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How do you read and write an entire text file's contents using the simplest File class methods? How does the 'using' statement (or 'using' declaration) ensure a FileStream is properly closed, even if an exception occurs? How do StreamReader and StreamWriter differ from directly using a FileStream, and when would you choose them? 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

What is the difference between File.ReadAllLines() and File.ReadLines() in terms of memory usage?

Beginner

Answer

File.ReadAllLines() reads the ENTIRE file and returns a fully-materialized string[] array all at once; File.ReadLines() returns a LAZY IEnumerable<string> that reads and yields one line at a time as you iterate, using far less memory for very large files since it never holds the whole file in memory simultaneously.
// Loads the entire file into memory at once
string[] allLines = File.ReadAllLines("huge.txt");

// Reads and processes one line at a time, low memory footprint
foreach (string line in File.ReadLines("huge.txt")) {
  ProcessLine(line);
}
Real-world example Processing a multi-gigabyte log file line by line without loading the entire file into memory.

Common follow-ups: Which method would you choose if you needed to know the total line count before processing?

Iterators & yield return
All 10 File I/O & Streams questions