using System;

using System.Collections.Generic;

​

​

public class SomeDataClass

{

​

    public IList<int> ListA { get; set; }

​

    public IList<int> ListB { get; set; }

​

    public string GetAsString()

    {

        return

            "A: " + (ListA == null ? "null\n" : string.Join(", ", ListA)) + '\n' +

            "B: " + (ListB == null ? "null\n" : string.Join(", ", ListB)) + '\n';

    }

}

​

​

public class Program

{

    private static void AppendValue(IList<int> li)

    {

        if (li == null)

        {

            li = new List<int>();

        }

​

        li.Add(42);

    }

​

    public static void Main()

    {

        var o = new SomeDataClass();

​

        if (o.ListA == null)

        {

            o.ListA = new List<int>();

        }

        o.ListA.Add(42);

​

        AppendValue(o.ListB);

​

​

        Console.WriteLine(o.GetAsString());            

    }

​

}

​

​