Generate 6 or 8 digit alpha numeric code using best performance in C#

Generating a 6 or 8-digit alphanumeric code in C# involves creating a string consisting of random letters and numbers. Here is a C# example using best practices and focusing on performance:

Use a StringBuilder for efficient string concatenation.
Use RandomNumberGenerator for cryptographic security.

Cache the character array to avoid creating it repeatedly.

using System;
using System.Security.Cryptography;
using System.Text;

public class AlphaNumericCodeGenerator
{
    private static readonly char[] chars =
        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789".ToCharArray();

    public static string GenerateCode(int length)
    {
        if (length != 6 && length != 8)
        {
            throw new ArgumentException("Code length must be either 6 or 8.", nameof(length));
        }

        using (var rng = RandomNumberGenerator.Create())
        {
            var bytes = new byte[length];
            rng.GetBytes(bytes);

            var result = new StringBuilder(length);
            foreach (var byteValue in bytes)
            {
                result.Append(chars[byteValue % chars.Length]);
            }

            return result.ToString();
        }
    }

    public static void Main()
    {
        string code6 = GenerateCode(6);
        Console.WriteLine($"6-digit code: {code6}");

        string code8 = GenerateCode(8);
        Console.WriteLine($"8-digit code: {code8}");
    }
}

Explanation:

Character Array: A cached array of characters (both upper and lower case letters and digits) to avoid creating it repeatedly.

Random Number Generator: RandomNumberGenerator is used for better randomness and security compared to Random.

StringBuilder: Used for efficient string concatenation.
Argument Validation: Ensures the length is either 6 or 8.

Random Bytes: Generates random bytes and maps each byte to a character in the chars array.

Modulo Operation: Maps each byte to a valid index in the chars array.

This code ensures the generated string is both secure and efficient.