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How do you read and write an entire text file's contents using the simplest File class methods? What is the difference between File.ReadAllLines() and File.ReadLines() in terms of memory usage? How does the 'using' statement (or 'using' declaration) ensure a FileStream is properly closed, even if an exception occurs? How do you efficiently copy data between two streams using Stream.CopyToAsync(), and why is this preferred over a manual byte-by-byte loop? How would you implement a custom Stream subclass to wrap another stream with additional behavior, like counting bytes read? How do you use FileSystemWatcher to monitor a directory for file changes, and what are its common reliability pitfalls? How would you implement memory-mapped file access using MemoryMappedFile for efficient random access to a very large file? How do you correctly implement async, cancellable file I/O using CancellationToken with FileStream operations? How would you implement a robust, atomic file-write operation that avoids leaving a corrupted or partially-written file if the process crashes mid-write?
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C# › File I/O & Streams

How do StreamReader and StreamWriter differ from directly using a FileStream, and when would you choose them?

Intermediate

Answer

FileStream operates on raw BYTES; StreamReader/StreamWriter wrap a Stream (like a FileStream) and add TEXT ENCODING handling (converting between bytes and characters using a specified or detected encoding, like UTF-8), providing convenient line-based text reading/writing methods like ReadLine() and WriteLine().
using StreamReader reader = new StreamReader("data.txt");
string? line;
while ((line = reader.ReadLine()) != null) {
  Console.WriteLine(line);
}

using StreamWriter writer = new StreamWriter("output.txt");
writer.WriteLine("Hello, World!");
Real-world example Reading a text-based configuration or CSV file line by line with automatic character encoding handling.

Common follow-ups: How would you specify a specific text encoding (like UTF-16) when creating a StreamReader?

String Handling & StringBuilder
All 10 File I/O & Streams questions