BinaryReader BinaryWriter Console.ReadLine Convert Dictionary Dictionary

`Dictionary`, binary file

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

Important types

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.

Detail Here you are asked to either Write the Dictionary and its contents to the file, or Read in a Dictionary from that file.

Detail In Write, we use the using statement on a FileStream and BinaryWriter. We implement the simple file format described.

Detail In Read, we use a FileStream and BinaryReader and then loop through the required number of 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.

And The file was 32 bytes, which means the characters were encoded with one byte each.

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.

Also Performance reading in the file is good. It doesn't require much parsing. The reads are sequential and thus exploit the cache.

Summary