When you want to extend another class in Fragment,

public class Main_Title_Fragment : Fragment
{
    ImageButton _Back_Button, _Exit_Button;
    TextView _Title_Text;
    FragmentTransaction _Main_Menu_Fragment;
    FragmentManager _Back_Stack;


    public override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        _Main_Menu_Fragment = ChildFragmentManager.BeginTransaction();
        Main_Menu_Fragment _Setting = new Main_Menu_Fragment();
        _Main_Menu_Fragment.Replace(Resource.Id._Adapter_Content, _Setting);
        _Main_Menu_Fragment.Commit();
        // Fragment that opens by default when running the screen
    }

    public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
    }