ＳJIS文字からJIS文字への変換(全角文字のみ対象)

Sjis2Jis.cs

public static byte[] Sjis2Jis(byte[] targetByte)
{
    if ((targetByte.Length % 2) != 0)
    {
        throw new Exception("2で割り切れません。");
    }

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

    foreach (byte[] changeByte in sjisList)
    {
        // 全角文字エリア
        if (((changeByte[0] >= 0x81 && changeByte[0] <= 0x9F)  ||
             (changeByte[0] >= 0xE0 && changeByte[0] <= 0xEF)) &&
             (changeByte[1] != 0x00 && changeByte[1] != 0xff))
        {
            int firstByte = (int)changeByte[0];
            int firstResult = 0;
            int i0xc1 = (int)0xC1; // 193
            int i0x81 = (int)0x81; // 129
            int i0x21 = (int)0x21; // 33
            int i0x9f = (int)0x9F; // 159
            int i0xff = (int)0xff; // 255
            int i0x01 = (int)0x01; // 1
            int i0x1E = (int)0x1E; // 30

            int secoundByte = (int)changeByte[1];
            int secondResult = 0;
            int i0x9e = (int)0x9E; // 158
            int i0x1f = (int)0x1F; // 31
            int i0x60 = (int)0x60; // 96
            int i0x20 = (int)0x20; // 32
            int i0x7e = (int)0x7E; // 126

            if (firstByte >= i0xc1)
            {
                firstResult = firstByte - i0xc1;
                firstResult = (firstResult * 2) + i0x21;
            }
            else
            {
                firstResult = firstByte - i0x81;
                firstResult = (firstResult * 2) + i0x21;
            }

            if (secoundByte >= i0x9f && secoundByte <= i0xff)
            {
                firstResult += 1;
            }

            if (secoundByte >= i0x01 && secoundByte <= i0x1E)
            {
                firstResult = i0xff;
            }

            if (secoundByte <= i0x9e)
            {
                if (secoundByte - i0x1f > i0x60)
                {
                    secondResult = secoundByte - i0x20;
                }
                else
                {
                    secondResult = secoundByte - i0x1f;
                }
            }
            else
            {
                secondResult = secoundByte - i0x7e;
            }

            byte jis01 = (byte)firstResult;
            byte jis02 = (byte)secondResult;

            jisList.Add(jis01);
            jisList.Add(jis02);
        }
        // その他
        else
        {
            jisList.AddRange(changeByte);
        }
    }

    return jisList.ToArray();
}