C#

string inputString = "The quick brown fox jumps over the lazy dog";
char[] chars = new char[inputString.Length];
for (int i = 0; i < inputString.Length; i++)
{
    if (inputString[i] >= 'a' && inputString[i] < 'h') // is it between a and g (lower case only)
        chars[i] = (char)(inputString[i] + 1);         // yes, add 1 to make it the next letter
    else
        chars[i] = inputString[i];                     // no, copy direct to the output buffer
}
Console.WriteLine(chars);

private const string transA = "abcdefghijklmnopqurstuvwxyz";
private const string transB = "bcdefghijklmnopqurstuvwxyza";

char[] chars = new char[inputString.Length];
for (int i = 0; i < inputString.Length; i++)
    {
    int index = transA.IndexOf(inputString[i]);
    chars[i] = index >= 0 ? transB[index] : inputString[i];
    }
string outputString = new string(chars);

OriginalGriff wrote:

Richard's solution will work fine, but a better solution is to have two "translation arrays":

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]
"If you just follow the bacon Eddy, wherever it leads you, then you won't have to think about politics." -- Some Bell.

string ss = string.Join("", inputString.Select(ch => (char)(ch + 1)));

0 1 2 3 4 5 6 7 8 9
9 8 7 6 5 4 3 2 1 0

int offset = '9' + '0';

string ciphered = string.Join("",
    inputString.Select(ch =>
        (char)
        (
            Char.IsDigit(ch) ? offset - ch : ch + 1
        )
    )
);

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Z A B C D E F G H I J K L M N O P Q R S T U V W X Y

a b c d e f g h i j k l m n o p q r s t u v w x y z
b c d e f g h i j k l m n o p q r s t u v w x y z a

0 1 2 3 4 5 6 7 8 9
9 8 7 6 5 4 3 2 1 0

string: TEXTcsharp#2367 
result: SDWSdtibsq#7632

int offset = '9' + '0';
string test = string.Join("", normal.Select(ch => (ch >= 'a' && ch < 'z') ? (char)(ch + 1) : (ch > 'A' && ch <= 'Z') ? (char)(ch - 1) : (ch == 'z') ? (char)'a' : (ch == 'A') ? (char)'Z' : ((char)(Char.IsDigit(ch) ? offset - ch : ch + 1))));

int offset = '9' + '0';
string test = string.Join("", normal.Select(ch => (ch >= 'a' && ch < 'z') ? (char)(ch + 1) : (ch > 'A' && ch <= 'Z') ? (char)(ch - 1) : (ch == 'z') ? (char)'a' : (ch == 'A') ? (char)'Z' : ((char)(Char.IsDigit(ch) ? offset - ch : ch + 1))));

public string Encode(string source)
{
    StringBuilder sb = new StringBuilder();

    int offset = '9' + '0';

    foreach (char ch in source)
    {
        if (Char.IsDigit(ch))
        {
            sb.Append((char) (offset - ch));
            continue;
        }

        if (Char.IsUpper(ch))
        {
            if (ch == 'Z')
            {
                sb.Append('Y');
            }
            else
            {
                sb.Append((char) (ch - 1));
            }

            continue;
        }

        if (ch == 'z')
        {
            sb.Append('a');
        }
        else
        {
            sb.Append((char) (ch + 1));
        }
    }

    return sb.ToString();
}

int offset = '9' + '0';

public char EncodeChar(char ch)
{
    return ch switch
    {
        Char c when Char.IsDigit(ch) =>
            ((char) (offset - ch)),

        Char c when Char.IsUpper(ch) =>
            ch == 'Z' ? 'Y' : ((char)(ch - 1)),

        Char c =>
            ch == 'z' ? 'a' : ((char) (ch + 1))
    };
}

public string Encode3(string source)
{
    StringBuilder sb = new StringBuilder();

    foreach (char ch in source)
    {
        sb.Append(EncodeChar(ch));
    }

    return sb.ToString();
}

private const string transA = "abcdefghijklmnopqurstuvwxyz0123456789";
private const string transB = "bcdefghijklmnopqurstuvwxyza9876543210";