using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var customList = new TransformedProperty<int>();

        customList.Property = "1,2,3,4,5";

        Console.WriteLine("Current length of ID's: " + customList.ValueList.Count);

        customList.ValueList.Add( 10 );

        Console.WriteLine("Current property value: " + customList.Property);

    }

}

​

public class TransformedProperty<TValueType> {

    // a single list, will always stay available

    private readonly IList<TValueType> _list = new List<TValueType>();

    // a property to view the current items in a "string" form

    public string Property {

        get {

            // just return all the items, and join them with ","

            return string.Join(",", ValueList);

        }

        set {

            // upon setting, clear the list

            ValueList.Clear();

            if (string.IsNullOrWhiteSpace(value)) {

                // no values -> can exist

                return;

            }

            // wouldn't use it in production code, but for the demo part, seems fine

            // split all values per ','

            foreach (var val in value.Split(',')) {

                // try to convert the single values to the TValueType

                // be aware this won't work for all types

                // it can throw an error

                // you might need to rethink in case you really want to use a principle like this (string is very restricting to show an object)

                ValueList.Add( (TValueType)Convert.ChangeType( val, typeof(TValueType) ) );

            }

        }

    }

    public IList<TValueType> ValueList {

        get {

            // return the list, new items can always be added through ValueList.Add it will never be empty

            // it is however not unique, when you need a unique list, think about HashSet<T> with ISet<T> as interface

            return _list;

        }

    }

}