using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class Test
{
	public static void Main()
	{
		List<int> list1 = new List<int>() {1,2,3};
		List<int> list2 = null;
		
		var agg = list1.Concat(list2.NeverNull());
	}
}

public static class Extensions
{
	public static IEnumerable<T> NeverNull<T>(this IEnumerable<T> list)
	{
		return list ?? Enumerable.Empty<T>();
	}
}