How does GC find out class implements destructor?

[voroninp]

As I understand destructors in C# are just syntactic sugar for protected virtual Finalize() method declared on the Object class.
However, classes with destructors are put into the finalization queue.

Does GC use reflection to check whether Finalize() was overridden in the hierarchy or how does it know there's a finilization logic?

[EgorBo]

You can access a bitflag from object's pMethodTable to see if it has a finalizer or not (obj->pMT->m_flags & 0x00100000). But I think GC doesn't query it explicitly. Instead, we register object in the finalizer queue on the allocation step. Objects with finalizers always use the slow path for allocations.

[voroninp]

Hm...

Objects with finalizers always use the slow path for allocations.

Could you provide more details, please?

[EgorBo]

Could you provide more details, please?

Normally, GC allocates large chunks of memory and VM/JIT are allowed to register new objects there themselves (basically, by bumping the pointer and setting pMT) while checking bounds and only use the slow path allocator when there is not enough room in the current chunk. It is not the case for object with finalizers as those must always use the slow path allocator so the actual allocation will happen by the GC.

[AustinWise]

If you want to see the implementation, search for GC_ALLOC_FINALIZE. You can see the allocation helpers set this flag and the GC checking for it to register objects for finalization.