unit Unit2;

interface

uses
  SysUtils, Dialogs, Generics.Collections;

type
  TMainClass = class
    A: integer;
    B: integer;
  end;

  TAddClass1 = class(TMainClass)
    function Calc: integer;
    Constructor Create;
  end;

  TAddClass2 = class(TMainClass)
    function Calc: integer;
    Constructor Create;
  end;

var
  List: TList<TMainClass>;

implementation

Constructor TAddClass1.Create;
begin
  A := 1;
  B := 2;
end;

function TAddClass1.Calc: integer;
begin
  Result := A + B;
end;

Constructor TAddClass2.Create;
begin
  A := 1;
  B := 2;
end;

function TAddClass2.Calc: integer;
begin
  Result := A * B;
end;

begin
  List := TList<TMainClass>.Create;
  List.Add(TAddClass1.Create);
  List.Add(TAddClass2.Create);
  List[0].Calc;
end.