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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 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 does the 'using' statement (or 'using' declaration) ensure a FileStream is properly closed, even if an exception occurs?

Intermediate

Answer

'using' automatically calls Dispose() on the resource when execution leaves its scope — whether normally or via an exception — guaranteeing the underlying file handle is released; this is essential for FileStream and other IDisposable resources like database connections, since forgetting to dispose them can leave files locked or exhaust OS handles.
using (FileStream stream = File.OpenRead("data.txt")) {
  // stream is guaranteed to be disposed when this block exits, even on exception
}

// Modern C# 8+ 'using declaration' (disposed at end of enclosing scope)
using FileStream stream2 = File.OpenRead("data.txt");
Real-world example Guaranteeing a file lock is released immediately after processing, even if the processing code throws an exception midway.

Common follow-ups: What interface must a type implement to be usable with a 'using' statement?

Exception Handling
All 10 File I/O & Streams questions