JIS文字からSJIS文字への変換(全角文字のみ対象)

Jis2Sjis.cs

public static byte[] Jis2Sjis(byte[] targetByte)
{
    if ((targetByte.Length % 2) != 0)
    {
        throw new Exception("2で割り切れません。");
    }

    List<byte[]> jisList  = new List<byte[]>();
    List<byte>   sjisList = new List<byte>();
    for (int i = 0; i < targetByte.Length; i = i + 2)
    {
        jisList.Add(new byte[] { targetByte[i], targetByte[i + 1] });
    }

    foreach (byte[] changeByte in jisList)
    {
        // 全角文字エリア
        if ((changeByte[0] >= 0x21 && changeByte[0] <= 0x7E) &&
            (changeByte[1] >= 0x21 && changeByte[1] <= 0x7E))
        {
            int firstByte = (int)changeByte[0];
            int i0x21 = (int)0x21;
            int i0x5e = (int)0x5e;
            int i0x81 = (int)0x81;
            int i0xc1 = (int)0xc1;
            int firstResult = ((firstByte - i0x21) / 2) + (firstByte <= i0x5e ? i0x81 : i0xc1);

            int secoundByte = (int)changeByte[1];
            int i0x5f = (int)0x5f;
            int i0x1f = (int)0x1f;
            int i0x20 = (int)0x20;
            int i0x7e = (int)0x7e;
            int secondResult = 0;

            if ((firstByte & 1) >= 1)
            {
                secondResult = secoundByte + ((secoundByte <= i0x5f) ? i0x1f : i0x20);
            }
            else
            {
                secondResult = secoundByte + i0x7e;
            }

            byte sjis01 = (byte)firstResult;
            byte sjis02 = (byte)secondResult;

            sjisList.Add(sjis01);
            sjisList.Add(sjis02);
        }
        // その他
        else
        {
            sjisList.AddRange(changeByte);
        }
    }

    return sjisList.ToArray();
}