// Example 6: Add a list

// This is just a dummy method that returns some ints
public static IEnumerable<int> GetAnotherListOfInts() 
=> new[] { 10, 11, 12, 13 };

// With this extension method...
public static void Add<T>(this List<T> list, IEnumerable<T> items)
=> list.AddRange(items);

// ...this is possible
var result = new List<int>
{
  1,
  2,
  GetAnotherListOfInts(),
  3
};

// Output each item, to see if things work correctly
foreach(var item in result){
  Console.WriteLine(item);
}