public static void RegisterRoutes(RouteCollection routes)
{
  //MVC
  routes.MapRoute(
    name: "Default",
    url: "{controller}/{action}/{id}",
    defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
  );
 
  //WebForm
  routes.MapPageRoute(
    routeName: "WebForm",
    routeUrl: "Admin/{user}",
    physicalFile: "~/Admin/User.aspx"
  );
}

public class UrlRoutingModule : IHttpModule {
  public RouteCollection RouteCollection {
    get {
      if (_routeCollection == null) {
        //全局的RouteCollection集合
        _routeCollection = RouteTable.Routes;
      }
      return _routeCollection;
    }
    set {
      _routeCollection = value;
    }
  }
 
  protected virtual void Init(HttpApplication application) {
    //注册PostResolveRequestCache事件
    application.PostResolveRequestCache += OnApplicationPostResolveRequestCache;
  }
 
  private void OnApplicationPostResolveRequestCache(object sender, EventArgs e) {
    //创建上下文
    HttpApplication app = (HttpApplication)sender;
    HttpContextBase context = new HttpContextWrapper(app.Context);
    PostResolveRequestCache(context);
  }
 
  public virtual void PostResolveRequestCache(HttpContextBase context) {
    //1.获取RouteData
    RouteData routeData = RouteCollection.GetRouteData(context);
    if (routeData == null) {
      return;
    }
    //2.获取IRouteHandler
    IRouteHandler routeHandler = routeData.RouteHandler;
    if (routeHandler == null) {
       
    }
     
    //RequestContext保证了HttpContext和RouteData,在后续使用
    RequestContext requestContext = new RequestContext(context, routeData);
 
    context.Request.RequestContext = requestContext;
 
    //3.获取IHttpHandler
    IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext);
 
    //重新映射到处理程序
    context.RemapHandler(httpHandler);
  }
}　　

public RouteData GetRouteData(HttpContextBase httpContext) {
  using (GetReadLock()) {
    foreach (RouteBase route in this) {
      RouteData routeData = route.GetRouteData(httpContext);
      if (routeData != null) {           
        return routeData;
      }
    }
  }
  return null;
}

public override RouteData GetRouteData(HttpContextBase httpContext) {
  string requestPath = httpContext.Request.AppRelativeCurrentExecutionFilePath.Substring(2) + httpContext.Request.PathInfo;
 
  //结合默认值，匹配url
  RouteValueDictionary values = _parsedRoute.Match(requestPath, Defaults);
 
  if (values == null) {
    return null;
  }
 
  //包装成RouteData，这里为什么不放在if后面呢？
  RouteData routeData = new RouteData(this, RouteHandler);
 
  //匹配约束
  if (!ProcessConstraints(httpContext, values, RouteDirection.IncomingRequest)) {
    return null;
  }
 
  //RouteData的Values和DataTokens都来自于Route
  foreach (var value in values) {
    routeData.Values.Add(value.Key, value.Value);
  }
  if (DataTokens != null) {
    foreach (var prop in DataTokens) {
      routeData.DataTokens[prop.Key] = prop.Value;
    }
  }
 
  return routeData;
}

public interface IRouteHandler {
  IHttpHandler GetHttpHandler(RequestContext requestContext);
}

public static Route MapRoute(this RouteCollection routes, string name, string url, object defaults, object constraints, string[] namespaces)
{
  //创建一个Route对象，它的IRouteHandler为MvcRouteHandler
  Route route = new Route(url, new MvcRouteHandler())
  {
    Defaults = CreateRouteValueDictionary(defaults),
    Constraints = CreateRouteValueDictionary(constraints),
    DataTokens = new RouteValueDictionary()
  };
 
  if ((namespaces != null) && (namespaces.Length > 0))
  {
    route.DataTokens["Namespaces"] = namespaces;
  }
 
  //将Route注册到RouteCollection中
  routes.Add(name, route);
 
  return route;
}

protected virtual IHttpHandler GetHttpHandler(RequestContext requestContext)
{
  //设置session状态
  requestContext.HttpContext.SetSessionStateBehavior(GetSessionStateBehavior(requestContext));
 
  //返回一个实现了IHttpHandler的MvcHandler
  return new MvcHandler(requestContext);
}

public class MvcHandler : IHttpAsyncHandler, IHttpHandler, IRequiresSessionState
{
   protected internal virtual IAsyncResult BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, object state)
  {
    IController controller;
    IControllerFactory factory;
 
    //这个方法里会激活Controller对象
    ProcessRequestInit(httpContext, out controller, out factory);
 
    IAsyncController asyncController = controller as IAsyncController;
    if (asyncController != null)
    {
      // asynchronous controller
      BeginInvokeDelegate beginDelegate = delegate(AsyncCallback asyncCallback, object asyncState)
      {
        try
        {
          //调用Controller的BeginExecute方法
          return asyncController.BeginExecute(RequestContext, asyncCallback, asyncState);
        }
        catch
        {
          factory.ReleaseController(asyncController);
          throw;
        }
      };
 
      EndInvokeDelegate endDelegate = delegate(IAsyncResult asyncResult)
      {
        try
        {
          asyncController.EndExecute(asyncResult);
        }
        finally
        {
          factory.ReleaseController(asyncController);
        }
      };
 
      SynchronizationContext syncContext = SynchronizationContextUtil.GetSynchronizationContext();
      AsyncCallback newCallback = AsyncUtil.WrapCallbackForSynchronizedExecution(callback, syncContext);
      return AsyncResultWrapper.Begin(newCallback, state, beginDelegate, endDelegate, _processRequestTag);
    }
    else
    {
      // synchronous controller
      Action action = delegate
      {
        try
        {
          controller.Execute(RequestContext);
        }
        finally
        {
          factory.ReleaseController(controller);
        }
      };
 
      return AsyncResultWrapper.BeginSynchronous(callback, state, action, _processRequestTag);
    }
  }
}