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Home Interviews How do you write to a file using memory-mapped I/O? – MindStick
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ICSM Computer
ICSM Computer 13 May 2025
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Writing to a file using memory-mapped I/O in C# is done with the MemoryMappedFile class. This technique allows you to treat a file as if it's in memory, which can offer performance benefits for large files or for inter-process communication.

Basic Example: Write using Memory-Mapped File

using System;
using System.IO;
using System.IO.MemoryMappedFiles;
using System.Text;

class Program
{
    static void Main()
    {
        string path = "example.dat";
        string text = "Hello from memory-mapped file!";
        byte[] data = Encoding.UTF8.GetBytes(text);

        // Create or overwrite the file and ensure it's large enough
        using (var file = new FileStream(path, FileMode.Create, FileAccess.ReadWrite, FileShare.None))
        {
            file.SetLength(data.Length); // Set file size
        }

        // Open the file as a memory-mapped file
        using (var mmf = MemoryMappedFile.CreateFromFile(path, FileMode.Open))
        {
            using (var accessor = mmf.CreateViewAccessor(0, data.Length, MemoryMappedFileAccess.Write))
            {
                accessor.WriteArray(0, data, 0, data.Length);
            }
        }

        Console.WriteLine("Data written to file using memory-mapped I/O.");
    }
}

Breakdown:

  1. MemoryMappedFile.CreateFromFile(...): Maps a file to memory.
  2. CreateViewAccessor(...): Gets a memory-accessible view of the file.
  3. accessor.WriteArray(...): Writes bytes directly into the mapped memory.

Notes:

  1. Ideal for large files or random access patterns.
  2. Can also be used for inter-process communication (IPC).
  3. You must pre-size the file using SetLength() to avoid IOException.

Gotchas:

  1. You need to manage memory ranges carefully — writing beyond allocated size causes exceptions.
  2. Not suitable for simple small file writes (overhead not worth it).
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