How to create this List<>?

List<baseType> myList = new List<baseType>();
myList.Add(someChild1);
myList.Add(someChild2);
 
foreach(myClass my in myClassList)
{
foreach(baseType b in myList)
{
  b.DoSomething(common1, common2, my.different1);
  b.DoSomething(common1, common2, my.different2);
  b.DoSomething(common1, common2, my.different3);
}
}
 
//////
abstract class baseType<T>
{
 public abstract void DoSomething(common1, common2, List<T>);
}
 
//////
class someChild1 : baseType<different1>
{
 public override void DoSomething(common1, common2, different1){...}
}
 
class someChild2 : baseType<different2>
{
 public override void DoSomething(common1, common2, different2){...}
}