using System;

public class Program

{

    //from https://stackoverflow.com/questions/23246591/generate-unique-number-as-image-name/23249727#23249727

    public char GenerateChar(System.Random random)

    {

        char randomChar;

        int next = random.Next(0, 62);

        if (next < 10)

        {

            randomChar = (char) (next + '0'); //0, 1, 2 ... 9

        }

        else if (next < 36)

        {

            randomChar = (char) (next - 10 + 'A'); //A, B, C ... Z

        }

        else

        {

            randomChar = (char) (next - 36 + 'a'); //a, b, c ... z

        }

        return randomChar;

    }

​

    public string GenerateString(System.Random random, int length)

    {

        char[] randomStr = new char[length];

        for (int i = 0; i < length; i++)

        {

            randomStr[i] = GenerateChar(random);

        }

        return new string(randomStr);

    }

    public static void Main()

    {

        var random = new System.Random();

        var program = new Program();

        Console.WriteLine(program.GenerateString(random, 12));

        Console.WriteLine(program.GenerateString(random, 12));

        Console.WriteLine(program.GenerateString(random, 12));

        Console.WriteLine(program.GenerateString(random, 12));

        Console.WriteLine(program.GenerateString(random, 12));

        Console.WriteLine(program.GenerateString(random, 12));

    }

}