How to use Specifications with the Repository Pattern

If you’re inclined to use the Repository Pattern, the specifications can help eliminate the common pain points and reduce the number of methods in the repositories. This library exposes an ISpecificationEvaluator evaluator implementation for EF Core and EF 6, which can be utilized to build the desired queries. The default evaluator implementation is a stateless object, and a singleton instance can be retrieved through SpecificationEvaluator.Default.

Let’s assume we have the following entity.

public class Hero
{
    public string Name { get; set; }
    public string SuperPower { get; set; }
    public bool IsAlive { get; set; }
    public bool IsAvenger { get; set; }
}

We can implement a repository for Hero as follows. Instead of having multiple methods (e.g. GetAllAliveHeroes, GetAllAvengerHeroes, etc.), we may have a single method that accepts a specification as a parameter.

public interface IHeroRepository
{
    List<Hero> ListHeroes(Specifcation<Hero> specification);
}

public class HeroRepository : IHeroRepository
{
    private readonly HeroDbContext _dbContext;

    public HeroRepository(HeroDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public List<Hero> ListHeroes(ISpecification<Hero> spec)
    {
        var query = SpecificationEvaluator.Default.GetQuery(_dbContext.Heroes, spec);
        return query.ToList();
    }
}

Once we have that in place, we may create and use various specifications to retrieve the desired data from the repository.

public class AliveHeroSpec : Specification<Hero>
{
    public AliveHeroSpec()
    {
        Query.Where(h => h.IsAlive == true);
    }
}

public class AliveAvengerHeroSpec : Specification<Hero>
{
    public AliveAvengerHeroSpec()
    {
        Query.Where(h => h.IsAlive == true && x.IsAvenger == true);
    }
}

var aliveHeroes = _repository.ListHeroes(new AliveHeroSpec());
var aliveAvengerHeroes = _repository.ListHeroes(new AliveAvengerHeroSpec());

How to use Specifications with the Repository Pattern

Further Reading