在执行一行代码之前CLR做的68件事

因为CLR是一个托管环境，所以运行时中有几个组件需要在执行任何代码之前初始化。本文将介绍EE（执行引擎）启动程序，并详细检查初始化过程。68只是一个粗略的指南，它取决于您使用的运行时版本、启用了哪些功能以及其他一些东西。

样例代码

假设你有一个最简单的C#程序，在CLR将“Hello World”输出到控制台之前会发生什么？

using System;

namespace ConsoleApplication
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
        }
    }
}

进入EE（执行引擎）的代码路径

当.NET可执行文件运行时，通过以下代码路径进入EE：

1. _CorExeMain() (the external entry point)
   • call to _CorExeMainInternal()
2. _CorExeMainInternal()
   • call to EnsureEEStarted()
3. EnsureEEStarted()
   • call to EEStartup()
4. EEStartup()
   • call to EEStartupHelper()
5. EEStartupHelper()

因此，我们在EEStartupHelper（）中结束，它在高层执行以下操作（来自ceemain.cpp中的注释）---EEStartup负责运行时的所有一次性初始化：

• 创建默认和共享的appdomains。
• 加载mscorlib.dll并加载基本类型（System.Object…）

EE（执行引擎）启动程序的主要阶段

下面的列表包含了从EEStartupHelper（）调用的所有单独函数（~500 L.O.C）。为了使它们更容易理解，我们将它们分为不同的阶段：

• 第1阶段-在运行其他任何东西之前，设置需要到位的基础设施
• 第2阶段-初始化核心低层组件
• 第3阶段-启动低级组件，即错误处理、Profiling  API、调试
• 阶段4-启动主要组件，即垃圾收集器（GC）、AppDomains、安全性
• 第5阶段-最终设置，然后通知其他组件EE已启动

注意：下面列表中的某些项仅在生成时定义了特定功能时才包含，这些项由ifdef语句中的包含来指示。还要注意的是，这些链接指向要调用的函数的代码，而不是EEStartupHelper（）中的代码行。

第1阶段-在运行其他任何东西之前，设置需要到位的基础设施

1. Wire-up console handling - SetConsoleCtrlHandler(..) (ifndef FEATURE_PAL)
2. Initialise the internal SString class (everything uses strings!) - SString::Startup()
3. Make sure the configuration is set-up, so settings that control run-time options can be accessed - EEConfig::Set-up() and InitializeHostConfigFile() (#if !defined(CROSSGEN_COMPILE))
4. Initialize Numa and CPU group information - NumaNodeInfo::InitNumaNodeInfo() and CPUGroupInfo::EnsureInitialized() (#ifndef CROSSGEN_COMPILE)
5. Initialize global configuration settings based on startup flags - InitializeStartupFlags()
6. Set-up the Thread Manager that gives the runtime access to the OS threading functionality (StartThread(), Join(), SetThreadPriority() etc) - InitThreadManager()
7. Initialize Event Tracing (ETW) and fire off the CLR startup events - InitializeEventTracing() and ETWFireEvent(EEStartupStart_V1) (#ifdef FEATURE_EVENT_TRACE)
8. Set-up the GS Cookie (Buffer Security Check) to help prevent buffer overruns - InitGSCookie()
9. Create the data-structures needed to hold the ‘frames’ used for stack-traces - Frame::Init()
10. Ensure initialization of Apphacks environment variables - GetGlobalCompatibilityFlags() (#ifndef FEATURE_CORECLR)
11. Create the diagnostic and performance logs used by the runtime - InitializeLogging() (#ifdef LOGGING) and PerfLog::PerfLogInitialize() (#ifdef ENABLE_PERF_LOG)

