C# casting to parent still running child methods

<com.example.ActivityMonitorItemView
  android:layout_width="wrap_content"
  android:layout_height="wrap_conetent"
  android:id="@+id/tester"/>

def demo
  @demo
  self
end

def demo
  @group = @demo
  self
end

report.live.add_money_to_gross(money)

report.add_money_to_gross(money)

class ReportItemArray < Array
  def add_money_to_net money
    item = find_item(money)
    if item
      item.net += money
    else
      self << ReportItem.new(currency: money.currency.to_s, net: money)
    end
  end
  def add_money_to_gross money
    item = find_item(money)
    if item
      item.gross += money
    else
      self << ReportItem.new(currency: money.currency.to_s, gross: money)
    end
  end
  def find_item money
    self.find {|s| s.currency == money.currency }
  end
end

class Report
  def initialize
    @demo = ReportItemArray.new
    @live = ReportItemArray.new
  end
  def live
    @live
  end
  def demo
    @demo
  end
end

report.live.add_money_to_gross(money)

int main()
{
    //I know for a fact that the item is a child
    std::vector<Parent> items;
    items.push_back(Child()); // <<<< You are copying and slicing here

    //Static cast to use child specific function doesn't work
    Child* childPtr = static_cast<Child*>(items[0]);
    childPtr->childSpecificFunction(); // <<<<< undefined behavior

    //Gives intellisence error: no suitable conversion from "Parent" to "Child *" exists
}

int main()
{
    std::vector<std::unique_ptr<Parent>> items;
    items.push_back(std::make_unique<Child>());  // <<< working with pointers,
                                                 //     no slicing.

    Child* childPtr = static_cast<Child*>(items[0].get());
    childPtr->childSpecificFunction();
 }

public interface IRepository<out T>{ ... }