using System;

using System.Linq;

using System.Collections.Generic;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(RandomString(50));

    }

    static string RandomString(int length, string allowedChars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")

    {

        if (length < 0) throw new ArgumentOutOfRangeException("length", "length cannot be less than zero.");

        if (string.IsNullOrEmpty(allowedChars)) throw new ArgumentException("allowedChars may not be empty.");

​

        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));

​

        // Guid.NewGuid and System.Random are not particularly random. By using a

        // cryptographically-secure random number generator, the caller is always

        // protected, regardless of use.

        using (var rng = new System.Security.Cryptography.RNGCryptoServiceProvider()) {

            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) {

                    // Divide the byte into allowedCharSet-sized groups. If the

                    // random value falls into the last group and the last group is

                    // too small to choose from the entire allowedCharSet, ignore

                    // the value in order to avoid biasing the result.

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

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

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

                }

            }

        return result.ToString();

    }

}

}