What items i need to dispose in each activity?

What items i need to dispose in each activity for free memory?
I'm using recyclerview,listview,textview,edidtext and buttons.

Is it good practice to set them null on destroy method?

 private List<InventoryPreviewClass> mItems;
    private ListView mlistview;
    private EditText mSearch;
    private EditText etSearchAlwaysOn;
    private LinearLayout mContainer;
    private bool mAnimatedDown;
    string dpPath;
    private bool mIsAnimating;
    private MyListViewAdapterInventory mAdapter;

    SQLiteConnection db;
    private TextView mTxtHeaderFirstName;
    private TextView mTxtHeaderLastName;
    private bool mFirstNameAscending;
    private bool mLastNameAscending;
    List<String> ID;
    List<String> Name;
    Spinner CategorySpinner;
    protected override void OnDestroy()
    {
        base.OnDestroy();
        mItems = null;
        mlistview = null;
        mSearch = null;
        etSearchAlwaysOn = null;
        mContainer = null;

        dpPath = null;
        mAdapter = null;
        db = null;
        mTxtHeaderFirstName = null;
        mTxtHeaderLastName = null;
        ID = null;
        Name = null;
        CategorySpinner = null;


    }

Is there need to dispose them all? Or it will automatically dispose those items if there no in use?