这里是document中的解释:
Error:
By default Json.NET will error if a reference loop is encountered (otherwise the serializer will get into an infinite loop).
Ignore:
Json.NET will ignore objects in reference loops and not serialize them. The first time an object is encountered it will be serialized as usual but if the object is encountered as a child object of itself the serializer will skip serializing it.
Serialize:
This option forces Json.NET to serialize objects in reference loops. This is useful if objects are nested but not indefinitely.