dic.cs

public enum SkillEffectType {
    ChangeCharacterHP,
    ChangeAllEnemyHP,
    //...
}

public partial class SkillEffect {

    private static Dictionary<SkillEffectType, Type> skill_types = new Dictionary<SkillEffectType, Type> {
        { SkillEffectType.ChangeCharacterHP, typeof(SkillEffect.ChangeCharacterHP) },
        { SkillEffectType.ChangeAllEnemyHP, typeof(SkillEffect.ChangeAllEnemyHP) },
    };
    
    public static SkillEffect build(SkillEffectType type, Dictionary<string, object> skilleffect) {
        return (SkillEffect)Activator.CreateInstance(SkillEffect.skill_types[type], new object[] { skilleffect });
    }
}

switch.cs

public enum SkillEffectType {
    ChangeCharacterHP,
    ChangeAllEnemyHP,
    //...
}


public partial class SkillEffect {
    public static SkillEffect build(SkillEffectType type, Dictionary<string, object> skilleffect) {
        switch (type) {
            case SkillEffectType.ChangeCharacterHP: return new SkillEffect.ChangeCharacterHP(skilleffect);
            case SkillEffectType.ChangeAllEnemyHP: return new SkillEffect.ChangeAllEnemyHP(skilleffect);
            //...
            default: throw new InvalidOperationException("unknown skill type");
        }
    }
}