using System;

bool? value = null;

Console.WriteLine($"{value & false} {value | true}");

using System;

try
{
    try { throw new Exception(); }
    finally { Console.Write("F"); }
}
catch (Exception) when (Check())
{
    Console.Write("C");
}

static bool Check()
{
    Console.Write("Q");
    return true;
}

using System;

var first = (X: 1, Y: 2);
var second = (A: 1, B: 2);

Console.WriteLine(first == second);

using System;

Show(null);
Show((object)null);

static void Show(params object[] items)
{
    Console.Write(
        items is null ? "null " : items.Length + " "
    );
}

using System;
using System.Threading.Tasks;

var gate = new TaskCompletionSource<bool>();

async Task Work()
{
    Console.Write("A");
    await gate.Task;
    Console.Write("C");
}

var task = Work();
Console.Write("B");
gate.SetResult(true);
await task;

using System;

var item = new Item();
item.Show();

interface IShow
{
    void Show() => Console.Write("I");
}

class Item : IShow { }

using System;

int[] values = { 1, 2 };

int copy = First(values);
ref int alias = ref First(values);

copy = 7;
alias = 9;

Console.WriteLine($"{values[0]} {copy}");

static ref int First(int[] items)
    => ref items[0];

using System;

var value = new Item();

Console.WriteLine(
    $"{value == null} {value is null}"
);

class Item
{
    public static bool operator ==(Item a, Item b)
        => true;

    public static bool operator !=(Item a, Item b)
        => false;

    public override bool Equals(object other)
        => ReferenceEquals(this, other);

    public override int GetHashCode() => 0;
}

using System;
using System.Collections.Generic;

var words = new[] { "Go", "GO", "go" };
var exact = new HashSet<string>(words, StringComparer.Ordinal);
var ignore = new HashSet<string>(words, StringComparer.OrdinalIgnoreCase);

Console.WriteLine($"{exact.Count} {ignore.Count}");

using System;

{
    using var first = new Resource("A");
    using var second = new Resource("B");
    Console.Write("X");
}

sealed class Resource(string name) : IDisposable
{
    public void Dispose() => Console.Write(name);
}

using System;

int calls = 0;
int[] values = null;
int? value = values?[Next()];

Console.WriteLine($"{calls} {value is null}");

int Next()
{
    calls++;
    return 0;
}