RandomStringHelper.cs

public static class RandomStringHelper
{
    public static string RandomString(int length)
    {
        const string valid = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        var res = new StringBuilder();
        using (var rng = new RNGCryptoServiceProvider())
        {
            byte[] uintBuffer = new byte[sizeof(uint)];

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

        return res.ToString();
    }
}