今天遇到一个问题，需要在API请求结束时，释放数据库链接，避免连接池被爆掉。
按照以往的经验，需要实现IHttpModule，具体不展开了。
但是实现了IHttpModule后，还得去web.config中增加配置，这有点麻烦了，就想有没有简单的办法。
其实是有的，就是在Global.asax.cs里面定义并实现 Application_EndRequest 方法，在这个方法里面去释放数据库连接即可，经过测试，确实能达到效果。
但是，为什么方法名必须是Application_EndRequest ？在这之前真不知道为什么，只知道baidu上是这么说的，也能达到效果。
还好我有一点好奇心，想搞清楚是怎么回事情，就把net framework的源码拉下来(其实源代码在电脑里面已经躺了N年了) 分析了一下，以下是分析结果。
省略掉前面N个调用
第一个需要关注的是 HttpApplicationFactory.cs
从名字就知道，这是HttpApplication的工厂类，大家看看Gloabal.asax.cs 里面，是不是这样定义的
public class MvcApplication : System.Web.HttpApplication { ..... } 两者结合起来看，可以推测，HttpApplicationFactory 是用来获取 MvcApplication 实例的，实际情况也是如此 上代码（来自HttpApplicationFactory）
internal class HttpApplicationFactory{ internal const string applicationFileName = "global.asax"; //看到这里，就知道为什么入口文件是global.asax了，因为这里定义死了 ... private void EnsureInited() { if (!_inited) { lock (this) { if (!_inited) { Init(); _inited = true; } } } } private void Init() { if (_customApplication != null) return; try { try { _appFilename = GetApplicationFile(); CompileApplication(); } finally { // Always set up global.asax file change notification, even if compilation // failed. This way, if the problem is fixed, the appdomain will be restarted. SetupChangesMonitor(); } } catch { // Protect against exception filters throw; } } private void CompileApplication() { // Get the Application Type and AppState from the global file _theApplicationType = BuildManager.GetGlobalAsaxType(); BuildResultCompiledGlobalAsaxType result = BuildManager.GetGlobalAsaxBuildResult(); if (result != null) { // Even if global.asax was already compiled, we need to get the collections // of application and session objects, since they are not persisted when // global.asax is compiled. Ideally, they would be, but since <object> tags // are only there for ASP compat, it's not worth the trouble. // Note that we only do this is the rare case where we know global.asax contains // <object> tags, to avoid always paying the price (VSWhidbey 453101) if (result.HasAppOrSessionObjects) { GetAppStateByParsingGlobalAsax(); } // Remember file dependencies _fileDependencies = result.VirtualPathDependencies; } if (_state == null) { _state = new HttpApplicationState(); } // Prepare to hookup event handlers via reflection ReflectOnApplicationType(); } private void ReflectOnApplicationType() { ArrayList handlers = new ArrayList(); MethodInfo[] methods; Debug.Trace("PipelineRuntime", "ReflectOnApplicationType"); // get this class methods methods = _theApplicationType.GetMethods(BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance | BindingFlags.Static); foreach (MethodInfo m in methods) { if (ReflectOnMethodInfoIfItLooksLikeEventHandler(m)) handlers.Add(m); } // get base class private methods (GetMethods would not return those) Type baseType = _theApplicationType.BaseType; if (baseType != null && baseType != typeof(HttpApplication)) { methods = baseType.GetMethods(BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static); foreach (MethodInfo m in methods) { if (m.IsPrivate && ReflectOnMethodInfoIfItLooksLikeEventHandler(m)) handlers.Add(m); } } // remember as an array _eventHandlerMethods = new MethodInfo[handlers.Count]; for (int i = 0; i < _eventHandlerMethods.Length; i++) _eventHandlerMethods[i] = (MethodInfo)handlers[i]; } private bool ReflectOnMethodInfoIfItLooksLikeEventHandler(MethodInfo m) { if (m.ReturnType != typeof(void)) return false; // has to have either no args or two args (object, eventargs) ParameterInfo[] parameters = m.GetParameters(); switch (parameters.Length) { case 0: // ok break; case 2: // param 0 must be object if (parameters[0].ParameterType != typeof(System.Object)) return false; // param 1 must be eventargs if (parameters[1].ParameterType != typeof(System.EventArgs) && !parameters[1].ParameterType.IsSubclassOf(typeof(System.EventArgs))) return false; // ok break; default: return false; } // check the name (has to have _ not as first or last char) String name = m.Name; int j = name.IndexOf('_'); if (j <= 0 || j > name.Length-1) return false; // special pseudo-events if (StringUtil.EqualsIgnoreCase(name, "Application_OnStart") || StringUtil.EqualsIgnoreCase(name, "Application_Start")) { _onStartMethod = m; _onStartParamCount = parameters.Length; } else if (StringUtil.EqualsIgnoreCase(name, "Application_OnEnd") || StringUtil.EqualsIgnoreCase(name, "Application_End")) { _onEndMethod = m; _onEndParamCount = parameters.Length; } else if (StringUtil.EqualsIgnoreCase(name, "Session_OnEnd") || StringUtil.EqualsIgnoreCase(name, "Session_End")) { _sessionOnEndMethod = m; _sessionOnEndParamCount = parameters.Length; } return true; } } 上面代码调用链路是EnsureInited->Init->CompileApplication->ReflectOnApplicationType->ReflectOnMethodInfoIfItLooksLikeEventHandler ,核心作用是：将MvcApplication中，方法名包含下划线、方法参数为空或者有2个参数(第一个参数的类型是Object,第二个参数的类型是EventArgs) 的方法加入到_eventHandlerMethods 中
