.NET：Microsoft.Practices.ServiceLocation 之 動態單例模式

背景

框架開發中，經常會用到“單例模式”，但是傳統的單例模式不支持多態和運行時變化，在關注測試的今天，這種模式是不可行的。為了應對這種情況，微軟又提供了另外一種模式，暫且將其稱為“動態單例模式”。 我也想統一我的框架對單例的使用模式，因此就寫了這篇文章。

Microsoft.Practices.ServiceLocation 核心代碼

看完代碼，如何使用這種模式就不用我多介紹了。

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;
        }
    }
}

動態單例模式的優點

1. 支持多態。
2. 運行時可變。
3. 支持其它級別范圍的單例，如：請求級、線程級和會話級等。
4. 支持對象池。

備注

Microsoft.Practices.ServiceLocation下載地址：http://commonservicelocator.codeplex.com/。