This issue has plagued me for so long that I've ceased thinking about it as a problem, and just automatically work around it... but it can't be how the designers intended the game to work, I expect... or can it?

When I click the button to "set hairstyle for all outfits", it never works; I have to go through all of them and manually set the hairstyle for each mode of dress. What I usually do is set up the outfits first, and then go back and choose a hair for each one, which is kind of backwards, since hair is first on the list of CAS chores. (I also sometimes design the rest of the face before the hair, if it's one of those hairdos that gets in the way, but that's another story.) I fully realize that with custom hairs, not every creator makes them possible for all the clothing styles (another mystery to me)... and I do use a ton of CC. But the selection should stick for the hairstyles that are available for every outfit... I would think. Is this a known issue, and there's a patch available? Could it just be a coincidence that every hairstyle I choose won't work for all clothing modes? If the latter is the case, I'd think that they would stick where possible, but revert to some basegame hair for the ones that the creator didn't account for. Instead, I think what happens is that when I click the "all clothing" button, it sets the pre-made hair for all outfits, instead of the one I picked for "everyday". Maybe that's how it's supposed to work, and I should just stop using those pre-made faces (which I only do occasionally, to save a little time in face sculpting, but which admittedly sometimes backfires, even for that purpose). Anyhow... please advise. It's quite possible that this has already been covered, but I didn't know what to search for in the forum archives. "CAS" generated 350 pages of results; "CAS and hair" generated zilch.

I find it to be the same, and I can't for the life of me figure out to change hairstyles for work wear (how to explain by storyline how my sim's dreadlocks vanish?), so I'll be interested to see any solutions or explanations!

What I do if I want all the hairs to be the same - I make sure the "set hair for all" is locked, then pick the hair. This locks it in. Otherwise you do have to go through and pick the hairs.

If I want one category different, I lock in the main hair like above, thn unlock it and pick the hairs I want for the other categories.

In most cases this works, if the hair is set for all categories. If it's not (has multiple states, or lacks categories), it can unfortunately only be fixed outside the game with SimPE or similar - if that's the problem, you can go through the hairs and make sure they're categorized the way you want them**).

Also, sometimes if you try to pick a different hair for Everyday, it messes up the choices, though. I just try to make sure the Everyday hair is the main hair I want the sim to have. Also make sure you don't mix binned and custom hairs (can occasionally work, but usually doesn't if you do the Everyday hair as custom).

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**Unless the hair has multiple states (which is a different can of worms***), you can set the "Category" to 137F in SimPE, for all the PropertySet resources, which covers all categories used ingame. There are also a few available recategorizers with tick boxes if you prefer those.

(***You can try to set the main hair to Category = 137F, and the other hair used for swim/etc. to Age+gender = 0, but hairs can absolutely be a very wriggly can of worms, so make sure to test out the hairs before putting them on your favorite sims. This trick does work best for temporarily hiding accessory recolors for certain ages/genders, and I've had the odd WTF moment when using it with hairs).

That's the thing, though... for me it doesn't always lock it in. When it does, it usually locks in a hairstyle I didn't pick, like the one that came on the premade head.
Unless... wait, I just re-read your answer, and I think I see what you're saying here. I'm probably doing it in the wrong order.
I should:
1. click the "all outfits" button first
2. then pick the style I want for most of the outfits
3. then change the ones I want afterwards (like for swimming or sleep) Yes? All along, I've been picking the everyday hairstyle, and then clicking the button to lock that one in, but the game locks in the premade one instead. I'm gonna try it that way, and see what happens. If it's something so simple, I'll gladly give myself a big dope slap. Doh!

Yep! That's the way I do it, anyway.

(But with some comments on when it won't work, mentioned above).