.NET：Microsoft.Practices.ServiceLocation 之 动态单例模式

背景

框架开发中，经常会用到“单例模式”，但是传统的单例模式不支持多态和运行时变化，在关注测试的今天，这种模式是不可行的。为了应对这种情况，微软又提供了另外一种模式，暂且将其称为“动态单例模式”。 我也想统一我的框架对单例的使用模式，因此就写了这篇文章。

Microsoft.Practices.ServiceLocation 核心代码

看完代码，如何使用这种模式就不用我多介绍了。

IServiceLocator

using System;
using System.Collections.Generic;

namespace Microsoft.Practices.ServiceLocation
{
    /// <summary>
    /// The generic Service Locator interface. This interface is used
    /// to retrieve services (instances identified by type and optional
    /// name) from a container.
    /// </summary>
    public interface IServiceLocator : IServiceProvider
    {
        /// <summary>
        /// Get an instance of the given <paramref name="serviceType"/>.
        /// </summary>
        /// <param name="serviceType">Type of object requested.</param>
        /// <exception cref="ActivationException">if there is an error resolving
        /// the service instance.</exception>
        /// <returns>The requested service instance.</returns>
        object GetInstance(Type serviceType);

        /// <summary>
        /// Get an instance of the given named <paramref name="serviceType"/>.
        /// </summary>
        /// <param name="serviceType">Type of object requested.</param>
        /// <param name="key">Name the object was registered with.</param>
        /// <exception cref="ActivationException">if there is an error resolving
        /// the service instance.</exception>
        /// <returns>The requested service instance.</returns>
        object GetInstance(Type serviceType, string key);

        /// <summary>
        /// Get all instances of the given <paramref name="serviceType"/> currently
        /// registered in the container.
        /// </summary>
        /// <param name="serviceType">Type of object requested.</param>
        /// <exception cref="ActivationException">if there is are errors resolving
        /// the service instance.</exception>
        /// <returns>A sequence of instances of the requested <paramref name="serviceType"/>.</returns>
        IEnumerable<object> GetAllInstances(Type serviceType);

        /// <summary>
        /// Get an instance of the given <typeparamref name="TService"/>.
        /// </summary>
        /// <typeparam name="TService">Type of object requested.</typeparam>
        /// <exception cref="ActivationException">if there is are errors resolving
        /// the service instance.</exception>
        /// <returns>The requested service instance.</returns>
        TService GetInstance<TService>();

        /// <summary>
        /// Get an instance of the given named <typeparamref name="TService"/>.
        /// </summary>
        /// <typeparam name="TService">Type of object requested.</typeparam>
        /// <param name="key">Name the object was registered with.</param>
        /// <exception cref="ActivationException">if there is are errors resolving
        /// the service instance.</exception>
        /// <returns>The requested service instance.</returns>
        TService GetInstance<TService>(string key);

        /// <summary>
        /// Get all instances of the given <typeparamref name="TService"/> currently
        /// registered in the container.
        /// </summary>
        /// <typeparam name="TService">Type of object requested.</typeparam>
        /// <exception cref="ActivationException">if there is are errors resolving
        /// the service instance.</exception>
        /// <returns>A sequence of instances of the requested <typeparamref name="TService"/>.</returns>
        IEnumerable<TService> GetAllInstances<TService>();
    }
}

ServiceLocatorProvider

namespace Microsoft.Practices.ServiceLocation
{
    /// <summary>
    /// This delegate type is used to provide a method that will
    /// return the current container. Used with the <see cref="ServiceLocator"/>
    /// static accessor class.
    /// </summary>
    /// <returns>An <see cref="IServiceLocator"/>.</returns>
    public delegate IServiceLocator ServiceLocatorProvider();
}

ServiceLocator

namespace Microsoft.Practices.ServiceLocation
{
    /// <summary>
    /// This class provides the ambient container for this application. If your
    /// framework defines such an ambient container, use ServiceLocator.Current
    /// to get it.
    /// </summary>
    public static class ServiceLocator
    {
        private static ServiceLocatorProvider currentProvider;

        /// <summary>
        /// The current ambient container.
        /// </summary>
        public static IServiceLocator Current
        {
            get { return currentProvider(); }
        }

        /// <summary>
        /// Set the delegate that is used to retrieve the current container.
        /// </summary>
        /// <param name="newProvider">Delegate that, when called, will return
        /// the current ambient container.</param>
        public static void SetLocatorProvider(ServiceLocatorProvider newProvider)
        {
            currentProvider = newProvider;
        }
    }
}

动态单例模式的优点

1. 支持多态。
2. 运行时可变。
3. 支持其它级别范围的单例，如：请求级、线程级和会话级等。
4. 支持对象池。

备注

Microsoft.Practices.ServiceLocation下载地址：http://commonservicelocator.codeplex.com/。