This is all in C# .NET Excel Interop Automation for Office 2007.

Say you create two excel apps and open the same workbook for each application:

app = new Excel.ApplicationClass();
app2 = new Excel.ApplicationClass();

string fileLocation = "myBook.xslx";

workbook = app.Workbooks.Open(fileLocation,
            Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing,
             Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing,
            Type.Missing, Type.Missing, Type.Missing, Type.Missing);

workbook2 = app2.Workbooks.Open(fileLocation,
            Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing,
             Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing,
            Type.Missing, Type.Missing, Type.Missing, Type.Missing);

Now, I want to replicate any changes that occur in workbook2, into workbook. I figured out I can hook up the SheetChanged event to capture cell changes:

app.SheetChange += new Microsoft.Office.Interop.Excel.AppEvents_SheetChangeEventHandler(app_SheetChange);

void app_SheetChange(object Sh, Microsoft.Office.Interop.Excel.Range Target)
{
     Excel.Worksheet sheetReadOnly = (Excel.Worksheet)Sh;

     string changedRange = Target.get_Address(missing, missing,
                Excel.XlReferenceStyle.xlA1, missing, missing);

     Console.WriteLine("The value of " + sheetReadOnly.Name + ":" +
                changedRange + " was changed to = " + Target.Value2);

     Excel.Worksheet sheet = workbook.Worksheets[sheetReadOnly.Index] as Excel.Worksheet;

     Excel.Range range = sheet.get_Range(changedRange, missing);

     range.Value2 = Target.Value2;
 }

How do you capture calculate changes? I can hook onto the calculate event but the only thing that is passed is the sheet, not the cells that were updated. I tried forcing an app.Calculate() or app.CalculateFullRebuild() but nothing updates in the other application. The change event does not get fired when formulas change (i.e. a slider control causes a SheetCalculate event and not a SheetChange event)

Is there a way to see what formulas were updated? Or is there an easier way to sync two workbooks programmatically in real time?