Trying to exit C# Excel Workbook without a dialog box

Try setting the Application.DisplayAlerts property to false. You may find it useful to set this property to false for the majority of your automation routines. Remember to restore the previous value before returning.

Application applicationInstance = ...;
var oldDisplayAlertsValue = applicationInstance.DisplayAlerts;
applicationInstance.DisplayAlerts = false;
try
{
    outputExcelWorkBook.Close(false, Missing.Value, Missing.Value);
}
finally
{
    appliationInstance.DisplayAlerts = oldDisplayAlertsValue;
}

This worked for me:

1. Initiate Excel

2. Open the workbook

3. Get the active sheet and make an edit (added "Text" to cell [2,2])

4. Close the workbook with a single parameter of true which means "save changes"

5. No dialog box is displayed.

Note: When I call Close without a parameter I am prompted to save changes.

    Microsoft.Office.Interop.Excel.Application excel = new Application();
    Microsoft.Office.Interop.Excel.Workbook workBook =
        excel.Workbooks.Open(fileLocation);
    Microsoft.Office.Interop.Excel.Worksheet sheet = workBook.ActiveSheet;
    sheet.Cells[2, 2] = "Text";
    workBook.Close(true);
    excel.Quit();