specification-pattern-generator

Specification Pattern Generator

Turn repeated query criteria into reusable, unit-testable Specification objects, with an evaluator that applies them to an EF IQueryable.

Input — point it at your code

Works on a target: a folder, project, or GitHub URL. Spot duplicated query logic to consolidate: grep -rn "\.Where(\|\.Include(\|\.OrderBy" --include=*.cs <target>.

Base specification

public abstract class Specification<T>
{
    public Expression<Func<T, bool>>? Criteria { get; protected set; }
    public List<Expression<Func<T, object>>> Includes { get; } = new();
    public Expression<Func<T, object>>? OrderBy { get; private set; }
    public int? Take { get; private set; }
    public int? Skip { get; private set; }
    public bool AsNoTracking { get; private set; } = true;

    protected void Where(Expression<Func<T, bool>> c) => Criteria = c;
    protected void Include(Expression<Func<T, object>> i) => Includes.Add(i);
    protected void Order(Expression<Func<T, object>> o) => OrderBy = o;
    protected void Page(int skip, int take){ Skip = skip; Take = take; }
    protected void Tracked() => AsNoTracking = false;
}

Evaluator

public static class SpecificationEvaluator
{
    public static IQueryable<T> Apply<T>(IQueryable<T> q, Specification<T> spec) where T : class
    {
        if (spec.AsNoTracking) q = q.AsNoTracking();
        if (spec.Criteria is not null) q = q.Where(spec.Criteria);
        q = spec.Includes.Aggregate(q, (cur, inc) => cur.Include(inc));
        if (spec.OrderBy is not null) q = q.OrderBy(spec.OrderBy);
        if (spec.Skip is not null) q = q.Skip(spec.Skip.Value);
        if (spec.Take is not null) q = q.Take(spec.Take.Value);
        return q;
    }
}

Example spec + usage

public class ActiveOrdersForCustomer : Specification<Order>
{
    public ActiveOrdersForCustomer(Guid customerId)
    {
        Where(o => o.CustomerId == customerId && o.Status == OrderStatus.Active);
        Include(o => o.Lines);
        Order(o => o.CreatedAt);
    }
}

var orders = await SpecificationEvaluator
    .Apply(db.Orders, new ActiveOrdersForCustomer(id)).ToListAsync(ct);