ASP.net Identity admin reset user password
One thing you should keep in mind is that when the Add fails, your user is left without a password as it's already been removed. When will this fail? Usually when your password does not pass the validation.

Of course, seeing how an admin is doing it, there's probably not so much of a problem as they can simply add a different one - as long as the message is clear enough to show there actually is a problem with adding the new password.

The way Microsoft wants you to tackle this is by using a PasswordResetToken.

var code = await _userManager.GeneratePasswordResetTokenAsync(user);
await _userManager.ResetPasswordAsync(user, code, newPassword);


For this you'll have to fiddle around a bit with the token generators that Identity provides.