StreamReader 读取文本文件乱码问题

public System.Text.Encoding GetFileEncodeType(string filename)
        {
            System.IO.FileStream fs = new System.IO.FileStream(filename, System.IO.FileMode.Open, System.IO.FileAccess.Read);
            System.IO.BinaryReader br = new System.IO.BinaryReader(fs);
            Byte[] buffer = br.ReadBytes(2);
            if (buffer[0] >= 0xEF)
            {
                if (buffer[0] == 0xEF && buffer[1] == 0xBB)
                {
                    return System.Text.Encoding.UTF8;
                }
                else if (buffer[0] == 0xFE && buffer[1] == 0xFF)
                {
                    return System.Text.Encoding.BigEndianUnicode;
                }
                else if (buffer[0] == 0xFF && buffer[1] == 0xFE)
                {
                    return System.Text.Encoding.Unicode;
                }
                else
                {
                    return System.Text.Encoding.Default;
                }
            }
            if (buffer[0] == 0x3c)//utf-8无bom格式
            {
                return System.Text.Encoding.UTF8;
            }
            else
            {
                return System.Text.Encoding.Default;
            }

        }