using System;

using System.Collections.Generic;

​

public static class Extension

{

    public static T FirstOrNew<T>(this IEnumerable<T> list) where T : new()

    {

        if (list == null) throw new ArgumentException("Value can't be null");

        var enumerator = list.GetEnumerator();

        return enumerator.MoveNext() ? enumerator.Current : new T();

    }

}

public class Program

{

    public static void Main()

    {

        var list1 = new List<int> { 5 };

        var result1 = list1.FirstOrNew(); //5

​

        var list2 = new List<int>();

        var result2 = list2.FirstOrNew(); //0

        Console.WriteLine(result1);

        Console.WriteLine(result2);

    }

}