trying to get currently open workbook path from excel. MyExcelAppObject.Application.ActiveWorkbook sometimes returns null

Question

I am trying to get currently open workbook name (path) from excel application. The method looks like this:

        var ExcelObj = (Microsoft.Office.Interop.Excel.Application)System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application");
        return ExcelObj.Application.ActiveWorkbook.FullName;

very often (almost 50% of attempts) it throws nullreferenceexception because ExcelObj.Application.ActiveWorkbook is null but sometimes it returns right value. I couldn't figure out what is the problem. Why does it work sometimes but sometimes doesn't.