework 2.0 的反编译源码，最终还是在 MSDN 上的一篇英文资料中找到了答案。

    设计这种模式，似乎并不是那么容易，需要设计许多类方可构建基于Provider的自定义服务。

    下面是一个基本Provider的自定义服务的示例，它公开了两个操作方法“RetrieveImage”和“SaveImage”。它有可以会使用不同的数据库，这样可以定义多种处理方法。只需要在 Web.Config 中进行配置，就可以让系统调用相应的行为来进行处理。
    1、 首先构建一个 ImageProvider 它继承了 ProviderBase 类。

ProviderBase#region ProviderBase 
public abstract class ImageProvider : ProviderBase
...{
// Properties 
public abstract string ApplicationName ...{ get; set; }
public abstract bool CanSaveImages ...{ get; }

// Methods 
public abstract Image RetrieveImage (string id);
public abstract void SaveImage (string id, Image image);
}

public class ImageProviderCollection : ProviderCollection
...{
public new ImageProvider this[string name]
...{
get ...{ return (ImageProvider) base[name]; }
}

public override void Add(ProviderBase provider)
...{
if (provider == null)
throw new ArgumentNullException("provider");

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

base.Add(provider);
}
}
#endregion

    2、 我们先定义一个使用 SQL Server 的处理方法。

SqlServer 处理方法#region SqlServer 处理方法
[SqlClientPermission (SecurityAction.Demand, Unrestricted=true)]
public class SqlImageProvider : ImageProvider
...{
 private string _applicationName;
 private string _connectionString;

 public override string ApplicationName
 ...{
get ...{ return _applicationName; }
 set ...{ _applicationName = value; }
 }

 public override bool CanSaveImages
 ...{
 get ...{ return false; }
 }

 public string ConnectionStringName
 ...{
 get ...{ return _connectionStringName; }
 set ...{ _connectionStringName = value; }
 }

 public override void Initialize (string name,
 NameValueCollection config)
 ...{
 // Verify that config isn't null 
 if (config == null)
 throw new ArgumentNullException ("config");

 // Assign the provider a default name if it doesn't have one 
 if (String.IsNullOrEmpty (name))
 name = "SqlImageProvider";

 // Add a default "description" attribute to config if the
 // attribute doesn't exist or is empty 
 if (string.IsNullOrEmpty (config["description"])) ...{
 config.Remove ("description");
 config.Add ("description",
 "SQL image provider");
 }

 // Call the base class's Initialize method 
 base.Initialize(name, config);

 // Initialize _applicationName 
 _applicationName = config["applicationName"];

 if (string.IsNullOrEmpty(_applicationName))
 _applicationName = "/";

 config.Remove["applicationName"];

 // Initialize _connectionString 
 string connect = config["connectionStringName"];

 if (String.IsNullOrEmpty (connect))
 throw new ProviderException
 ("Empty or missing connectionStringName");

 config.Remove ("connectionStringName");

 if (WebConfigurationManager.ConnectionStrings[connect] == null)
 throw new ProviderException ("Missing connection string");

 _connectionString = WebConfigurationManager.ConnectionStrings
 [connect].ConnectionString;

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

 // Throw an exception if unrecognized attributes remain 
 if (config.Count > 0) ...{
 string attr = config.GetKey (0);
 if (!String.IsNullOrEmpty (attr))
 throw new ProviderException
 ("Unrecognized attribute: " + attr);
 }
 }

 public override Image RetrieveImage (string id)
 ...{
 // TODO: Retrieve an image from the database using
 // _connectionString to open a database connection 
 }
 public override void SaveImage (string id, Image image)
 ...{
 throw new NotSupportedException ();
 }
}
#endregion

    配置基于Provider的自定义服务,现在可以看看如何在 Web.Config 中配置它所需的节点。这里在 <System.Web> 节中添加了 <ImageService> 节，在属性 defaultProvider 中指定了它使用的默认 Provider 服务。

