using RNGCryptoServiceProvider to generate random string

static string RandomString(int length)
{
    const string valid = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
    StringBuilder res = new StringBuilder();
    using (RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider())
    {
        byte[] uintBuffer = new byte[sizeof(uint)];

        while (length-- > 0)
        {
            rng.GetBytes(uintBuffer);
            uint num = BitConverter.ToUInt32(uintBuffer, 0);
            res.Append(valid[(int)(num % (uint)valid.Length)]);
        }
    }

    return res.ToString();
}