保存文件的时候，如何添加BOM

保存文件的时候，怎么添加BOM

C# code

using (StreamWriter sw = new StreamWriter("abc.txt")){         sw.Write(str);          sw.Close();}

C# code

using (Stream stream = File.Open("E:\\abc.txt", FileMode.Create))using (StreamWriter sw = new StreamWriter(stream)){    stream.WriteByte(Convert.ToByte("EF", 16));    stream.WriteByte(Convert.ToByte("BB", 16));    stream.WriteByte(Convert.ToByte("BF", 16));    sw.Write(str);}