• It allows to encapsulate some piece of domain knowledge into a single unit (specification) and reuse it in different parts of the code base.
• There are 3 main use cases for the Specification pattern:
  • Looking up data in the database.
    • That is finding records that match the specification we have in hand.
  • Validating objects in the memory.
    • In other words, checking that an object we retrieved or created fits the spec.
  • Creating a new instance that matches the criteria.
    • This is useful in scenarios where you don’t care about the actual content of the instances, but still, need it to have certain attributes.
• Generic specifications are a bad practice.
  • If a specification allows you to indicate an arbitrary condition, it becomes just a container for the information which is passed to it by its client and doesn’t solve the underlying problem of domain knowledge encapsulation.
  • Such specifications simply don’t contain any knowledge themselves.
  • We can resolve this by using strongly-typed specifications that hard code the domain knowledge, with little or no possibility to alter it from the outside.

public abstract class Specification<T>
{
    public abstract Expression<Func<T, bool>> ToExpression();

    public bool IsSatisfiedBy(T entity)    
	  {
        Func<T, bool> predicate = ToExpression().Compile();
        return predicate(entity);
    }
}

public class MovieRatingAtMostSpecification : Specification<Movie>
{
    private readonly MovieRating _rating;

    public MovieRatingAtMostSpecification(MovieRating rating) => _rating = rating; 

    public override Expression<Func<Movie, bool>> ToExpression() => (movie => movie.MovieRating <= _rating);
}

public void SomeMethod() // Controller
{
    var gRating = new MovieRatingAtMostSpecification(MovieRating.G);

    var isOk = gRating.IsSatisfiedBy(movie); // Exercising a single movie

    IReadOnlyList<Movie> movies = repository.Find(gRating);
}

public IReadOnlyList<T> Find(Specification<T> specification) // Repository
{
    using (ISession session = SessionFactory.OpenSession())
        return session.Query<T>().Where(specification.ToExpression()).ToList();
}

• With this approach, we lift the domain knowledge to the class level making it much easier to reuse.
  • No need to keep track of spec instances anymore: creating additional specification objects doesn’t lead to domain knowledge duplication, so we can do it freely.
  • Also, it’s really easy to combine the specifications using And, Or, and Not methods.

public abstract class Specification<T>
{
    public Specification<T> And(Specification<T> specification) => new AndSpecification<T>(this, specification);		
		public Specification<T> Or(Specification<T> specification) => new OrSpecification<T>(this, specification);
}

public class AndSpecification<T> : Specification<T>
{
    private readonly Specification<T> _left;
    private readonly Specification<T> _right; 

    public AndSpecification(Specification<T> left, Specification<T> right)
    {
        _right = right;
        _left = left;
    } 

    public override Expression<Func<T, bool>> ToExpression()
    {
        Expression<Func<T, bool>> leftExpression = _left.ToExpression();
        Expression<Func<T, bool>> rightExpression = _right.ToExpression();
 
        BinaryExpression andExpression = Expression.AndAlso(leftExpression.Body, rightExpression.Body);
        return Expression.Lambda<Func<T, bool>>(andExpression, leftExpression.Parameters.Single());
    }
}

public class OrSpecification<T> : Specification<T> 
{
	private readonly Specification<T> _left;
	private readonly Specification<T> _right;

	public OrSpecification(Specification<T> left, Specification<T> right) 
	{
		_right = right;
		_left = left;
	}

	public override Expression<Func<T, bool>> ToExpression() 
	{
		var leftExpression = _left.ToExpression();
		var rightExpression = _right.ToExpression();
		var paramExpr = Expression.Parameter(typeof(T));
		var exprBody = Expression.OrElse(leftExpression.Body, rightExpression.Body);
		exprBody = (BinaryExpression)new ParameterReplacer(paramExpr).Visit(exprBody);
		var finalExpr = Expression.Lambda<Func<T, bool>>(exprBody, paramExpr);

		return finalExpr;
	}
}

• A question that is somewhat related to the specification pattern is: can repositories just return an **IQueryable<T>**?
  • Wouldn’t it be easier to allow clients to query data from the backing store the way they want?
  • This approach has essentially the same drawback as our initial specification pattern implementation.
    • It encourages us to violate the DRY principle by duplicating the domain knowledge.
    • This technique doesn’t offer us anything in terms of consolidating it in a single place.
  • The second drawback here is that we are getting database notions leaking out of repositories.
    • The implementation of IQueryable<T> highly depends on what LINQ provider is used behind the scene, so the client code should be aware that there potentially are queries which can’t be compiled into SQL.
  • We are also getting a potential LSP violation.
    • **IQueryables** are evaluated lazily, so we need to keep the underlying connection opened during the whole business transaction.
    • Otherwise, the method will blow up with an exception.