NMSSpecification - Specification pattern on .net

What is the Specification Pattern?

When selecting a subset of objects, it allows to separate the statement of what kind of objects can be selected from the object that does the selection.

Ex:

A cargo has a separate storage specification to describe what kind of container can contain it. The specification object has a clear and limited responsibility, which can be separated and decoupled from the domain object that uses it.

What can you do with Specification Pattern?

1. Validate an object or check that only suitable objects are used for a certain role
2. Select a subset of objects based a specified criteria, and refresh the selection at various times
3. Describe what an object might do, without explaining the details of how the object does it, but in such a way that a candidate might be built to fulfill the requirement

How to use:

1: Linq expression based Specification

use Specification<T> as parent class:

public class ExpensiveProductSpecification : Specification<Product>
{
    public override Expression<Func<Product, bool>> AsExpression()
    {
        return x=>x.Price>=100000; //criteria
    }
}

//----------------------------------------------------------------

public class GoldColorProductSpecification : Specification<Product>
{
    public override Expression<Func<Product, bool>> AsExpression()
    {
        return x=>x.Color == ProductColor.Gold;
    }
}

2: Composition based Specification

use CompositeSpecification<T> as parent class:

public class LuxuryProductSpecification : CompositeSpecification<Product>
{
    public override bool IsSatisfiedBy(Product candidate)
    {
        var specs =  new ExpensiveProductSpecification().And(new GoldColorProductSpecification());
        return specs.IsSatisfiedBy(candidate);
     }
}

References:

http://martinfowler.com/apsupp/spec.pdf

https://domainlanguage.com/ddd/