long m_nBorderStyle;
Remarks
The data member in your control class that stores the control's border style. If you choose the BORDERSTYLE option from the Stock Properties tab in the ATL Object Wizard, the wizard automatically creates this data member in your control class, creates put and get methods for the property, and supports IPropertyNotifySink to automatically notify the container when the property changes.
CComControl Overview | Class Members
See Also CComControl::m_bBorderVisible, CComControl::m_clrBorderColor, CComControl::m_nBorderWidth