// Get application instance
IHttpHandler app = HttpApplicationFactory.GetApplicationInstance(context);

internal static IHttpHandler GetApplicationInstance(HttpContext context) { if (_customApplication != null)return _customApplication; // Check to see if it's a debug auto-attach requestif (context.Request.IsDebuggingRequest) return new HttpDebugHandler();    _theApplicationFactory.EnsureInited();_theApplicationFactory.EnsureAppStartCalled(context);return _theApplicationFactory.GetNormalApplicationInstance(context);
}

// the only instance of application factory
private static HttpApplicationFactory _theApplicationFactory = new HttpApplicationFactory();

private void EnsureInited() {if (!_inited) {lock (this) { if (!_inited) {Init(); _inited = true; }} }
}

_appFilename = GetApplicationFile();
CompileApplication();

private void FireApplicationOnStart(HttpContext context) { if (_onStartMethod != null) { HttpApplication app = GetSpecialApplicationInstance();app.ProcessSpecialRequest(context,_onStartMethod,_onStartParamCount, this,EventArgs.Empty, null); RecycleSpecialApplicationInstance(app); }
}

private HttpApplication GetNormalApplicationInstance(HttpContext context) {HttpApplication app = null; lock (_freeList) {if (_numFreeAppInstances > 0) {app = (HttpApplication)_freeList.Pop(); _numFreeAppInstances--;if (_numFreeAppInstances < _minFreeAppInstances) { _minFreeAppInstances = _numFreeAppInstances;} }}if (app == null) { // If ran out of instances, create a new oneapp = (HttpApplication)HttpRuntime.CreateNonPublicInstance(_theApplicationType); using (new ApplicationImpersonationContext()) {app.InitInternal(context, _state, _eventHandlerMethods); }}return app;
}

internal void InitInternal(HttpContext context, HttpApplicationState state, MethodInfo[] handlers) { Debug.Assert(context != null, "context != null");// Remember state_state = state; PerfCounters.IncrementCounter(AppPerfCounter.PIPELINES); try {try { // Remember context for config lookups_initContext = context;_initContext.ApplicationInstance = this;// Set config path to be application path for the application initializationcontext.ConfigurationPath = context.Request.ApplicationPathObject; // keep HttpContext.Current working while running user codeusing (new DisposableHttpContextWrapper(context)) { // Build module list from configif (HttpRuntime.UseIntegratedPipeline) {Debug.Assert(_moduleConfigInfo != null, "_moduleConfigInfo != null");Debug.Assert(_moduleConfigInfo.Count >= 0, "_moduleConfigInfo.Count >= 0"); try {context.HideRequestResponse = true; _hideRequestResponse = true; InitIntegratedModules();}finally { context.HideRequestResponse = false;_hideRequestResponse = false; } }else { InitModules();// this is used exclusively for integrated modeDebug.Assert(null == _moduleContainers, "null == _moduleContainers"); }// Hookup event handlers via reflection if (handlers != null)HookupEventHandlersForApplicationAndModules(handlers); // Initialization of the derived class_context = context;if (HttpRuntime.UseIntegratedPipeline && _context != null) { _context.HideRequestResponse = true;} _hideRequestResponse = true; try { Init();}catch (Exception e) {RecordError(e); }} if (HttpRuntime.UseIntegratedPipeline && _context != null) {_context.HideRequestResponse = false; }_hideRequestResponse = false;_context = null;_resumeStepsWaitCallback= new WaitCallback(this.ResumeStepsWaitCallback); // Construct the execution steps array
            if (HttpRuntime.UseIntegratedPipeline) { _stepManager = new PipelineStepManager(this);} else {_stepManager = new ApplicationStepManager(this);}_stepManager.BuildSteps(_resumeStepsWaitCallback);} finally { _initInternalCompleted = true;// Reset config pathcontext.ConfigurationPath = null;// don't hold on to the context _initContext.ApplicationInstance = null;_initContext = null; } }catch { // Protect against exception filters throw;}
}