C# Files and Directories

Variables and collections hold program state while the process runs. When it ends, that in-memory state is gone unless the program writes it somewhere durable. A text file is the simplest persistence boundary and helps reveal issues that do not exist with hardcoded data: paths, encoding, permissions, partial writes, and malformed content.

A file is bytes with a name in a filesystem. Text APIs encode characters into bytes and decode them back. Modern .NET convenience methods use UTF-8 defaults appropriate for much text, but interoperability may require an explicit encoding agreed with another system.

File.WriteAllText replaces a file with one string, AppendAllText adds text, WriteAllLines writes a sequence of lines, and matching Read methods retrieve content. These methods open, use, and close the file internally, making them a good fit for small settings, exports, and learning projects.

Replacing and appending are different data policies. An activity log usually appends; a generated report may replace; a critical settings file may require writing a temporary file and atomically replacing the old one. Choose deliberately rather than using the first method autocomplete suggests.

string path = "visits.txt";
File.AppendAllText(path, DateTime.UtcNow + Environment.NewLine);

string content = File.ReadAllText(path);
Console.WriteLine(content);

Hardcoding separators such as \ or / makes platform assumptions and creates doubled or missing separators. Path.Combine joins segments according to the current platform. AppContext.BaseDirectory locates the application base, while Environment.SpecialFolder APIs identify user locations when that is truly where data belongs.

Never combine untrusted text into a sensitive base path and assume it stays inside that directory. Inputs containing rooted paths or parent traversal segments can escape the intended location. Validate allowed file names, resolve the full path, and confirm it remains within the approved root for real upload or file-serving features.

string folder = Path.Combine("data", "reports");
Directory.CreateDirectory(folder);

string reportPath = Path.Combine(folder, "weekly.txt");
File.WriteAllText(reportPath, "Completed: 7");

ReadAllText loads an entire file. For a large log, streaming one line at a time uses bounded memory and lets processing begin before the end is read. StreamReader and StreamWriter expose that control and must be disposed with using.

File APIs face external change. A file can disappear after File.Exists returns true, so existence checks do not replace handling the operation's exceptions. Catch specific failures where the program can offer recovery, and do not reveal internal server paths to untrusted users.

using (StreamReader reader = File.OpenText("events.txt"))
{
  string line;
  while ((line = reader.ReadLine()) != null)
  {
      Console.WriteLine(line);
  }
}