Поиск объекта класса в листе — C#(Си шарп)

class PassiveAbility
    {
 
    }
 
    class TestClass1 : PassiveAbility
    {
        public int a = 2;
    }
 
    class TestClass2 : PassiveAbility
    {
        public int c = 2;
    }
 
    class TestClass3 : PassiveAbility
    {
        public int d = 2;
    }
 
 
    class Output
    {
        static void Main()
        {
            
            List<PassiveAbility> PassiveList = new List<PassiveAbility>();
            PassiveList.Add(new TestClass1());
            PassiveList.Add(new TestClass2());
            PassiveList.Add(new TestClass3());
 
        }
    }

PassiveList.OfType<TestClass1>().First().a = 5;