Random string with no duplicates

Question

I'm trying to generate a 16 chars random string with NO DUPLICATE CHARS. I thoght that it shouldn't be to hard but I'm stuck.

I'm using 2 methods, one to generate key and another to remove duplicate chars. In main I've created a while loop to make sure that generated string is 16 chars long.

There is something wrong with my logic because it just shoots up 16-char string with duplicates. Just can't get it right.

The code:

public string RemoveDuplicates(string s)
{
    string newString = string.Empty;
    List<char> found = new List<char>();
    foreach (char c in s)
    {
        if (found.Contains(c))
            continue;

        newString += c.ToString();
        found.Add(c);
    }
    return newString;
}

public static string GetUniqueKey(int maxSize)
{
    char[] chars = new char[62];
    chars =
    "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
    byte[] data = new byte[1];
    RNGCryptoServiceProvider crypto = new RNGCryptoServiceProvider();
    crypto.GetNonZeroBytes(data);
    data = new byte[maxSize];
    crypto.GetNonZeroBytes(data);
    StringBuilder result = new StringBuilder(maxSize);
    foreach (byte b in data)
    {
        result.Append(chars[b % (chars.Length)]);

    }
    return result.ToString();
}

string builder = "";

do
{                       

    builder = GetUniqueKey(16);
    RemoveDuplicates(builder);

    lblDir.Text = builder;
    Application.DoEvents();


} while (builder.Length != 16);

Rhodos

Answer 1

Consider implementing shuffle algorithm with which you will shuffle your string with unique characters and then just pick up first 16 characters.

You can do this in-place, by allocating single StringBuffer which will contain your initial data ("abc....") and just use Durstenfeld's version of the algorithm to mutate your buffer, than return first 16 chars.

Hemlock

Thank You! So simple and elegant. Just what I wanted. – Rhodos

Answer 2

There are many algorithms for this.

One easy one is:

1. Fill an array of chars with the available chars.
2. Shuffle the array.
3. Take the first N items (where N is the number of characters you need).

Sample code:

using System;

namespace ConsoleApplication2
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
            Random rng = new Random();

            for (int i = 0; i < 10; ++i)
            {
                string randomString = RandomString(16, chars, rng);
                Console.WriteLine(randomString);
            }
        }

        public static string RandomString(int n, char[] chars, Random rng)
        {
            Shuffle(chars, rng);
            return new string(chars, 0, n);
        }

        public static void Shuffle(char[] array, Random rng)
        {
            for (int n = array.Length; n > 1; )
            {
                int k = rng.Next(n);
                --n;
                char temp = array[n];
                array[n] = array[k];
                array[k] = temp;
            }
        }
    }
}

Justiciary

Answer 3

const string chars = 
               "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
var r = new Random();
var s = new string(chars.OrderBy(x => r.Next()).Take(16).ToArray());

Lauralauraceous

Answer 4

I am using a GUID generation method it itself generates random strings and you can modify it if a number appears in the beginning, use the code given below:

string guid = System.Guid.NewGuid().ToString("N");
            while (char.IsDigit(guid[0]))
                guid = System.Guid.NewGuid().ToString("N");

Hope that helps.

Sofko

Guids are not random. They may be unique, but they are not random. There is nothing in the .NET spec that says guids are anything but unique. – Vinyl

Answer 5

See if this helps:

    RandomString()
    {
        string randomStr = Guid.NewGuid().ToString();
        randomStr = randomStr.Replace("-", "").Substring(0, 16);   
        Console.WriteLine(randomStr);
    }

This returns alpha-numeric string.

Calebcaledonia