ExampleInCS.md

Example in C#

Example WRONG code

The "wrong" code violates the Single Responsibility Principle (SRP) because the Journal class is handling two separate concerns: managing journal entries and managing the persistence of those entries to a file.

using System.Diagnostics;

// Just stores a couple of journal entries and ways of
// working with them
public class Journal
{
    // secret journal entries
    private readonly List<string> entries = new List<string>();

    private static int count = 0;

    public int AddEntry(string text)
    {
        entries.Add($"{++count}: {text}");
        return count; // memento pattern!
    }

    public void RemoveEntry(int index)
    {
        entries.RemoveAt(index);
    }

    public override string ToString()
    {
        return string.Join(Environment.NewLine, entries);
    }

    // breaks single responsibility principle
    public void Save(string filename, bool overwrite = false)
    {
        File.WriteAllText(filename, ToString());
    }

    // breaks single responsibility principle
    public void Load(string filename)
    {
        File.ReadAllText(filename).Split('\n');
    }
}

public class Demo
{
    static void Main(string[] args)
    {
        var j = new Journal();
        j.AddEntry("I cried today.");
        j.AddEntry("I ate a bug.");
        Console.WriteLine(j);

        var filename = @"c:\temp\journal.txt";
        j.Save(filename);
        Process.Start(filename);
    }
}

Example FIXED code

The Journal class is responsible not just for maintaining the journal entries, but also for saving and loading them from a file. This violates SRP because a change in how we store the journal entries would require changes to the Journal class.

using System.Diagnostics;

// Just stores a couple of journal entries and ways of
// working with them
public class Journal
{
    private readonly List<string> entries = new List<string>();

    private static int count = 0;

    public int AddEntry(string text)
    {
        entries.Add($"{++count}: {text}");
        return count; // memento pattern!
    }

    public void RemoveEntry(int index)
    {
        entries.RemoveAt(index);
    }

    public override string ToString()
    {
        return string.Join(Environment.NewLine, entries);
    }
}
// handles the responsibility of persisting objects
public class Persistence
{
    public void SaveToFile(Journal journal, string filename, bool overwrite = false)
    {
        if (overwrite || !File.Exists(filename))
            File.WriteAllText(filename, journal.ToString());
    }
}

public class Demo
{
    static void Main(string[] args)
    {
        var j = new Journal();
        j.AddEntry("I cried today.");
        j.AddEntry("I ate a bug.");
        Console.WriteLine(j);

        var p = new Persistence();
        var filename = @"c:\temp\journal.txt";
        p.SaveToFile(j, filename);
        Process.Start(filename);
    }
}

The Journal class now only handles journal entries, while a new Persistence class is introduced to handle saving the Journal object to a file. This adheres to the SRP because each class now has a single responsibility: Journal for managing entries, and Persistence for managing file operations.

Now, if you need to change how journal entries are stored, you only need to modify the Persistence class, not Journal. Similarly, if the way we manage journal entries changes, we only update Journal, not Persistence. This separation of concerns makes the code easier to maintain and understand.