Casing assembly having a configuration for blocking extraneous material

A casing assembly having at least one configuration for blocking extraneous material, the casing assembly comprising:

• a first shell part, comprising a contact portion and a first recess portion;
• and
• a second shell part, comprising a step portion and a protrusion portion, the step portion being in contact with the contact portion of the first shell part and forming a decorative slot exposed to the outside with the contact portion, the first recess portion located on a side of the contact portion that is located away from the decorative slot, the protrusion portion located on a side of the step portion that is located away from the decorative slot and located in the first recess portion;
• wherein, the protrusion portion has a top surface facing toward the first recess portion, the first recess portion has a bottom surface, and an idle cavity is formed between the top surface of the protrusion portion and the bottom surface of the first recess portion.