C#

private ApplicationClass app = null;
private Workbook book = null;
private Worksheet sheet = null;

...

private void openExcel(string fileName)
{
	if (app != null)
		closeExcel();
	app = new ApplicationClass();

	app.Visible = false;
	app.ScreenUpdating = false;
	app.DisplayAlerts = false;
	book = app.Workbooks.Open(fileName, Missing.Value, Missing.Value, Missing.Value,
									   Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value,
									   Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value,
									   Missing.Value);
	sheet = (Worksheet)book.Sheets[1];
}

void closeExcel()
{
	if (sheet != null)
	{
		System.Runtime.InteropServices.Marshal.ReleaseComObject(sheet);
		sheet = null;
	}

	if (book != null)
	{
		book.Close(false, Missing.Value, Missing.Value);
		System.Runtime.InteropServices.Marshal.ReleaseComObject(book);
		book = null;
	}
	

	if (app != null)
	{
		app.Quit();
		System.Runtime.InteropServices.Marshal.ReleaseComObject(app);
	}
	app = null;

	GC.Collect();
}