MergeDocument() crash C4D

chuanzhen

Hi,

c4d.documents.MergeDocument(doc, path_str,c4d.SCENEFILTER_OBJECTS | c4d.SCENEFILTER_MATERIALS,None)

this code,c4d always crashed

Anlv

Hi, @chuanzhen

This code runs fine on my end (Cinema 4D 2026.3.4). Analyzing the cause of the failure may require more detailed information, such as the Cinema 4D version number and the real path (path resolution may occur), and corruption of the path_str file is also one possibility.

Cheers,
Anlv

chuanzhen

@Anlv Hi
Here is a video demonstrating how C4D crashes. For a saved document (or a loaded document), using MergeDocument() is fine, but for a new document that hasn't been saved, using MergeDocument() will definitely crash
2026.3.3 win11

Anlv

Hi, @chuanzhen

I tested the same code as you in both saved projects and unsaved new documents, but no crash occurred. This might be a bug, possibly related to the ObjectData plugin in your project. I can't be certain that's the cause. You could try ruling out the plugin's influence first. But this is beyond my capability, so let's wait for more professional personnel to help.

Cheers,
Anlv

chuanzhen

@Anlv Thanks for your reply.can't reproduce this issue on your side, it seems I need to test it on other computers. It's a puzzling problem

ferdinand

Hey @chuanzhen,

we will need both the scene you are merging and merging into to make here any sensible statement. We will also need more than this one sole line of code.

And while a crash always indicates that we could do something better, my hunch would be that that fault lies mostly with some plugin. Because Cinema itself uses heavily MergeDocument and if there would be a principal bug with the very common flags c4d.SCENEFILTER_OBJECTS | c4d.SCENEFILTER_MATERIALS, we would have found and fixed it before.

The first thing that comes to mind with your line of code is that you are off-main thread and then run into problems because you try to let the code run on the MT by passing None for thread. But off-main-thread, merging documents would be a bit unusual (and also illegal if one of the participating documents is a loaded document). You can use c4d.threading.GeGetCurrentThread() to get the current thread and pass it for thread.

Another cause could be some NodeData plugin which holds data outside of its data container and which does not correctly serialize that extra data (does not implement NodeData::Read, ::Write, and ::CopyTo) which can then lead to crashes when Cinema tries to run the merged scene with only partially copied node data (assuming the plugin in questions also fails to do sanity/existence checks on its internal extra data).

Cheers,
Ferdinand

chuanzhen

@ferdinand Thanks for reply.
I tested a simple C4D file (such as one with only a cube, where the merge command works well).
But I still have a question. If it's a plugin issue ( CopyTo Read Write function is not implemented), then why can the Merge command in C4D merge files normally?

（As an aside, I found this issue in one of my plugins (which uses MergeDocument). This plugin was developed in C4D 2024, and the same code worked in older versions but crashes in 2026. Even with the same script code and merge same file, it works normally in 2024 but crashes in 2026）

c4d.documents.MergeDocument(doc, path_str,c4d.SCENEFILTER_OBJECTS | c4d.SCENEFILTER_MATERIALS,c4d.threading.GeGetCurrentThread())

ferdinand

@chuanzhen said in MergeDocument() crash C4D:

But I still have a question. If it's a plugin issue ( CopyTo Read Write function is not implemented), then why can the Merge command in C4D merge files normally?

What I meant was this: Imagine you have an ObjectData plugin Foo which has a class instance attribute called _data. Many of Foo's methods rely on _data, as for example GetVirtualObjects, GetDDescription, etc. When you now copy an instance of Foo, Cinema will copy its data container but not _data when you have not implemented CopyTo. On the copied instance of Foo, the _data attribute will either not exist at all or be in uninitialized state. Cinema can then for example crash or freeze when you raise an AttributeError (because you try to access a non-existing _data; or any other error) in a GetDDescription call in some corner cases. _data being misaligned with the data container of a node could also lead to all sorts of bugs when your code assumes that they are somehow aligned.

In short, Cinema can always read, write, and copy nodes, no matter what you do. But with CopyTo you can make sure that data outside of the data container of the node (BaseList2D.GetDataInstance) is also correctly copied. The lesson here is that when you have fields such as self._my_data on a node, and that field cannot be reestablished on the fly, you must implement Read, Write, and CopyTo, so that _my_data can be written, read, and copied from/to/between scene files. You therefore also usually implement all three of these methods and not just one of them when you need them.

Cheers,
Ferdinand

chuanzhen

@ferdinand Thanks for your detailed explanation.

ferdinand

（As an aside, I found this issue in one of my plugins (which uses MergeDocument). This plugin was developed in C4D 2024, and the same code worked in older versions but crashes in 2026. Even with the same script code and merge same file, it works normally in 2024 but crashes in 2026）

things can change in the backend of Cinema, without concrete code (the plugin) and example files, I cannot help you much. It could very well be that we added some corner case bug.