var excel = new Microsoft.Office.Interop.Excel.Application();
excel.Workbooks.Open(filePath);
excel.Run("RefreshAllStaticData");

var excel = new Microsoft.Office.Interop.Excel.Application();
var addIn = ex.AddIns.Add(filename);
addIn.Installed = true;
Console.WriteLine(addIn.IsOpen);

//From http://blogs.officezealot.com/whitechapel/archive/2005/04/10/4514.aspx
public class ExcelInteropService
{
    private const string EXCEL_CLASS_NAME = "EXCEL7";

    private const uint DW_OBJECTID = 0xFFFFFFF0;

    private static Guid rrid = new Guid("{00020400-0000-0000-C000-000000000046}");

    public delegate bool EnumChildCallback(int hwnd, ref int lParam);

    [DllImport("Oleacc.dll")]
    public static extern int AccessibleObjectFromWindow(int hwnd, uint dwObjectID, byte[] riid, ref Microsoft.Office.Interop.Excel.Window ptr);

    [DllImport("User32.dll")]
    public static extern bool EnumChildWindows(int hWndParent, EnumChildCallback lpEnumFunc, ref int lParam);

    [DllImport("User32.dll")]
    public static extern int GetClassName(int hWnd, StringBuilder lpClassName, int nMaxCount);

    public static Microsoft.Office.Interop.Excel.Application GetExcelInterop(int? processId = null)
    {
        var p = processId.HasValue ? Process.GetProcessById(processId.Value) : Process.Start("excel.exe");
        try
        {
            return new ExcelInteropService().SearchExcelInterop(p);
        }
        catch (Exception)
        {
            Debug.Assert(p != null, "p != null");
            return GetExcelInterop(p.Id);
        }
    }

    private bool EnumChildFunc(int hwndChild, ref int lParam)
    {
        var buf = new StringBuilder(128);
        GetClassName(hwndChild, buf, 128);
        if (buf.ToString() == EXCEL_CLASS_NAME) { lParam = hwndChild; return false; }
        return true;
    }

    private Microsoft.Office.Interop.Excel.Application SearchExcelInterop(Process p)
    {
        Microsoft.Office.Interop.Excel.Window ptr = null;
        int hwnd = 0;

        int hWndParent = (int)p.MainWindowHandle;
        if (hWndParent == 0) throw new ExcelMainWindowNotFoundException();

        EnumChildWindows(hWndParent, EnumChildFunc, ref hwnd);
        if (hwnd == 0) throw new ExcelChildWindowNotFoundException();

        int hr = AccessibleObjectFromWindow(hwnd, DW_OBJECTID, rrid.ToByteArray(), ref ptr);
        if (hr < 0) throw new AccessibleObjectNotFoundException();

        return ptr.Application;
    }
}

var excel = ExcelInteropService.GetExcelInterop();
excel.Workbooks.Open(filename);
excel.Run("RefreshAllStaticData");
// works "like a charm"