gistfile1.cs

Application xlApp;
Workbooks xlBooks;
Workbook xlBook;
Worksheet xlSheet;

public IndexUpdaterExcelService()
{
    try
    {
        try
        {
            xlApp = (Application)Marshal.GetActiveObject("Excel.Application");
        }
        catch
        {
            xlApp = new Application();
        }

        xlApp.DisplayAlerts = false;
        xlBooks = xlApp.Workbooks;

        xlBook = xlBooks.Count > 0
            ? xlBooks[1]
            : xlBooks.Open(_fileNameAndPath);

        xlSheet = (Worksheet)xlBook.Worksheets[1];

        xlApp.Visible = true;
    }
    catch (Exception e)
    {
        Log.Error(e.Message);
        Log.Error(@"Unable to Connect to '{0}' Please open the file and restart this app.".FormatWith(_fileNameAndPath));
    }
}