All 10 File I/O & Streams questions
More in File I/O & Streams
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 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 you read and write an entire text file's contents using the simplest File class methods?

Beginner

Answer

File.ReadAllText() reads an entire file's contents into a single string; File.WriteAllText() writes a string to a file, creating it if it doesn't exist or overwriting it if it does — both are synchronous and load the ENTIRE file into memory at once, fine for small files.
string content = File.ReadAllText("data.txt");
File.WriteAllText("output.txt", "Hello, World!");

// Async equivalents
string content2 = await File.ReadAllTextAsync("data.txt");
await File.WriteAllTextAsync("output.txt", "Hello, World!");
Real-world example Reading a small configuration file's entire content in one call, without manually managing a stream.

Common follow-ups: Why should you prefer the Async versions of these methods in a server application?

Asynchronous Programming
All 10 File I/O & Streams questions