How would you implement a "trait" design-pattern in C#?

Question

I know the feature doesn't exist in C#, but PHP recently added a feature called Traits which I thought was a bit silly at first until I started thinking about it.

Say I have a base class called Client. Client has a single property called Name.

Now I'm developing a re-usable application that will be used by many different customers. All customers agree that a client should have a name, hence it being in the base-class.

Now Customer A comes along and says he also need to track the client's Weight. Customer B doesn't need the Weight, but he wants to track Height. Customer C wants to track both Weight and Height.

With traits, we could make the both the Weight and the Height features traits:

class ClientA extends Client use TClientWeight
class ClientB extends Client use TClientHeight
class ClientC extends Client use TClientWeight, TClientHeight

Now I can meet all my customers' needs without adding any extra fluff to the class. If my customer comes back later and says "Oh, I really like that feature, can I have it too?", I just update the class definition to include the extra trait.

How would you accomplish this in C#?

Interfaces don't work here because I want concrete definitions for the properties and any associated methods, and I don't want to re-implement them for each version of the class.

(By "customer", I mean a literal person who has employed me as a developer, whereas by "client" I'm referring a programming class; each of my customers has clients that they want to record information about)

Answer 1

You can get the syntax by using marker interfaces and extension methods.

Prerequisite: the interfaces need to define the contract which is later used by the extension method. Basically the interface defines the contract for being able to "implement" a trait; ideally the class where you add the interface should already have all members of the interface present so that no additional implementation is required.

public class Client {
  public double Weight { get; }

  public double Height { get; }
}

public interface TClientWeight {
  double Weight { get; }
}

public interface TClientHeight {
  double Height { get; }
}

public class ClientA: Client, TClientWeight { }

public class ClientB: Client, TClientHeight { }

public class ClientC: Client, TClientWeight, TClientHeight { }

public static class TClientWeightMethods {
  public static bool IsHeavierThan(this TClientWeight client, double weight) {
    return client.Weight > weight;
  }
  // add more methods as you see fit
}

public static class TClientHeightMethods {
  public static bool IsTallerThan(this TClientHeight client, double height) {
    return client.Height > height;
  }
  // add more methods as you see fit
}

Use like this:

var ca = new ClientA();
ca.IsHeavierThan(10); // OK
ca.IsTallerThan(10); // compiler error

Edit: The question was raised how additional data could be stored. This can also be addressed by doing some extra coding:

public interface IDynamicObject {
  bool TryGetAttribute(string key, out object value);
  void SetAttribute(string key, object value);
  // void RemoveAttribute(string key)
}

public class DynamicObject: IDynamicObject {
  private readonly Dictionary<string, object> data = new Dictionary<string, object>(StringComparer.Ordinal);

  bool IDynamicObject.TryGetAttribute(string key, out object value) {
    return data.TryGet(key, out value);
  }

  void IDynamicObject.SetAttribute(string key, object value) {
    data[key] = value;
  }
}

And then, the trait methods can add and retrieve data if the "trait interface" inherits from IDynamicObject:

public class Client: DynamicObject { /* implementation see above */ }

public interface TClientWeight, IDynamicObject {
  double Weight { get; }
}

public class ClientA: Client, TClientWeight { }

public static class TClientWeightMethods {
  public static bool HasWeightChanged(this TClientWeight client) {
    object oldWeight;
    bool result = client.TryGetAttribute("oldWeight", out oldWeight) && client.Weight.Equals(oldWeight);
    client.SetAttribute("oldWeight", client.Weight);
    return result;
  }
  // add more methods as you see fit
}

Note: by implementing IDynamicMetaObjectProvider as well the object would even allow to expose the dynamic data through the DLR, making the access to the additional properties transparent when used with the dynamic keyword.

Answer 2

This is really an suggested extension to Lucero's answer where all the storage was in the base class.

How about using dependency properties for this?