第2阶段-初始化核心低层组件

1. Write to the log ===================EEStartup Starting===================
2. Ensure that the Runtime Library functions (that interact with ntdll.dll) are enabled - EnsureRtlFunctions() (#ifndef FEATURE_PAL)
3. Set-up the global store for events (mutexes, semaphores) used for synchronisation within the runtime - InitEventStore()
4. Create the Assembly Binding logging mechanism a.k.a Fusion - InitializeFusion() (#ifdef FEATURE_FUSION)
5. Then initialize the actual Assembly Binder infrastructure - CCoreCLRBinderHelper::Init() which in turn calls AssemblyBinder::Startup() (#ifdef FEATURE_FUSION is NOT defined)
6. Set-up the heuristics used to control Monitors, Crsts, and SimpleRWLocks - InitializeSpinConstants()
7. Initialize the InterProcess Communication with COM (IPC) - InitializeIPCManager() (#ifdef FEATURE_IPCMAN)
8. Set-up and enable Performance Counters - PerfCounters::Init() (#ifdef ENABLE_PERF_COUNTERS)
9. Set-up the CLR interpreter - Interpreter::Initialize() (#ifdef FEATURE_INTERPRETER), turns out that the CLR has a mode where your code is interpreted instead of compiled!
10. Initialise the stubs that are used by the CLR for calling methods and triggering the JIT - StubManager::InitializeStubManagers(), also Stub::Init() and StubLinkerCPU::Init()
11. Set up the core handle map, used to load assemblies into memory - PEImage::Startup()
12. Startup the access checks options, used for granting/denying security demands on method calls - AccessCheckOptions::Startup()
13. Startup the mscorlib binder (used for loading “known” types from mscorlib.dll) - MscorlibBinder::Startup()
14. Initialize remoting, which allows out-of-process communication - CRemotingServices::Initialize() (#ifdef FEATURE_REMOTING)
15. Set-up the data structures used by the GC for weak, strong and no-pin references - Ref_Initialize()
16. Set-up the contexts used to proxy method calls across App Domains - Context::Initialize()
17. Wire-up events that allow the EE to synchronise shut-down - g_pEEShutDownEvent->CreateManualEvent(FALSE)
18. Initialise the process-wide data structures used for reader-writer lock implementation - CRWLock::ProcessInit() (#ifdef FEATURE_RWLOCK)
19. Initialize the debugger manager - CCLRDebugManager::ProcessInit() (#ifdef FEATURE_INCLUDE_ALL_INTERFACES)
20. Initialize the CLR Security Attribute Manager - CCLRSecurityAttributeManager::ProcessInit() (#ifdef FEATURE_IPCMAN)
21. Set-up the manager for Virtual call stubs - VirtualCallStubManager::InitStatic()
22. Initialise the lock that that GC uses when controlling memory pressure - GCInterface::m_MemoryPressureLock.Init(CrstGCMemoryPressure)
23. Initialize Assembly Usage Logger - InitAssemblyUsageLogManager() (#ifndef FEATURE_CORECLR)

第3阶段-启动低级组件，即错误处理、Profiling API、调试

1. Set-up the App Domains used by the CLR - SystemDomain::Attach() (also creates the DefaultDomain and the SharedDomain by calling SystemDomain::CreateDefaultDomain() and SharedDomain::Attach())
2. Start up the ECall interface, a private native calling interface used within the CLR - ECall::Init()
3. Set-up the caches for the stubs used by delegates - COMDelegate::Init()
4. Set-up all the global/static variables used by the EE itself - ExecutionManager::Init()
5. Initialise Watson, for windows error reporting - InitializeWatson(fFlags) (#ifndef FEATURE_PAL)
6. Initialize the debugging services, this must be done before any EE thread objects are created, and before any classes or modules are loaded - InitializeDebugger() (#ifdef DEBUGGING_SUPPORTED)
7. Activate the Managed Debugging Assistants that the CLR provides - ManagedDebuggingAssistants::EEStartupActivation() (ifdef MDA_SUPPORTED)
8. Initialise the Profiling API - ProfilingAPIUtility::InitializeProfiling() (#ifdef PROFILING_SUPPORTED)
9. Initialise the exception handling mechanism - InitializeExceptionHandling()
10. Install the CLR global exception filter - InstallUnhandledExceptionFilter()
11. Ensure that the initial runtime thread is created - SetupThread() in turn calls SetupThread(..)
12. Initialise the PreStub manager (PreStub’s trigger the JIT) - InitPreStubManager() and the corresponding helpers StubHelpers::Init()
13. Initialise the COM Interop layer - InitializeComInterop() (#ifdef FEATURE_COMINTEROP)
14. Initialise NDirect method calls (lazy binding of unmanaged P/Invoke targets) - NDirect::Init()
15. Set-up the JIT Helper functions, so they are in place before the execution manager runs - InitJITHelpers1() and InitJITHelpers2()
16. Initialise and set-up the SyncBlock cache - SyncBlockCache::Attach() and SyncBlockCache::Start()
17. Create the cache used when walking/unwinding the stack - StackwalkCache::Init()

