using System.Collections.Generic;
public static void Main()
List<string> x = new List<string>();
var s1 = x.FirstOrDefault(a => a == "Hello");
Console.WriteLine(s1.ToUpper());
catch(NullReferenceException e2) {
Console.WriteLine("Error on s1");
Console.WriteLine(e2.Message);
catch(ArgumentNullException e1) {
Console.WriteLine("Error for x");
Console.WriteLine(e1.Message);
var s2 = y.FirstOrDefault(a => a == "Hello");
Console.WriteLine(s2.ToUpper());
catch(NullReferenceException e4) {
Console.WriteLine("Error on s2");
Console.WriteLine(e4.Message);
catch(ArgumentNullException e3) {
Console.WriteLine("Error for y");
Console.WriteLine(e3.Message);
Console.WriteLine("Done");