一：看下面一些概念
1MethodTable
MethodTable可以说在CLR里面无处不在，这个东西主要是作为对象的数据类型存在，主要包含了EEClass
模块地址，类型名称，模块路径等。
2.EEClass
EEclass描述了实例对象和类型里面的函数描述符起始块地址，起始块以MethodDescChunk描述。EEclass
成员变量m_pChunks存放了起始块的地址。
3.MethodDesc
看这个名称就知道，它是函数描述符。主要包含了函数名称，函数所述模块，函数是否被jit编译等属性。
4.FixUpPreCode
从字面理解修理预代码，其实它也是这个功能。就是它描述了托管函数被编译前和编译后的变化。编译前
会调用非托管PrecodeFixupThunk函数，编译后直接跳到编译的结果。
二：它们是如何组织的
MethodTable会通过CLR生成实例化对象。在这个生成的过程中，会初始化EEClass。然后会给MethodDescChunks
以及MethodDesc分配相应的地址，MethodDesc跟MethodDescChunks地址是连接在一起的，类似于MethodDescChunks
->MethodDes(1)->MehtodDesc(2)->MethodDesc(3)...... 这种形式。下面就是把MethodDescChunks当MethodDesc分配
完成的时候，会遍历MethodDesc，没遍历一个，就会出初始化一个FixUpPreCode结构体。用以描述函数没被编译之前
的状态。他们的关联方式如下图所示：
三：CLR是如何被运行出来的
作为一个.Net 程式核心部分，这一块是一个重点也是一个难点
主要的步骤有以下几步：
1.CLR会加载当前项目bin/debug/文件夹下面的【目名称.ConsoleApp15.runtimeconfig.json】的这个文件，以读取runtime 的配置
2.CLR 会获取当前runtime运行时的指针
3.通过runtime运行时指针函数，传递进去需要加载的模块，类名称，以及函数名称，获取到需要调用的函数指针
4.获取到函数指针之后，就可以释放掉上面运行步骤所占用的内存
5.调用获取到的函数指针调用函数，比如Main函数，这个是程式入口。
到了这一步不多说了，因为所有的.net程序都是从Main函数开始的。这样流程就会被C#代码所接管。
四：代码是如何构造的
由于CLR代码长达几十万行甚至几百万行，所以无法一一展示。
这里取一部分看看是如何执行的
1.MethodTable的构建过程(MethodTablebuilder.cpp 12163行）
MethodTableBuilder builder(
NULL,
pClass,
&GetThread()->m_MarshalAlloc,
pamTracker);
pMT = builder.BuildMethodTableThrowing(
pAllocator,
pLoaderModule,
pModule,
cl,
pInterfaceBuildInfo,
pLayoutRawFieldInfos,
pParentMethodTable,
&genericsInfo,
parentInst,
(WORD)cInterfaces);
END_SO_INTOLERANT_CODE;
RETURN(TypeHandle(pMT));
2.EEclass构建过程（methodtablebuilder.cpp 11957行）
EEClass * pClass = MethodTableBuilder::CreateClass(
pModule,
cl,
fHasLayout,
fIsDelegate,
fIsEnum,
&genericsInfo,
pAllocator,
pamTracker);
3.MethodDescchunks和MethodDesc的构建过程是在buildmethodtablethrowing函数里面调用 AllocAndInitMethodDescs();，先看看 AllocAndInitMethodDescs();函数（methodtablebuilder.cpp 1666行）
VOID MethodTableBuilder::AllocAndInitMethodDescs()
{
STANDARD_VM_CONTRACT;
if (sizeOfMethodDescs != 0)
{
AllocAndInitMethodDescChunk(startIndex, NumDeclaredMethods() - startIndex, sizeOfMethodDescs);
}
}
4.继续看 AllocAndInitMethodDescChunk函数（methodtablebuilder.cpp 6836行），很明显看到了里面构建了methoddescchunks和methoddesc。
VOID MethodTableBuilder::AllocAndInitMethodDescChunk(COUNT_T startIndex, COUNT_T count, SIZE_T sizeOfMethodDescs)
{
CONTRACTL {
STANDARD_VM_CHECK;
PRECONDITION(sizeOfMethodDescs <= MethodDescChunk::MaxSizeOfMethodDescs);
} CONTRACTL_END;
void * pMem = GetMemTracker()->Track(
GetLoaderAllocator()->GetHighFrequencyHeap()->AllocMem(S_SIZE_T(sizeof(TADDR) + sizeof(MethodDescChunk) + sizeOfMethodDescs)));
// Skip pointer to temporary entrypoints
MethodDescChunk * pChunk = (MethodDescChunk *)((BYTE*)pMem + sizeof(TADDR));
COUNT_T methodDescCount = 0;
SIZE_T offset = sizeof(MethodDescChunk);
#ifdef _PREFAST_
#pragma warning(push)
#pragma warning(disable:22019) // Suppress PREFast warning about integer underflow
#endif // _PREFAST_
for (COUNT_T i = 0; i < count; i++)
