void ClassLoader::PropagateCovariantReturnMethodImplSlots(MethodTable* pMT)
{
    CONTRACTL
    {
        STANDARD_VM_CHECK;
        PRECONDITION(CheckPointer(pMT));
    }
    CONTRACTL_END;

    // 以下代码的逻辑是如果 MethodImpl 具有 PreserveBaseOverridesAttribute 特性，则将重写的 MethodImpl 传播到所有适用的虚表空间槽。在 C# 的抽象或虚拟方法都相当于定义了函数的虚表，存放在虚表空间槽。 这是为了确保如果我们使用基类型方法之一的签名来调用覆盖方法，我们仍然执行覆盖方法。
    // 例如下面注释的代码例子
    //
    // Propagate an overriding MethodImpl to all applicable vtable slots if the MethodImpl
    // has the PreserveBaseOverrides attribute. This is to ensure that if we use the signature of one of
    // the base type methods to call the overriding method, we still execute the overriding method.
    //
    // Consider this case:
    //
    //      class A 
    //      {
    //          RetType VirtualFunction() { }
    //      }
    //      class B : A 
    //      {
    //          [PreserveBaseOverrides]
    //          DerivedRetType VirtualFunction() { .override A.VirtualFuncion }
    //      }
    //      class C : B 
    //      {
    //          MoreDerivedRetType VirtualFunction() { .override A.VirtualFunction }
    //      }
    //
    // NOTE: Typically the attribute would be added to the MethodImpl on C, but was omitted in this example to
    //       illustrate how its presence on a MethodImpl on the base type can propagate as well. In other words,
    //       think of it as applying to the vtable slot itself, so any MethodImpl that overrides this slot on a
    //       derived type will propagate to all other applicable vtable slots.
    //
    // Given an object of type C, the attribute will ensure that:
    //      callvirt RetType A::VirtualFunc()               -> executes the MethodImpl on C
    //      callvirt DerivedRetType B::VirtualFunc()        -> executes the MethodImpl on C
    //      callvirt MoreDerivedRetType C::VirtualFunc()    -> executes the MethodImpl on C
    //
    // Without the attribute, the second callvirt would normally execute the MethodImpl on B (the MethodImpl on
    // C does not override the vtable slot of B's MethodImpl, but only overrides the declaring method's vtable slot.
    //

    // Validation not applicable to interface types and value types, since these are not currently
    // supported with the covariant return feature

    if (pMT->IsInterface() || pMT->IsValueType())
        return;

    MethodTable* pParentMT = pMT->GetParentMethodTable();
    if (pParentMT == NULL)
        return;

    // Propagate overriding MethodImpls to applicable vtable slots if the declaring method has the attribute

    if (pMT->GetClass()->HasVTableMethodImpl())
    {
        MethodTable::MethodDataWrapper hMTData(MethodTable::GetMethodData(pMT, FALSE));

        for (WORD i = 0; i < pParentMT->GetNumVirtuals(); i++)
        {
            if (pMT->GetRestoredSlot(i) == pParentMT->GetRestoredSlot(i))
            {
                // The real check is that the MethodDesc's must not match, but a simple VTable check will
                // work most of the time, and is far faster than the GetMethodDescForSlot method.
                _ASSERTE(pMT->GetMethodDescForSlot(i) == pParentMT->GetMethodDescForSlot(i));
                continue;
            }

            MethodDesc* pMD = pMT->GetMethodDescForSlot(i);
            MethodDesc* pParentMD = pParentMT->GetMethodDescForSlot(i);
            if (pMD == pParentMD)
                continue;

            // If the bit is not set on this method, but we reach here because it's been set on the method at the same slot on
            // the base type, set the bit for the current method to ensure any future overriding method down the chain gets checked.
            if (!pMD->RequiresCovariantReturnTypeChecking() && pParentMD->RequiresCovariantReturnTypeChecking())
                pMD->SetRequiresCovariantReturnTypeChecking();

            // The attribute is only applicable to MethodImpls. For anything else, it will be treated as a no-op
            if (!pMD->IsMethodImpl())
                continue;

            // Search if the attribute has been applied on this vtable slot, either by the current MethodImpl, or by a previous
            // MethodImpl somewhere in the base type hierarchy.
            bool foundAttribute = false;
            MethodTable* pCurrentMT = pMT;
            while (!foundAttribute && pCurrentMT != NULL && i < pCurrentMT->GetNumVirtuals())
            {
                MethodDesc* pCurrentMD = pCurrentMT->GetMethodDescForSlot(i);

                // The attribute is only applicable to MethodImpls. For anything else, it will be treated as a no-op
                if (pCurrentMD->IsMethodImpl())
                {
                	// 下面两个变量是没有使用的，只是让 GetCustomAttribute 函数可以调
                    BYTE* pVal = NULL;
                    ULONG cbVal = 0;
                    // 这里就是判断是否存在特性，如果存在，那么设置 foundAttribute 变量
                    if (pCurrentMD->GetCustomAttribute(WellKnownAttribute::PreserveBaseOverridesAttribute, (const void**)&pVal, &cbVal) == S_OK)
                        foundAttribute = true;
                }

                pCurrentMT = pCurrentMT->GetParentMethodTable();
            }

            if (!foundAttribute)
                continue;

            // Search for any vtable slot still pointing at the parent method, and update it with the current overriding method
            for (WORD j = i; j < pParentMT->GetNumVirtuals(); j++)
            {
                MethodDesc* pCurrentMD = pMT->GetMethodDescForSlot(j);
                if (pCurrentMD == pParentMD)
                {
                    // This is a vtable slot that needs to be updated to the new overriding method because of the
                    // presence of the attribute.
                    pMT->SetSlot(j, pMT->GetSlot(i));
                    _ASSERT(pMT->GetMethodDescForSlot(j) == pMD);

                    hMTData->UpdateImplMethodDesc(pMD, j);
                }
            }
        }
    }
}