Dictionary, binary file

A C# Dictionary can be stored in a binary file. We develop a custom file format that allows for efficient persistence.

By combining FileStream, BinaryReader and BinaryWriter, we can write a Dictionary to the disk. This approach is not the only possible one, but it works.

Example

The binary file contains a single 32-bit integer at the start. This tells us how many pairs are found in the rest of the file. Then we can read in that many pairs.

using System;
using System.Collections.Generic;
using System.IO;

class Program
{
    static void Main()
    {
        while (true)
        {
            Console.WriteLine("1=Write, 2=Read");
            string value = Console.ReadLine();
            if (value == "1")
            {
                var dictionary = new Dictionary<string, string>();
                dictionary["perls"] = "dot";
                dictionary["net"] = "perls";
                dictionary["dot"] = "net";
                Write(dictionary, "C:\\dictionary.bin");
            }
            else if (value == "2")
            {
                var dictionary = Read("C:\\dictionary.bin");
                foreach (var pair in dictionary)
                {
                    Console.WriteLine(pair);
                }
            }
        }
    }

    static void Write(Dictionary<string, string> dictionary, string file)
    {
        using (FileStream fs = File.OpenWrite(file))
        using (BinaryWriter writer = new BinaryWriter(fs))
        {
            // Put count.
            writer.Write(dictionary.Count);
            // Write pairs.
            foreach (var pair in dictionary)
            {
                writer.Write(pair.Key);
                writer.Write(pair.Value);
            }
        }
    }

    static Dictionary<string, string> Read(string file)
    {
        var result = new Dictionary<string, string>();
        using (FileStream fs = File.OpenRead(file))
        using (BinaryReader reader = new BinaryReader(fs))
        {
            // Get count.
            int count = reader.ReadInt32();
            // Read in all pairs.
            for (int i = 0; i < count; i++)
            {
                string key = reader.ReadString();
                string value = reader.ReadString();
                result[key] = value;
            }
        }
        return result;
    }
}
1=Write, 2=Read
1
1=Write, 2=Read
2
[perls, dot]
[net, perls]
[dot, net]
1=Write, 2=Read

Output details

You can see the Dictionary was written to the file. The Dictionary instances are separate in Main. I checked the C:\dictionary.bin file that was used.

Discussion

There are advantages to using a custom binary format to persist a collection. If you are using integers instead of strings, a binary format can be used to efficiently encode values.

A Dictionary can be written to and read from a binary file. We developed a simple file format that stores the number of pairs in the first integer in the binary file.