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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 StreamReader and StreamWriter differ from directly using a FileStream, and when would you choose them? 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 you efficiently copy data between two streams using Stream.CopyToAsync(), and why is this preferred over a manual byte-by-byte loop?

Intermediate

Answer

CopyToAsync() efficiently transfers data from a source stream to a destination stream using an internally-managed BUFFER (reading a chunk, writing it, repeating), fully asynchronously — far more efficient and less error-prone than manually implementing your own read/write loop with a byte array.
using FileStream source = File.OpenRead("large-file.dat");
using FileStream destination = File.Create("copy.dat");
await source.CopyToAsync(destination); // efficient, buffered, fully async
Real-world example Copying a large uploaded file from a temporary location to permanent storage without loading the entire file into memory.

Common follow-ups: Can CopyToAsync() also be used between fundamentally different stream types, like a network stream and a file stream?

Asynchronous Programming
All 10 File I/O & Streams questions