Specification Pattern

The Specification Pattern. Yet another one I find useful in daily business. But a few enhancements can make it even more useful and a lot nicer to handle.

1, 2, many!

The CompositeSpecification is most often implemented with two fields for spec1 and spec2 or left and right. The AndSpecification and OrSpecifications evaluate these fields with their respective operator.

public class Or<T> : CompositeSpecification<T>
{
  // ...

  public override bool IsSatisfiedBy(T candidate)
  {
    return spec1.IsSatisfiedBy(candidate) || spec2.IsSatisfiedBy(candidate);
  }
}

This can result in a degenerated tree structure when you link many specifications with the same operator (e.g. a.Or(b).Or(c)...Or(n)).

To avoid this I decided to change the behavior of the composite. Instead of two fields it uses a list of children. If you try to link two specifications the code checks wether either one of them is of the same type as the composite and adds the other composites children instead of the composite itself to the list.

Sounds more complicated than it is. Let’s see some code.

public abstract class CompositeSpecification<T> : Specification<T>
{
  private readonly List<Specification<T>> children;
  public CompositeSpecification()
  {
    this.children = new List<Specification<T>>();
  }
  public IEnumerable<Specification<T>> Children
  {
    get { return children; }
  }
  public int Count
  {
    get { return this.children.Count; }
  }
  protected void Add(Specification<T> specification)
  {
    this.children.Add(specification);
  }
  protected void AddRange(IEnumerable<Specification<T>> specifications)
  {
    this.children.AddRange(specifications);
  }
}

public class Or<T> : CompositeSpecification<T>
{
  public Or(Specification<T> specification, Specification<T> other)
  {
    this.Include(specification);
    this.Include(other);
  }
  public override string Description
  {
    get { return " || "; }
  }
  public override bool IsSatisfiedBy(T candidate)
  {
    foreach (Specification<T> specification in this.Children)
    {
      if (specification.IsSatisfiedBy(candidate))
      {
        return true;
      }
    }
    return false;
  }
  private void Include(Specification<T> specification)
  {
    Or<T> or = specification as Or<T>;
    if (or != null)
    {
      this.AddRange(or.Children);
    }
    else
    {
      this.Add(specification);
    }
  }
}

And this is how two specifications can be linked with an Or operator.

public abstract class Specification<T>
{
  // ...
  
  public abstract bool IsSatisfiedBy(T candidate);
  public Specification<T> Or(Specification<T> other)
  {
    return new Or<T>(this, other);
  }
}

In the end this will put all successive Or’s in a single list, which makes finding the right specification in the tree a lot easier.

What do we have?

Generating a human readable representation of the specification tree can be tedious but is often beneficial if you need to see “what you have”. The easiest way to traverse a tree structure is a Visitor. The same pattern is used by Microsoft in their expression trees.

We add an Accept method and a property for the description of the specification to the base classes

public abstract class Specification<T>
{
  // ...
  public abstract string Description { get; }
  public virtual void Accept(SpecificationVisitor<T> visitor)
  {
    visitor.Visit(this);
  }
}

public abstract class CompositeSpecification<T> : Specification<T>
{
  // ...
  public override void Accept(SpecificationVisitor<T> visitor)
  {
    visitor.Visit(this);
  }
}

public class Or<T> : CompositeSpecification<T>
{
  // ...
  public override string Description
  {
    get { return " || "; }
  }
}

Define a base class for the SpecificationVisitor

public abstract class SpecificationVisitor<T>
{
  public abstract void Visit(Specification<T> specification);
  public abstract void Visit(CompositeSpecification<T> composite);
}

And the implementation of a PrettyPrinter becomes as simple as that

public class PrettyPrinter<T> : SpecificationVisitor<T>
{
  private readonly StringBuilder sb;
  public PrettyPrinter()
  {
    this.sb = new StringBuilder(250);
  }
  public override void Visit(Specification<T> specification)
  {
    this.sb.Append(specification.Description);
  }
  public override void Visit(CompositeSpecification<T> composite)
  {
    this.sb.Append("(");
    foreach (Specification<T> child in composite.Children)
    {
      child.Accept(this);
      this.sb.Append(composite.Description);
    }
    int l = composite.Description.Length;
    this.sb.Remove(sb.Length - l, l);
    this.sb.Append(")");
  }
  public override string ToString()
  {
    return this.sb.ToString();
  }
}

And this gives you a nice and friendly printout of your graph

var spec = new AlwaysFalse().Or(new AlwaysTrue().And(new Odd())).Or(new AlwaysTrue());
var printer = new PrettyPrinter<T>();
spec.Accept(printer);
string friendly = printer.ToString(); // (false || (true && Odd) || true)

First time I tried to attach some zipped source code and found out that WordPress won’t let me… Get the source code here (project TecX.Common folder Specifications and the test suite that puts them to use in TecX.Common.Test).

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Rebuttal: Composite Pattern

Oren Eini (aka Ayende Rahien) has an interesting series on his blog where he reviews the GoF Design Patterns and how they apply (or don’t) to modern software development.

As part of this series he also reviews the Composite Pattern and finishes with the following advice:

Recommendation: If your entire dataset can easily fit in memory, and it make sense, go ahead. Most of the time, you probably should stay away.

I find his conclusion somewhat limited as he only seems to take data storage/structures into account. But what about code that “does” things?

If you consider the following interface:

public interface IWriter
{
  void Write(Foo foo);
}

there may be a lot of implementations that write somewhere. Like the local file system. A database. A WCF service or any number of other targets. And what if you want to write to several of those targets and not only one? Do you want to change your code to handle a List<IWriter>? Everywhere? No? Me neither. Instead I prefer to leave my existing code that knows how to write to a single target as is and introduce a composite writer that does the job of writing to multiple targets for me.

public class CompositeWriter : IWriter
{
  private readonly List<IWriter> writers = new List<IWriter>();
  public void Write(Foo foo)
  {
    foreach(var writer in this.writers)
    {
      writer.Write(foo);
    }
  }
  public void Add(IWriter writer)
  {
    this.writers.Add(writer);
  }
}

The sample ignores validation or error handling (Does the composite ignore errors in its targets and just continues? Do the targets have to handle the errors themselves etc.). It also ignores the possible complexity of the Foo it has to write. This is where I agree with Oren. If this structure is large and you need to duplicate it for every Write() you are screwed.

I also ignore the delay that might be introduced by writing to an arbitrarily large set of targets. But you can also handle that inside the composite. Maybe you introduce timeouts for calling a single writer. Or you write a decorator that calls a writer on a background thread if that fits your needs. You can make your solution as complex or simple as you have to. The fact is: Your original code won’t know about that complexity. It just calls an implementation of your IWriter interface. I think this is what SRP is about.

You isolate the callers from the possibly complex task of calling out to multiple targets. Your composite is responsible for dealing with that complexity. And that is it’s only responsibility.