using System;

using System.Collections.Generic;

using System.Xml.Serialization;

using System.IO;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var data = new Type

        {

                SLOW = new List<Slow>

                {

                    new Slow{Value = 1},

                    new Slow{Value = 2},

                    new Slow{Value = 3},

                    new Slow{Value = 4},

                },

                        FAST = new List<Fast>

                {

                    new Fast{Value = 1},

                    new Fast{Value = 2},

                    new Fast{Value = 3},

                }

        };

​

        XmlSerializer xmlSerializer = new XmlSerializer(data.GetType());

​

        using (StringWriter textWriter = new StringWriter())

        {

           xmlSerializer.Serialize(textWriter, data);

           var result = textWriter.ToString();

           result.Dump();

        }

    }

​

public class Type

    {

        private List<Slow> slowField;

​

        private List<Fast> FastField;

​

        [XmlElement("SLOW", Type = typeof(Slow))]

        [XmlElement("FAST", Type = typeof(Fast))]

        public List<Base> Complete => GetMergedList();

​

        private List<Base> GetMergedList()

        {

            int minLen = Math.Min(SLOW.Count, FAST.Count);

​

            var list = SLOW.Take(minLen)

            .Zip(FAST.Take(minLen), (a, b) => new Base[] { a, b })

            .SelectMany(array => array)

            .Concat(SLOW.Skip(minLen))

            .Concat(FAST.Skip(minLen));

            return list.ToList();

        }

        /// <remarks/>

        [XmlIgnore]

        public List<Slow> SLOW

        {

            get

            {

                return this.slowField;

            }

            set

            {

                this.slowField = value;

            }

        }

​

        /// <remarks/>

        //[System.Xml.Serialization.XmlElementAttribute("FAST", Order = 2)]

        [XmlIgnore]

        public List<Fast> FAST

        {

            get

            {

                return this.FastField;

            }

            set

            {

                this.FastField = value;

            }

        }

    }

​

​

​

    public class Base { }

​

    public class Fast : Base

    {

        public int Value { get; set; }

    }

​

    public class Slow : Base

    {

        public int Value { get; set; }

    }

}

​

​