A caster is surrounded by enemies. He casts Hypnotic pattern, centering the 30-foot cube on himself in order to target a maximum of foes.
Hypnotic pattern:
Each creature in the area who sees the pattern must make a Wisdom saving throw. On a failed save, the creature becomes charmed for the duration.
Being a creature in the area, the caster should be subject to the effect if he sees the pattern.
Can he shut his eyes in order to avoid being affected by his own spell?
There is no language in the spell that requires you to "see" your target location, just that you pick a point within 120' of you.
In addition, you know exactly where you are, so you could choose to go "blind" and cast the spell.
You may now be under the Blinded condition until the start of your next turn. Whether or not your DM will allow you to re-open your eyes will likely be under their control. Hypnotic Pattern "appears for a moment" so you may be able to reopen, but it would also be very understandable that you need to have your eyes closed for your turn in order to avoid the effects. This would put you under the Blinded condition until the start of your next turn.
Keeping your eyes shut in a stressful situation (for example combat) requires willpower. The character might be able to do it, but they might not.
To model this, make a WIS saving throw. If you pass then the character kept their eyes closed and the spell does not affect them. If you fail, they opened their eyes at the wrong time and the spell affects them.
In other words, the spell's saving throw is already modelling an attempt to close eyes and avoid the spell.
The rules do not include the caster automatically succeeding in a saving throw for a spell just because they cast it. You could allow it as a house rule, but that rule could leads to situations like the folloiwng. A mage casts fireball at their feet and takes no damage "because I cast it I know how to dodge it" or they cast cloudkill and no damage "because I cast it, I know when to hold my breath."
Actually, you cannot cast a cube centered on yourself. From the Player's Handbook, the rules for casting spells with a cube area of effect are:
You select a cube's point of origin, which lies anywhere on a face of the cubic effect. The cube's size is expressed as the length of each side.
A cube's point of origin is not included in the cube's area of effect, unless you decide otherwise.
You do not have to close your eyes since you can simply choose for it to not affect you. On the other hand, you cannot target all enemies that have surrounded you.
