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Serialization depth limit#

Unity serializes C# types by value. This means that even with reference types it will not serialize null. Value types in C# cannot have circular references (references to themselves within themselves), and the same goes for by value serialization. Unity cannot prevent serializable classes referencing themselves, so to prevent an infinite loop it implements a serialization depth limit.

Resolution#

Choose a resolution:

Remove the reference loop#

Avoid having a serialized class reference itself, or referencing other classes in a way that cause a loop.

Use UnityEngine.Object types#

You can use ScriptableObject or MonoBehaviour references instead of plain C# classes to prevent a loop. Unlike plain C# classes, references to UnityEngine.Object subtypes are by reference, and are null/none by default.

Use SerializeReference#

You can use [SerializeReference] to serialize by reference instead of by value. SerializeReference is selective about what it can serialize, so be sure to read the manual. It also doesn't show an editor by default, and this may be incompatible with your setup.

I have a package, Vertx.SerializeReferenceDropdown, that adds a type selection dropdown for serialize reference fields.