#ifdef _PREFAST_
#pragma warning(pop)
#endif // _PREFAST_
{
bmtMDMethod * pMDMethod = (*bmtMethod)[static_cast<SLOT_INDEX>(startIndex + i)];
MethodDesc * pMD = (MethodDesc *)((BYTE *)pChunk + offset);
pMD->SetChunkIndex(pChunk);
InitNewMethodDesc(pMDMethod, pMD);
#ifdef _PREFAST_
#pragma warning(push)
#pragma warning(disable:22018) // Suppress PREFast warning about integer underflow
#endif // _PREFAST_
offset += pMD->SizeOf();
#ifdef _PREFAST_
#pragma warning(pop)
#endif // _PREFAST_
methodDescCount++;
// If we're a value class, we want to create duplicate slots
// and MethodDescs for all methods in the vtable
// section (i.e. not non-virtual instance methods or statics).
// In the name of uniformity it would be much nicer
// if we created _all_ value class BoxedEntryPointStubs at this point.
// However, non-virtual instance methods only require unboxing
// stubs in the rare case that we create a delegate to such a
// method, and thus it would be inefficient to create them on
// loading: after all typical structs will have many non-virtual
// instance methods.
//
// Unboxing stubs for non-virtual instance methods are created
// in code:MethodDesc::FindOrCreateAssociatedMethodDesc.
if (NeedsTightlyBoundUnboxingStub(pMDMethod))
{
MethodDesc * pUnboxedMD = (MethodDesc *)((BYTE *)pChunk + offset);
//////////////////////////////////
// Initialize the new MethodDesc
// <NICE> memcpy operations on data structures like MethodDescs are extremely fragile
// and should not be used. We should go to the effort of having proper constructors
// in the MethodDesc class. </NICE>
memcpy(pUnboxedMD, pMD, pMD->SizeOf());
// Reset the chunk index
pUnboxedMD->SetChunkIndex(pChunk);
if (bmtGenerics->GetNumGenericArgs() == 0) {
pUnboxedMD->SetHasNonVtableSlot();
}
//////////////////////////////////////////////////////////
// Modify the original MethodDesc to be an unboxing stub
pMD->SetIsUnboxingStub();
////////////////////////////////////////////////////////////////////
// Add the new MethodDesc to the non-virtual portion of the vtable
if (!bmtVT->AddUnboxedMethod(pMDMethod))
BuildMethodTableThrowException(IDS_CLASSLOAD_TOO_MANY_METHODS);
pUnboxedMD->SetSlot(pMDMethod->GetUnboxedSlotIndex());
pMDMethod->SetUnboxedMethodDesc(pUnboxedMD);
offset += pUnboxedMD->SizeOf();
methodDescCount++;
}
}
_ASSERTE(offset == sizeof(MethodDescChunk) + sizeOfMethodDescs);
pChunk->SetSizeAndCount((ULONG)sizeOfMethodDescs, methodDescCount);
GetHalfBakedClass()->AddChunk(pChunk);
}
5.关于构建fixupprecode (methodtablebuilder.cpp 10497行）
{
for (MethodDescChunk *pChunk = GetHalfBakedClass()->GetChunks(); pChunk != NULL; pChunk = pChunk->GetNextChunk())
{
// Make sure that temporary entrypoints are create for methods. NGEN uses temporary
// entrypoints as surrogate keys for precodes.
pChunk->EnsureTemporaryEntryPointsCreated(GetLoaderAllocator(), GetMemTracker());
}
}