StringCollection.cs

using System.Collections.Generic;
using System.Linq;
using System.Threading;

public class StringCollection
{
    private List<string> collection;

    public StringCollection()
    {
        Interlocked.CompareExchange(ref collection, new List<string>(), null);
    }

    public void AddString(string item)
    {
        while (true)
        {
            var original = Interlocked.CompareExchange(ref collection, null, null);

            var copy = original.ToList();
            copy.Add(item);

            var result = Interlocked.CompareExchange(ref collection, copy, original);
                
            if (result == original)
            {
                break;
            }
        }
    }

    public override string ToString()
    {
        var original = Interlocked.CompareExchange(ref collection, null, null);

        return string.Join(",", original);
    }
}