31. File, Directory and Path — quick file operations

string folder = Path.Combine(AppContext.BaseDirectory, "data");   // builds paths with the right separator (\ or /)
Directory.CreateDirectory(folder);                                  // no error if it already exists
string file = Path.Combine(folder, "notes.txt");

File.WriteAllText(file, "Line 1\n");               // create or overwrite
File.AppendAllText(file, "Line 2\n");              // add to the end
File.WriteAllLines(file, ["a", "b", "c"]);
string all = File.ReadAllText(file);
string[] lines = File.ReadAllLines(file);           // whole file in memory
foreach (var line in File.ReadLines(file))          // lazy — one line at a time, good for big files
    Console.WriteLine(line);

if (File.Exists(file)) File.Copy(file, Path.Combine(folder, "backup.txt"), overwrite: true);
File.Move(file, Path.Combine(folder, "renamed.txt"));
File.Delete(Path.Combine(folder, "backup.txt"));

foreach (var csv in Directory.EnumerateFiles(folder, "*.csv", SearchOption.AllDirectories))
    Console.WriteLine(Path.GetFileName(csv));

Console.WriteLine(Path.GetExtension("report.pdf"));            // .pdf
Console.WriteLine(Path.GetFileNameWithoutExtension("report.pdf")); // report
string temp = Path.GetTempFileName();

await File.WriteAllTextAsync(file, "async version");          // async versions for servers/UI apps