This would have the effect of making the client classes light weight at run time when you have many properties that are not always set by every descendant. This is because the values are stored in a static member.

using System.Windows;

public class Client : DependencyObject
{
    public string Name { get; set; }

    public Client(string name)
    {
        Name = name;
    }

    //add to descendant to use
    //public double Weight
    //{
    //    get { return (double)GetValue(WeightProperty); }
    //    set { SetValue(WeightProperty, value); }
    //}

    public static readonly DependencyProperty WeightProperty =
        DependencyProperty.Register("Weight", typeof(double), typeof(Client), new PropertyMetadata());


    //add to descendant to use
    //public double Height
    //{
    //    get { return (double)GetValue(HeightProperty); }
    //    set { SetValue(HeightProperty, value); }
    //}

    public static readonly DependencyProperty HeightProperty =
        DependencyProperty.Register("Height", typeof(double), typeof(Client), new PropertyMetadata());
}

public interface IWeight
{
    double Weight { get; set; }
}

public interface IHeight
{
    double Height { get; set; }
}

public class ClientA : Client, IWeight
{
    public double Weight
    {
        get { return (double)GetValue(WeightProperty); }
        set { SetValue(WeightProperty, value); }
    }

    public ClientA(string name, double weight)
        : base(name)
    {
        Weight = weight;
    }
}

public class ClientB : Client, IHeight
{
    public double Height
    {
        get { return (double)GetValue(HeightProperty); }
        set { SetValue(HeightProperty, value); }
    }

    public ClientB(string name, double height)
        : base(name)
    {
        Height = height;
    }
}

public class ClientC : Client, IHeight, IWeight
{
    public double Height
    {
        get { return (double)GetValue(HeightProperty); }
        set { SetValue(HeightProperty, value); }
    }

    public double Weight
    {
        get { return (double)GetValue(WeightProperty); }
        set { SetValue(WeightProperty, value); }
    }

    public ClientC(string name, double weight, double height)
        : base(name)
    {
        Weight = weight;
        Height = height;
    }

}

public static class ClientExt
{
    public static double HeightInches(this IHeight client)
    {
        return client.Height * 39.3700787;
    }

    public static double WeightPounds(this IWeight client)
    {
        return client.Weight * 2.20462262;
    }
}

Answer 3

C# language (at least to version 5) does not have support for Traits.

However, Scala has Traits and Scala runs on the JVM (and CLR). Therefore, it's not a matter of run-time, but simply that of the language.

Consider that Traits, at least at the Scala sense, can be thought of as "pretty magic to compile in proxy methods" (they do not affect the MRO, which is different from Mixins in Ruby). In C# the way to get this behavior would be to use interfaces and "lots of manual proxy methods" (e.g. composition).

This tedious process could be done with a hypothetical processor (perhaps automatic code generation for a partial class via templates?), but that's not C#.

Happy coding.

Answer 4

This sounds like PHP's version of Aspect Oriented Programming. There are tools to help like PostSharp or MS Unity in some cases. If you want to roll-your-own, code-injection using C# Attributes is one approach, or as suggested extension methods for limited cases.

Really depends how complicated you want to get. If you are trying to build something complex I'd be looking at some of these tools to help.

Answer 5

Traits can be implemented in C# 8 by using default interface methods. Java 8 introduced default interface methods for this reason too.

Using C# 8, you can write almost exactly what you proposed in the question. The traits are implemented by the IClientWeight, IClientHeight interfaces that provide a default implementation for their methods. In this case, they just return 0:

public interface IClientWeight
{
    int getWeight()=>0;
}

public interface IClientHeight
{
    int getHeight()=>0;
}

public class Client
{
    public String Name {get;set;}
}

ClientA and ClientB have the traits but don't implement them. ClientC implements only IClientHeight and returns a different number, in this case 16 :

class ClientA : Client, IClientWeight{}
class ClientB : Client, IClientHeight{}
class ClientC : Client, IClientWeight, IClientHeight
{
    public int getHeight()=>16;
}

When getHeight() is called in ClientB through the interface, the default implementation is called. getHeight() can only be called through the interface.

ClientC implements the IClientHeight interface so its own method is called. The method is available through the class itself.

public class C {
    public void M() {        
        //Accessed through the interface
        IClientHeight clientB = new ClientB();        
        clientB.getHeight();

        //Accessed directly or through the class
        var clientC = new ClientC();        
        clientC.getHeight();
    }
}

This SharpLab.io example shows the code produced from this example

Many of the traits features described in the PHP overview on traits can be implemented easily with default interface methods. Traits (interfaces) can be combined. It's also possible to define abstract methods to force classes to implement certain requirements.

