Question

A Refresh button/method in an Excel Add-In needs to be invoked via an external winform application. Here is where I am up to:

private Microsoft.Office.Interop.Excel.Application excel = new Microsoft.Office.Interop.Excel.Application();
public FormMain()
{
InitializeComponent();
RefreshExcelSheet(@C:\a.xls");
} 

private bool RefreshExcelSheet(string path)
        {
           using (var wb = excel.Workbooks.Open(path).WithComCleanup())

How do I click the Refresh button or simply invoke its event?

I was looking at these articles but they are using VBA, I want a Winform app to open the spreadsheets and click the button:
Accessing a VSTO application-addin types from VBA (Excel)
Expose VSTO functionality to VBA w/o local admin

Answer 1

The simplest way to solve this problem is to just expose the existing custom .net method as a COM method by making is a com callable wrapper object (CCW).
VSTO provides you with a simple method to expose the COM Automation server.

http://blogs.msdn.com/b/andreww/archive/2007/01/15/vsto-add-ins-comaddins-and-requestcomaddinautomationservice.aspx

EDIT BY OP:
Andrew Whitechapel has updated the article, but I cannot get it to work. Even with StandardOleMarshalObject and Register for COM interop I still get the same error message as detailed in this article: http://blogs.msdn.com/b/andreww/archive/2008/08/11/why-your-comaddin-object-should-derive-from-standardolemarshalobject.asp