Dateien, JSON und Datum/Zeit

string ordner = Path.Combine(Path.GetTempPath(), "kurs-demo");
Directory.CreateDirectory(ordner);
string datei = Path.Combine(ordner, "notiz.txt");

File.WriteAllText(datei, "Zeile 1\nZeile 2\n");
File.AppendAllText(datei, "Zeile 3\n");
string inhalt = File.ReadAllText(datei);
string[] zeilen = File.ReadAllLines(datei);
Console.WriteLine($"{zeilen.Length} Zeilen, {inhalt.Length} Zeichen");
Console.WriteLine(File.Exists(datei) + " " + Path.GetFileName(datei) + " " + Path.GetExtension(datei));

foreach (var z in File.ReadLines(datei).Where(z => z.Contains('3'))) Console.WriteLine(z.Trim());    // zeilenweise, lazy

using (var schreiber = new StreamWriter(Path.Combine(ordner, "stream.txt")))
{
    schreiber.WriteLine("über einen Stream");
}
using (var leser = new StreamReader(Path.Combine(ordner, "stream.txt")))
{
    string? zeile;
    while ((zeile = leser.ReadLine()) != null) Console.WriteLine(zeile);
}
Console.WriteLine(string.Join(",", Directory.GetFiles(ordner).Select(Path.GetFileName).OrderBy(n => n)));
Directory.Delete(ordner, recursive: true);
Console.WriteLine(Directory.Exists(ordner));

using System.Text.Json;
using System.Text.Json.Serialization;

var mia = new Nutzer("Mia", 17, ["Lesen", "Rad"]);
var optionen = new JsonSerializerOptions { WriteIndented = true, PropertyNamingPolicy = JsonNamingPolicy.CamelCase };

string json = JsonSerializer.Serialize(mia, optionen);
Console.WriteLine(json);

Nutzer? zurueck = JsonSerializer.Deserialize<Nutzer>(json, optionen);
Console.WriteLine($"{zurueck?.Name} {zurueck?.Hobbys.Count}");

var liste = JsonSerializer.Deserialize<List<Nutzer>>("""[{"name":"Tom","alter":19,"hobbys":[]}]""", optionen);
Console.WriteLine(liste![0].Name);

try { JsonSerializer.Deserialize<Nutzer>("{kaputt"); }
catch (JsonException) { Console.WriteLine("ungültiges JSON"); }

using var dok = JsonDocument.Parse("""{"a":{"b":[10,20,30]}}""");
Console.WriteLine(dok.RootElement.GetProperty("a").GetProperty("b")[1].GetInt32());

record Nutzer(string Name, int Alter, List<string> Hobbys);

var d = new DateTime(2025, 3, 8, 14, 30, 0);
Console.WriteLine($"{d:dd.MM.yyyy HH:mm} {d.DayOfWeek} {d.DayOfYear} {DateTime.IsLeapYear(2024)}");
Console.WriteLine(d.AddDays(30).ToString("yyyy-MM-dd") + " " + d.AddMonths(-1).ToString("MMMM yyyy", System.Globalization.CultureInfo.InvariantCulture));

var diff = new DateTime(2025, 12, 24) - new DateTime(2025, 3, 8);
Console.WriteLine($"{diff.Days} Tage, {diff.TotalHours} Stunden");

DateOnly tag = new(2025, 12, 24);
TimeOnly zeit = new(18, 0);
Console.WriteLine($"{tag} {zeit} {tag.DayOfWeek}");
Console.WriteLine(DateTime.Parse("2025-03-08T14:30:00").Hour + " " + DateTime.TryParse("kein Datum", out _));

var utc = new DateTimeOffset(2025, 7, 1, 12, 0, 0, TimeSpan.Zero);
Console.WriteLine(utc.ToOffset(TimeSpan.FromHours(2)).ToString("HH:mm zzz"));

var spanne = TimeSpan.FromMinutes(135);
Console.WriteLine($"{spanne} {spanne.TotalHours:F2}");

Console.WriteLine(Environment.ProcessorCount > 0);
Console.WriteLine(Environment.GetEnvironmentVariable("GIBT_ES_NICHT") ?? "nicht gesetzt");
Console.WriteLine(Environment.GetCommandLineArgs().Length >= 1);
Console.WriteLine(OperatingSystem.IsLinux() || OperatingSystem.IsWindows() || OperatingSystem.IsMacOS());