SecureTokenGenerator

main.cs

using System;
using System.Linq;

using (var generator = new SecureTokenGenerator(TokenAlphabet.All, 30))
    foreach (string token in generator.Tokens.Take(40))
        Console.WriteLine(token);

SecureTokenGenerator.cs

using System;
using System.Collections.Generic;

public class SecureTokenGenerator : IDisposable
{
    private bool disposed = false;
    private readonly CryptoRandom random;
    private readonly string alphabet;
    private readonly int size;

    private const int DefaultSize = 16;
    private static readonly string DefaultAlphabet = TokenAlphabet.AsciiLetters + TokenAlphabet.Digits;

    internal SecureTokenGenerator(CryptoRandom random, string alphabet, int size)
    {
        this.random = random;
        this.alphabet = alphabet;
        this.size = size;
    }

    public SecureTokenGenerator(string alphabet, int size) : this(new CryptoRandom(), alphabet, size)
    {
    }

    public SecureTokenGenerator(string alphabet) : this(alphabet, DefaultSize)
    {
    }

    public SecureTokenGenerator(int size) : this(DefaultAlphabet, size)
    {
    }

    public SecureTokenGenerator() : this(DefaultSize)
    {
    }

    public string GetNextToken()
    {
        char[] chars = new char[this.size];

        for (int i = 0; i < this.size; i++)
        {
            var index = this.random.Next(this.alphabet.Length);
            chars[i] = this.alphabet[index];
        }

        return new string(chars);
    }

    public IEnumerable<string> Tokens
    {
        get
        {
            while (true)
                yield return this.GetNextToken();
        }
    }

    protected virtual void Dispose(bool disposing)
    {
        if (disposed)
            return;

        if (disposing)
            this.random.Dispose();

        this.disposed = true;
    }

    public void Dispose()
    {
        this.Dispose(true);
        GC.SuppressFinalize(this);
    }
}

TokenAlphabet.cs

public static class TokenAlphabet
{
    public const string AsciiLowercase = "abcdefghijklmnopqrstuvwxyz";
    public const string AsciiUppercase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    public const string Digits = "0123456789";
    public const string Symbols = "~!@#$%^&*()_+`-={}|[]\\:\"'<>?,./";
    public static readonly string AsciiLetters = AsciiLowercase + AsciiUppercase;
    public static readonly string All = AsciiLowercase + AsciiUppercase + Digits + Symbols;
}