So I did a very simple xls workbook:

public static class test
    {
        public static void excel()
        {
            String filename = @"c:\Test\test.xls";

            Workbook wb = new Workbook(WorkbookFormat.Excel97To2003);
            Worksheet ws = wb.Worksheets.Add("test");

            ws.Rows[0].Cells[0].Value = "testing";
            ws.Rows[0].Cells[1].Value = DateTime.Now;

            wb.Save(filename);
        }
    }

this worked.  So I am going to start commenting out parts of my code to figure if it is the creation of the rows and cells.  I attached the class, this used to work, I made no changes.

I'm glad that you found the problem :)

Horen Kirazyan

After stepping through code I found that my function to create the formula string for a cell was all messed up.  Fix the function and now saves.  Weird how it worked before, but if it wasn't for the save function failing I would have never found my bug.

Anyways, thanks for your help.  I apologize for the time you wasted.  It was not your control, but my code that was the issue.

Thanks