在ASP.NET MVC 的源码一些实体对象（比如 ControllerBuilder，ControllerFactory, Filters, ViewEngines 等）不再直接通过关键字new来创建实体，而是委托到 IResolver接口，而且 IResolver接口本身也比较简单，这样也提高了框架的可扩展性。由于IResolver<T>是internal,因此不会对外暴漏，在外边应该程序中是不能调用到的。
internal interface IResolver<T> { T Current { get; } }
在源码中实现了IResolver接口有2个类，SingleServiceResolver 和MultiServiceResolver ，用的最多的主要是SingleServiceResolver 类
internal class SingleServiceResolver<TService> :IResolver<TService> where TService : class
{
.......
public TService Current
{
get { return _currentValueFromResolver.Value ?? _currentValueThunk() ?? _defaultValue; }
}
.......
}
在SingleServiceResolver实现接口的方法很简单，这这段代码中_currentValueFromResolver，_currentValueThunk，_defaultValue 字段的赋值都是在SingleServiceResolver构造函数中。
public SingleServiceResolver(Func<TService> currentValueThunk, TService defaultValue, string callerMethodName)
{
if (currentValueThunk == null){ throw new ArgumentNullException("currentValueThunk");}
if (defaultValue == null){ throw new ArgumentNullException("defaultValue");}
_resolverThunk = () => DependencyResolver.Current;
_currentValueFromResolver = new Lazy<TService>(GetValueFromResolver);
_currentValueThunk = currentValueThunk;
_defaultValue = defaultValue;
_callerMethodName = callerMethodName;
}
private TService GetValueFromResolver()
{
TService result = _resolverThunk().GetService<TService>();
if (result != null && _currentValueThunk() != null){
throw new InvalidOperationException(String.Format(CultureInfo.CurrentCulture, MvcResources.SingleServiceResolver_CannotRegisterTwoInstances, typeof(TService).Name.ToString(),
_callerMethodName));
}
return result;
}
在构造函数中_currentValueFromResolver = new Lazy<TService>(GetValueFromResolver)中构造了 Lazy<T> 类的新实例，参数为返回值为TService的委托GetValueFromResolver方法_currentValueThunk 也是一个返回值为TService的委托方法，_defaultValue 为TService 的默认值，这个3个字段都是对应的IResolver <T>接口中的Current;在GetValueFromResolver方法中首先调用的是DependencyResolver.Current类中的GetService<TService>();构造函数中的callerMethodName是记录GetValueFromResolver方法出错的异常信息。
DependencyResolver依赖解析器类
DependencyResolver的作用在msdn上是这样注释的：“为实现 System.Web.Mvc.IDependencyResolver 或公共服务定位器 IServiceLocator 接口的依赖关系解析程序提供一个注册点。”；对于IDependencyResolver接口只有2个方法，主要密度是获取服务对象（object GetService(Type serviceType);）或是服务对象列表（ IEnumerable<object> GetServices(Type serviceType);）。
实现 IDependencyResolver接口的类有三个CacheDependencyResolver，DefaultDependencyResolver，DelegateBasedDependencyResolver类
private class DefaultDependencyResolver : IDependencyResolver
{
public object GetService(Type serviceType)
{
if (serviceType.IsInterface || serviceType.IsAbstract) { return null;}
try{
return Activator.CreateInstance(serviceType); }
catch
{
return null; }
}
public IEnumerable<object> GetServices(Type serviceType)
{ return Enumerable.Empty<object>(); }
}
在DefaultDependencyResolver类中的GetService方法中当参数serviceType为接口或抽象类时直接返回null，否则通过反射来创建对象；GetServices直接返回空的列表；
private class DelegateBasedDependencyResolver : IDependencyResolver
{
private Func<Type, object> _getService;
private Func<Type, IEnumerable<object>> _getServices;
public DelegateBasedDependencyResolver(Func<Type, object> getService, Func<Type, IEnumerable<object>> getServices)
{
_getService = getService;
_getServices = getServices;
}
public object GetService(Type type)
{
try
{
return _getService.Invoke(type);
}
catch
{
return null;
}
}
public IEnumerable<object> GetServices(Type type)
{
return _getServices(type);
}
}
DelegateBasedDependencyResolver 类的构造函数接收2个委托方法参数，一个是返回值为object，参数为Type的类型的委托函数，一个是返回值为 IEnumerable<object>，参数为Type的类型的委托函数，在GetService或GetServices方法中执行委托函数的Invoke方法从而创造对象，这样做的函数是把对象的创建放在外部，提高程序的扩展性。
private sealed class CacheDependencyResolver : IDependencyResolver { private readonly ConcurrentDictionary<Type, object> _cache = new ConcurrentDictionary<Type, object>(); private readonly ConcurrentDictionary<Type, IEnumerable<object>> _cacheMultiple = new ConcurrentDictionary<Type, IEnumerable<object>>(); private readonly Func<Type, object> _getServiceDelegate; private readonly Func<Type, IEnumerable<object>> _getServicesDelegate; private readonly IDependencyResolver _resolver; public CacheDependencyResolver(IDependencyResolver resolver) { _resolver = resolver; _getServiceDelegate = _resolver.GetService; _getServicesDelegate = _resolver.GetServices; } public object GetService(Type serviceType) { return _cache.GetOrAdd(serviceType, _getServiceDelegate); } public IEnumerable<object> GetServices(Type serviceType) { return _cacheMultiple.GetOrAdd(serviceType, _getServicesDelegate); } }
对于CacheDependencyResolver 类就可以看出是做了一个缓存，当创建对象时，缓存里面有的话直接从缓存里面取值；
关于DefaultDependencyResolver类与IDefaultDependencyResolver类时怎么关联呢？
public class DependencyResolver
{
private static DependencyResolver _instance = new DependencyResolver();
private IDependencyResolver _current;
/// <summary>
/// Cache should always be a new CacheDependencyResolver(_current).
/// </summary>
private CacheDependencyResolver _currentCache;
public DependencyResolver()
{
InnerSetResolver(new DefaultDependencyResolver());
}
public static IDependencyResolver Current
{
get { return _instance.InnerCurrent; }
}
internal static IDependencyResolver CurrentCache
{
get { return _instance.InnerCurrentCache; }
}
public IDependencyResolver InnerCurrent
{
get { return _current; }
}
/// <summary>
/// Provides caching over results returned by Current.
/// </summary>
internal IDependencyResolver InnerCurrentCache
{
get { return _currentCache; }
}
}
DefaultDependencyResolver类的属性Current就是返回IDefaultDependencyResolver接口对象，在DefaultDependencyResolver类的构造函数中默认用的DefaultDependencyResolver的类，InnerSetResolver方法主要是进行一些字段的赋值操作，而且还可以通过SetResolver方法来对_current，_currentCache字段进行赋值操作。