using System.Reflection; // For Missing.Value and BindingFlags
using System.Runtime.InteropServices; // For COMException
using Excel;

....................................................................................

private Excel.ApplicationClass exelApp = null;
private object objMissing = System.Reflection.Missing.Value;

....................................................................................
System.Windows.Forms.OpenFileDialog objExcelFileDialog;
objExcelFileDialog = new System.Windows.Forms.OpenFileDialog();
objExcelFileDialog.Multiselect = false;
objExcelFileDialog.Title =  "Выберете файл для конвертирования данных" ;
objExcelFileDialog.Filter =  "Таблицы Excel (*.xls)|*.xls" ;
if(objExcelFileDialog.ShowDialog() == DialogResult.OK)
{

	exelApp = new ApplicationClass();
	exelApp.Visible = true;
	Workbooks workbooks = exelApp.Workbooks;

	_Workbook workbook = workbooks.Open(objExcelFileDialog.FileName,objMissing,false,objMissing,objMissing,objMissing,objMissing,objMissing,objMissing,objMissing,objMissing,objMissing,objMissing,objMissing,objMissing);
	Sheets sheets = workbook.Worksheets;
	_Worksheet worksheet = (_Worksheet) sheets.get_Item( 1 );

	Range r1 = worksheet.get_Range( "A10" , Missing.Value);
	r1.CurrentRegion.Select();
	//Далее какая-то обработка.....................................
}