How to focus or select a cell in Excel

Question

How to focus a cell in Excel VSTO using C# or to select first cell using C# in VSTO?

Answer 1

 Excel.Application ExcelApp = (Excel.Application)Marshal.GetActiveObject("Excel.Application");
            Excel.Workbook book = ExcelApp.ActiveWorkbook;
            Excel.Worksheet sheet = book.ActiveSheet;
            Excel.Range ExcelRange = sheet.get_Range("A1");
            ExcelRange.Select();