using System;

using System.Text;

using System.Collections.Generic;           

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var s = RandomString(8);

        Console.WriteLine(s);

    }

    public static string RandomString(int length, string allowedChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789") {

​

    const int byteSize = 0x100;

    var allowedCharSet = new HashSet<char>(allowedChars).ToArray();

    if (byteSize < allowedCharSet.Length) throw new ArgumentException(String.Format("allowedChars may contain no more than {0} characters.", byteSize));

​

    using (var rng = System.Security.Cryptography.RandomNumberGenerator.Create()) {

        var result = new StringBuilder();

        var buf = new byte[128];

        while (result.Length < length) {

            rng.GetBytes(buf);

            for (var i = 0; i < buf.Length && result.Length < length; ++i) {

                var outOfRangeStart = byteSize - (byteSize % allowedCharSet.Length);

                if (outOfRangeStart <= buf[i]) continue;

                result.Append(allowedCharSet[buf[i] % allowedCharSet.Length]);

            }

        }

        return result.ToString();

    }

}

}