那么事件是怎么绑定的呢？继续上代码
internal class HttpApplicationFactory{ ...... using (new ApplicationImpersonationContext()) { app.InitInternal(context, _state, _eventHandlerMethods); } ...... ...... using (new ApplicationImpersonationContext()) { app.InitInternal(context, _state, _eventHandlerMethods); } ...... } // HttpApplication.cs public class HttpApplication{ internal void InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context) { ..... if (handlers != null) { HookupEventHandlersForApplicationAndModules(handlers); } ..... } internal void InitInternal(HttpContext context, HttpApplicationState state, MethodInfo[] handlers) { ..... if (handlers != null) HookupEventHandlersForApplicationAndModules(handlers); ..... } private void HookupEventHandlersForApplicationAndModules(MethodInfo[] handlers) { _currentModuleCollectionKey = HttpApplicationFactory.applicationFileName; if(null == _pipelineEventMasks) { Dictionary<string, RequestNotification> dict = new Dictionary<string, RequestNotification>(); BuildEventMaskDictionary(dict); if(null == _pipelineEventMasks) { _pipelineEventMasks = dict; } } for (int i = 0; i < handlers.Length; i++) { MethodInfo appMethod = handlers[i]; String appMethodName = appMethod.Name; int namePosIndex = appMethodName.IndexOf('_'); String targetName = appMethodName.Substring(0, namePosIndex); // Find target for method Object target = null; if (StringUtil.EqualsIgnoreCase(targetName, "Application")) target = this; else if (_moduleCollection != null) target = _moduleCollection[targetName]; if (target == null) continue; // Find event on the module type Type targetType = target.GetType(); EventDescriptorCollection events = TypeDescriptor.GetEvents(targetType); string eventName = appMethodName.Substring(namePosIndex+1); EventDescriptor foundEvent = events.Find(eventName, true); if (foundEvent == null && StringUtil.EqualsIgnoreCase(eventName.Substring(0, 2), "on")) { eventName = eventName.Substring(2); foundEvent = events.Find(eventName, true); } MethodInfo addMethod = null; if (foundEvent != null) { EventInfo reflectionEvent = targetType.GetEvent(foundEvent.Name); Debug.Assert(reflectionEvent != null); if (reflectionEvent != null) { addMethod = reflectionEvent.GetAddMethod(); } } if (addMethod == null) continue; ParameterInfo[] addMethodParams = addMethod.GetParameters(); if (addMethodParams.Length != 1) continue; // Create the delegate from app method to pass to AddXXX(handler) method Delegate handlerDelegate = null; ParameterInfo[] appMethodParams = appMethod.GetParameters(); if (appMethodParams.Length == 0) { // If the app method doesn't have arguments -- // -- hookup via intermidiate handler // only can do it for EventHandler, not strongly typed if (addMethodParams[0].ParameterType != typeof(System.EventHandler)) continue; ArglessEventHandlerProxy proxy = new ArglessEventHandlerProxy(this, appMethod); handlerDelegate = proxy.Handler; } else { // Hookup directly to the app methods hoping all types match try { handlerDelegate = Delegate.CreateDelegate(addMethodParams[0].ParameterType, this, appMethodName); } catch { // some type mismatch continue; } } // Call the AddXXX() to hook up the delegate try { addMethod.Invoke(target, new Object[1]{handlerDelegate}); } catch { if (HttpRuntime.UseIntegratedPipeline) { throw; } } if (eventName != null) { if (_pipelineEventMasks.ContainsKey(eventName)) { if (!StringUtil.StringStartsWith(eventName, "Post")) { _appRequestNotifications |= _pipelineEventMasks[eventName]; } else { _appPostNotifications |= _pipelineEventMasks[eventName]; } } } } } } 核心方法：HookupEventHandlersForApplicationAndModules，其作用就是将前面获取到的method与HttpApplication的Event进行绑定(前提是方法名是以Application_开头的)，
后面就是向IIS注册事件通知了，由于看不到IIS源码，具体怎么做的就不知道了。
最后安利一下，还是用net core吧，更加清晰、直观，谁用谁知道。
