Hi, refer the following code,
AcadApplication Acad = (AcadApplication)Marshal.GetActiveObject("AutoCAD.Application");
string ActiveDocumentName = Acad.ActiveDocument.Name;
string ActiveDocumentPath = Acad.ActiveDocument.FullName;
and refer the following article,
Quick Start: AutoCAD 2D Drawing using C# with AutoCAD COM API[
^]