Let's say we want our traits to have sayHeight() and sayWeight() methods that return a string with the height or weight. They'd need some way to force exhibiting classes (term stolen from the PHP guide) to implement a method that returns the height and weight :

public interface IClientWeight
{
    abstract int getWeight();
    String sayWeight()=>getWeight().ToString();
}

public interface IClientHeight
{
    abstract int getHeight();
    String sayHeight()=>getHeight().ToString();
}

//Combines both traits
public interface IClientBoth:IClientHeight,IClientWeight{}

The clients now have to implement thet getHeight() or getWeight() method but don't need to know anything about the say methods.

This offers a cleaner way to decorate

SharpLab.io link for this sample.

Answer 6

I'd like to point to NRoles, an experiment with roles in C#, where roles are similar to traits.

NRoles uses a post-compiler to rewrite the IL and inject the methods into a class. This allows you to write code like that:

public class RSwitchable : Role
{
    private bool on = false;
    public void TurnOn() { on = true; }
    public void TurnOff() { on = false; }
    public bool IsOn { get { return on; } }
    public bool IsOff { get { return !on; } }
}

public class RTunable : Role
{
    public int Channel { get; private set; }
    public void Seek(int step) { Channel += step; }
}

public class Radio : Does<RSwitchable>, Does<RTunable> { }

where class Radio implements RSwitchable and RTunable. Behind the scenes, Does<R> is an interface with no members, so basically Radio compiles to an empty class. The post-compilation IL rewriting injects the methods of RSwitchable and RTunable into Radio, which can then be used as if it really derived from the two roles (from another assembly):

var radio = new Radio();
radio.TurnOn();
radio.Seek(42);

To use radio directly before rewriting happened (that is, in the same assembly as where the Radio type is declared), you have to resort to extensions methods As<R>():

radio.As<RSwitchable>().TurnOn();
radio.As<RTunable>().Seek(42);

since the compiler would not allow to call TurnOn or Seek directly on the Radio class.

Answer 7

There is an academic project, developed by Stefan Reichart from the Software Composition Group at the University of Bern (Switzerland), which provides a true implementation of traits to the C# language.

Have a look at the paper (PDF) on CSharpT for the full description of what he has done, based on the mono compiler.

Here is a sample of what can be written:

trait TCircle
{
    public int Radius { get; set; }
    public int Surface { get { ... } }
}

trait TColor { ... }

class MyCircle
{
    uses { TCircle; TColor }
}

Answer 8

Building on what Lucero suggested, I came up with this:

internal class Program
{
    private static void Main(string[] args)
    {
        var a = new ClientA("Adam", 68);
        var b = new ClientB("Bob", 1.75);
        var c = new ClientC("Cheryl", 54.4, 1.65);

        Console.WriteLine("{0} is {1:0.0} lbs.", a.Name, a.WeightPounds());
        Console.WriteLine("{0} is {1:0.0} inches tall.", b.Name, b.HeightInches());
        Console.WriteLine("{0} is {1:0.0} lbs and {2:0.0} inches.", c.Name, c.WeightPounds(), c.HeightInches());
        Console.ReadLine();
    }
}

public class Client
{
    public string Name { get; set; }

    public Client(string name)
    {
        Name = name;
    }
}

public interface IWeight
{
    double Weight { get; set; }
}

public interface IHeight
{
    double Height { get; set; }
}

public class ClientA : Client, IWeight
{
    public double Weight { get; set; }
    public ClientA(string name, double weight) : base(name)
    {
        Weight = weight;
    }
}

public class ClientB : Client, IHeight
{
    public double Height { get; set; }
    public ClientB(string name, double height) : base(name)
    {
        Height = height;
    }
}

public class ClientC : Client, IWeight, IHeight
{
    public double Weight { get; set; }
    public double Height { get; set; }
    public ClientC(string name, double weight, double height) : base(name)
    {
        Weight = weight;
        Height = height;
    }
}

public static class ClientExt
{
    public static double HeightInches(this IHeight client)
    {
        return client.Height * 39.3700787;
    }

    public static double WeightPounds(this IWeight client)
    {
        return client.Weight * 2.20462262;
    }
}

Output:

Adam is 149.9 lbs.
Bob is 68.9 inches tall.
Cheryl is 119.9 lbs and 65.0 inches.

It isn't quite as nice as I'd like, but it's not too bad either.