阶段4-启动主要组件，即垃圾收集器（GC）、AppDomains、安全性

1. Start up security system, that handles Code Access Security (CAS) - Security::Start() which in turn calls SecurityPolicy::Start()
2. Wire-up an event to allow synchronisation of AppDomain unloads - AppDomain::CreateADUnloadStartEvent()
3. Initialise the ‘Stack Probes’ used to setup stack guards InitStackProbes() (#ifdef FEATURE_STACK_PROBE)
4. Initialise the GC and create the heaps that it uses - InitializeGarbageCollector()
5. Initialise the tables used to hold the locations of pinned objects - InitializePinHandleTable()
6. Inform the debugger about the DefaultDomain, so it can interact with it - SystemDomain::System()->PublishAppDomainAndInformDebugger(..) (#ifdef DEBUGGING_SUPPORTED)
7. Initialise the existing OOB Assembly List (no idea?) - ExistingOobAssemblyList::Init() (#ifndef FEATURE_CORECLR)
8. Actually initialise the System Domain (which contains mscorlib), so that it can start executing - SystemDomain::System()->Init()

第5阶段最终设置，然后通知其他组件EE已经启动了第4阶段-启动主要组件，即垃圾收集器（GC）、AppDomains、Security

1. Tell the profiler we’ve stated up - SystemDomain::NotifyProfilerStartup() (#ifdef PROFILING_SUPPORTED)
2. Pre-create a thread to handle AppDomain unloads - AppDomain::CreateADUnloadWorker() (#ifndef CROSSGEN_COMPILE)
3. Set a flag to confirm that ‘initialisation’ of the EE succeeded - g_fEEInit = false
4. Load the System Assemblies (‘mscorlib’) into the Default Domain - SystemDomain::System()->DefaultDomain()->LoadSystemAssemblies()
5. Set-up all the shared static variables (and String.Empty) in the Default Domain - SystemDomain::System()->DefaultDomain()->SetupSharedStatics(), they are all contained in the internal class SharedStatics.cs
6. Set-up the stack sampler feature, that identifies ‘hot’ methods in your code - StackSampler::Init() (#ifdef FEATURE_STACK_SAMPLING)
7. Perform any once-only SafeHandle initialization - SafeHandle::Init() (#ifndef CROSSGEN_COMPILE)
8. Set flags to indicate that the CLR has successfully started - g_fEEStarted = TRUE, g_EEStartupStatus = S_OK and hr = S_OK
9. Write to the log ===================EEStartup Completed===================

 一旦这些都完成了，CLR现在就可以执行您的代码了！

执行用户代码

您的代码将通过以下代码流执行（在第一次被“JITted”之后）：

1. CorHost2::ExecuteAssembly()
   • calling ExecuteMainMethod()
2. Assembly::ExecuteMainMethod()
   • calling RunMain()
3. RunMain() (in assembly.cpp)
   • eventually calling into you main() method
   • full explanation of the ‘call’ process

转自：https://mattwarren.org/2017/02/07/The-68-things-the-CLR-does-before-executing-a-single-line-of-your-code/