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How would you implement memory-mapped file access using MemoryMappedFile for efficient random access to a very large file?

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Answer

MemoryMappedFile maps a file's contents directly into the process's virtual address space, letting you access arbitrary portions of even a very large file (larger than available RAM) as if it were an in-memory array, with the OS handling paging data in/out transparently — far more efficient than repeated Seek/Read calls for random-access patterns.
using var mmf = MemoryMappedFile.CreateFromFile("huge-file.dat", FileMode.Open);
using var accessor = mmf.CreateViewAccessor(0, 1000);
byte firstByte = accessor.ReadByte(0);
accessor.Write(0, (byte)42); // efficient random access, OS-managed paging
Real-world example Efficiently performing random-access reads/writes on a multi-gigabyte database or index file without loading it entirely into memory.

Common follow-ups: Why is memory-mapped access particularly well-suited for scenarios needing frequent RANDOM (not sequential) access to a huge file?

Memory & Garbage Collection
All 10 File I/O & Streams questions