```cs
using System.Linq;

public static class CollectionExtentions {
	public IEnumerable<T> Show<T>(this IEnumerable<T> collection) {
		foreach (var item in collection) {
			Console.WriteLine(item);
		}
		return collection;
	}
	
	public static void Hello(this object obj) {
		Console.WriteLine("Hello {0}")
	}
}

2.Hello();
double[] t = { 5, 8, 5, 2, 3 };
var a = new A() { Id = 10 };
a.Hello();
t.Where(x => x % 2 == 0).Show<double>();
var t = new List<A> {
	l.Add(new A() { Id = 10 });
	l.Add(new A() { Id = 1 });
}
l.OrderBy(x => x.Id).Show();
l.Add(new A());
l.AddRange(l);
l.Contains(new A()).Show();
l.Any(x => x.Id == 10).Hello();
l.All(x => x.Id == 10).Show();
if (l.Count == 0) {
	Console.WriteLine();
}
l.Clear();
t.Order()
Array.Sort(t);
t.Show();
IEnumerable<int> Get() {
	yield return 1;
	yield return 2;
	for (int i = 0; i < 3; i++) {
		yield return i;
	}
}

Get().Show();
```

```cs
static void Lesson5Task() {
	var sw = new Stopwatch();
	sw.Start();
	long l1 = 0;
	for (int i = 0; i < 1_000_000_000; i++) {
		l1 += i;
	}
	sw.Stop();
	Thread t1 = new Thread(() => {
	
	});
	t1.Start();
	
	long l2 = 0;
	for (int i = 1_000_000_000; i > 1_000_000_000 / 2; i--) {
		l2 += i;
	}
	sw.Stop();
	Thread t2 = new Thread(() => {
	
	});
	t2.Start();
	
	t1.Join();
	t2.Join();
	
	l = l1 + l2;
	sw.Stop();
}
```

```cs
const int MAX = 1_000_000_000;
long Sum(int start, int stop);

async Task Foo(int delay) {
	Thread.Sleep(delay);
	await Task.Delay(1000);
	Thread.Sleep(delay);
}
sw.Reset();
sw.Start();
await Foo();
await Foo();
try {
	await Foo(100);
}
catch (Exception e) {
	Console.WriteLine(e.message);
}
Task.WhenAny(Foo(), Foo());
var task1 = Task.Run(() => Sum(0, MAX / 2));
var task2 = Task.Run(() => Sum(MAX / 2, MAX));
await Task.WhenAll(task1, task2);
sw.Stop();
```