c# - Generics List<List> -

If I have list gt; is the type of MyClass , for example list & lt; List & lt; MyClass & gt; & Gt; and MyClass to MyClassB is the original class. Why can not I do the following?

  list & lt; List & lt; MyClass & gt; & Gt; All List = New List & lt; List & lt; MyClass & gt; & Gt; (); & Lt; MyClassB & gt; MyList = new list & lt; MyClassB & gt; (); MyList.Add (New MyClassB ()); // And now the point all lists do not work. Add (my list);   
 If I apply a method, then I can say  SomeClass & lt; T & gt; ... where t: myclass , is there anything like my list problem?  
 So that I will add any child category list to my first-level list?   
 
 
 The reason for this is not that you can do this: think  allLists.Add (MyList)  then the compiler will know that  all the list [0]  a  list , so the following would be OK:  
  all the listings [0]. Add (New MyClassX ());   
 This will be a runtime error, because  all the list [0]  is actually a  list . It is not capable of holding  MyClassX  objects  
 If you change your code, then  myList  a  list , your code will work:  
  list & lt; MyClass & gt; MyList = new list & lt; MyClass & gt; (); MyList.Add (New MyClassB ()); // This works because MyClassB expands MyClass allLists. Add (myList); // This also works,