How to add existing Excel add-in in VSTO

Question

I'm working on a VSTO add-in. I don't want to write UDFs in it, although i know how to write it following COM stuff, or using ExcelDNA. Instead, i just want to include one of the existing xlam add-ins, which contains loads of UDFs. So when people install the VSTO add-in, i also want the xlam add-in to be installed. so i've written this code in ThisAddin_startup event.

        bool isExist=false;
        foreach (Excel.AddIn a in Application.AddIns)
        {                
            if (a.Name=="foobar.xlam") 
            {      
                if (!a.Installed)
                    a.Installed = true;
                isExist = true;
                break;
            }                
        }

        if (isExist==false)    
           this.Application.AddIns.Add(pathtotheaddin).Installed = true; //it is inside the resources folder
    

But i keep getting "Add method of AddIns class failed" error. What am i doing wrong?

Answer 1

I've found it finally. It's because there are no workbooks open yet, so it can't install any add-ins. You know that when there is no workbook open yet, add-ins button is disabled.