几种方式

using Excel = Microsoft.Office.Interop.Excel;

Excel.ApplicationClass excel = new Excel.ApplicationClass();
Excel.Application app = excel.Application;
Excel.Range all = app.get_Range("A1:H10", Type.Missing);

要么

Excel.Range last = sheet.Cells.SpecialCells(Excel.XlCellType.xlCellTypeLastCell, Type.Missing);
Excel.Range range = sheet.get_Range("A1", last);

int lastUsedRow = last.Row;
int lastUsedColumn = last.Column;