当IIS收到一个http请求，把请求信息发给对应的HttpModel（实际是实现类UrlRoutingModule），在HttpModel中会注册HttpApplication 类中的PostResolveRequestCache事件，通过此事件来动态选择映射HttpHandler处理程序。通过匹配到的RouteData类的RouteHandler属性得到IRouteHandler对象（MVC4是MvcRouteHandler、asp.net原生的是PageRouteHandler），通过这个对象的GetHttpHandler方法就可以得到HttpHandler处理程序。具体代码如下：
public virtual void PostResolveRequestCache(object sender, EventArgs e) {
HttpApplication app = (HttpApplication)sender;
HttpContextBase context = new HttpContextWrapper(app.Context);
//详细见附1
RouteData routeData = RouteCollection.GetRouteData(context);
if (routeData == null) {
return;
}
//详细见附1
IRouteHandler routeHandler = routeData.RouteHandler;
if (routeHandler is StopRoutingHandler) {
return;
}
RequestContext requestContext = new RequestContext(context, routeData);
context.Request.RequestContext = requestContext;
IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext);
if (httpHandler is UrlAuthFailureHandler) {
if (FormsAuthenticationModule.FormsAuthRequired) {
UrlAuthorizationModule.ReportUrlAuthorizationFailure(HttpContext.Current, this);
return;
}
else {
throw new HttpException(401, SR.GetString(SR.Assess_Denied_Description3));
}
}
context.RemapHandler(httpHandler);//动态指定HttpHandler
}
附1：
RouteCollection.GetHttpHandler方法获取匹配当前路由信息的RouteData，此方法是循环遍历RouteCollection集合并调用集合中RouteBase（实现类为Route）的GetRouteData方法，并返回第一个不为Null的Roudata。Roudata实在RoudataBase的GetRouteData方法中被创建的，详见如下代码：
public override RouteData GetRouteData(HttpContextBase httpContext) {
string requestPath = httpContext.Request.AppRelativeCurrentExecutionFilePath.Substring(2) + httpContext.Request.PathInfo;
RouteValueDictionary values = _parsedRoute.Match(requestPath, Defaults);
if (values == null) {
return null;
}
//为Roudata指定IRouteHandler对象
RouteData routeData = new RouteData(this, RouteHandler);
if (!ProcessConstraints(httpContext, values, RouteDirection.IncomingRequest)) {
return null;
}
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 class RouteData {
private IRouteHandler _routeHandler;
public RouteData() {
}
public RouteData(RouteBase route, IRouteHandler routeHandler) {
Route = route;
RouteHandler = routeHandler;
}
//更多代码
}
MVC4:动态指定到HttpHandler后，就是处理Controller的创建和Action的执行了
HttpHandler根据Routedata获取到Controller的名称，然后通过ControllerBuilder的静态方法GetControllerFactory获取IControllerFactory的实现类（默认是DefaultControllerFactory ）来创建请求的IController的实现类的实例。最后执行IController的Execute方法并传入请求上下文，方法Execute实现在System.Web.Mvc.ControllerBase这个抽象类中，Execute最终调用的是System.Web.Mvc.Controller中的ExecuteCore方法，ExecuteCore方法主要是保存TempData然后根据路由数据调用执行Action。代码如下：
protected override void ExecuteCore()
{
PossiblyLoadTempData();
try
{
string actionName = GetActionName(RouteData);
if (!ActionInvoker.InvokeAction(ControllerContext, actionName))
{
HandleUnknownAction(actionName);
}
}
finally
{
PossiblySaveTempData();
}
}
原文：http://www.cnblogs.com/lxiang/p/3953744.html