如何在c#凯撒密码中使用for循环重复字母表

我正在制作凯撒密码，我想让字母在一个循环中，例如，如果字母'z'需要被移动，它应该回到'a'大写和小写。

//Array that holds each char in the plaintext inputed is declared and initiated
char[] chars = plainTextInput.ToCharArray();
//For loop that will go through each letter and change the value of each letter by adding the shiftAmount
for (int i = 0; i < plainTextInput.Length; ++i)
{   
    chars[i] = (char)(((int)chars[i]) + shiftAmount);
    if (chars[i] >= 97 && chars[i] <= 122)
    {
        if (chars[i] > 122)
        {
            int x = chars[i] - 123;
            chars[i] = (char)((int)(97 + x));
        }
    }
}  
//Variable for ciphertext output holds char array chars as a string
cipherTextOutput = new string(chars); 

如果我输入'xyz'并移动1，我得到'yz{'

new_pos = (current_pos + shift) % 26

if ('A' <= c && c <= 'Z')      // uppercase
{
    current_pos = (int) c - (int) 'A';
}
else if ('a' <= c && c <= 'z') // lowercase
{
    current_pos = (int) c - (int) 'a';
}