public static str randomAlphaNumericStr(int _size)
    {
        System.String sourceStr = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
        System.Char[] chars = sourceStr.ToCharArray();

        System.Byte[] data = new System.Byte[4 * _size]();
        using (RNGCryptoServiceProvider crypto = new RNGCryptoServiceProvider())
        {
            crypto.GetBytes(data);
        }
        System.Text.StringBuilder result = new System.Text.StringBuilder(_size);
        for (int i = 0; i < _size; i++)
        {            
            var rnd = System.BitConverter::ToUInt32(data, i * 4);
            var idx = rnd mod chars.Length;

            result.Append(chars.GetValue(idx));
        }
        return result.ToString();
    }