    3. Web.Config 文件中配置 Image Service

<configuration >
 <connectionStrings>
 <add name="ImageServiceConnectionString" connectionString="" />
 </connectionStrings>
 <system.web>
 <imageService defaultProvider="SqlImageProvider">
 <providers>
 <add name="SqlImageProvider" type="SqlImageProvider"
 connectionStringName="ImageServiceConnectionString"/>
 </providers>
 </imageService>
 </system.web>
</configuration> 

    结构节点<ImageServer> 现在系统是不可识别的，所有必需还要有一个相应的类用来描述 <ImageServer> 配置节。

    4. <imageServer> 配置节的描述类

描述类#region 描述类
using System;
using System.Configuration;

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

 [StringValidator(MinLength = 1)]
 [ConfigurationProperty("defaultProvider",
 DefaultValue = "SqlImageProvider")]
 public string DefaultProvider
 ...{
 get ...{ return (string) base["defaultProvider"]; }
 set ...{ base["defaultProvider"] = value; }
 }
}
#endregion

    这一下可以在 Web.Config 中注册 <imageService> 节了，并且它会被系统识别。

    5. 创建 <imageService> 这个配置节的处理类

<configuration >
 <configSections>
 <sectionGroup name="system.web">
 <section name="imageService"
 type="ImageServiceSection, CustomSections"
 allowDefinition="MachineToApplication"
 restartOnExternalChanges="true" />
 </sectionGroup>
 </configSections>
 <connectionStrings>
 <add name="ImageServiceConnectionString" connectionString="" />
 </connectionStrings>
 <system.web>
 <imageService defaultProvider="SqlImageProvider">
 <providers>
 <add name="SqlImageProvider" type="SqlImageProvider"
 connectionStringName="ImageServiceConnectionString"/>
 </providers>
 </imageService>
 </system.web>
</configuration> 

    现在可以加载并初始化自定义的 Providers 上面的事情都完成后，就可以实现这个 ImageService 了，它将根据 Web.Config 加载配置中默认的ImageProvider ，可以在 ImageService 类中直接使用它。

    6、创建 ImageService 类，它将使用配置中的实例来处理

ImageService类#region ImageService类
using System;
using System.Drawing;
using System.Configuration;
using System.Configuration.Provider;
using System.Web.Configuration;
using System.Web;

public class ImageService
...{
 private static ImageProvider _provider = null;
 private static ImageProviderCollection _providers = null;
 private static object _lock = new object();

 public ImageProvider Provider
 ...{
 get ...{ return _provider; }
 }

 public ImageProviderCollection Providers
 ...{
 get ...{ return _providers; }
 }

 public static Image RetrieveImage(int imageID)
 ...{
 // Make sure a provider is loaded 
 LoadProviders();

 // Delegate to the provider 
 return _provider.RetrieveImage(imageID);
 }

 public static void SaveImage(Image image)
 ...{
 // Make sure a provider is loaded 
 LoadProviders();

 // Delegate to the provider 
 _provider.SaveImage(image);
 }

 private static void LoadProviders()
 ...{
 // Avoid claiming lock if providers are already loaded 
 if (_provider == null)
 ...{
 lock (_lock)
 ...{
 // Do this again to make sure _provider is still null 
 if (_provider == null)
 ...{
 // Get a reference to the <imageService> section 
 ImageServiceSection section = (ImageServiceSection)
 WebConfigurationManager.GetSection
 ("system.web/imageService");

 // Load registered providers and point _provider
 // to the default provider 
 _providers = new ImageProviderCollection();
 ProvidersHelper.InstantiateProviders
 (section.Providers, _providers,
 typeof(ImageProvider));
 _provider = _providers[section.DefaultProvider];

 if (_provider == null)
 throw new ProviderException
 ("Unable to load default ImageProvider");
 }
 }
 }
 }
}
#endregion

    这些在 Asp.NET 2.0 中被支持。