我偶然尝试在 LINQ 查询中使用我的规范。这里的麻烦在于我的参数规范。
让我们伪造一个简单的场景：
public class Car {
public Guid Id { get; set; }
public string Color { get; set; }
public int UsedPieces { get; set; }
// whatever properties
}
public class Piece {
public Guid Id { get; set; }
public string Color { get; set; }
// whatever properties
}
public static class PieceSpecifications : ISpecification<Piece> {
public static ISpecification<Piece> WithColor(string color) {
return new Specification<Piece>(p => p.Color == color);
}
}
我真正想做的事
// Get accepts ISpecification and returns IQueryable<Car> to force just one call to database
var carWithPieces = _carRepository.Get(CarSpecifications.UsedPiecesGreaterThan(10));
var piecesWithColor = from p in _pieceRepository.Get()
let car = carWithPieces.FirstOrDefault() // entire query will does one call to database
where PieceSpecifications.WithColor(car.Color).IsSatisfiedBy(p) // unfortunately it isn't possible
// where p.Color == car.Color -> it works, but it's not what I want
select p;
我知道这有点令人困惑，但我试图避免在我的真实（大）场景中进行大量往返，而且我知道实际上不可能将原始 LINQ 与实体框架一起使用。我厌倦了尝试这么多博客和失败的（我的）方法。有人知道一些真正的好方法。还有另一种方法吗？
错误
System.NotSupportedException：LINQ to Entities 无法识别方法“Boolean IsSatisfiedBy(App.Model.Piece)”方法，并且此方法无法转换为存储表达式。
更新
基本规格模式
public class Specification<T> : ISpecification<T> {
private readonly Expression<Func<T, bool>> _predicate;
public Specification(Expression<Func<T, bool>> predicate) {
_predicate = predicate;
}
public Expression<Func<T, bool>> Predicate {
get { return _predicate; }
}
public bool IsSatisfiedBy(T entity) {
return _predicate.Compile().Invoke(entity);
}
}
更新
如果我这样做很容易整洁
// call to database
var car = _carRepository
.Get(CarSpecifications.UsedPiecesGreaterThan(10))
.FirstOrDefault();
// Whoah! look I'm working, but calling to database again.
var piecesWithColor = _pieceRepository
.Get(PieceSpecifications.WithColor(car.Color))
.ToArray();
存储库
// The Get function inside repository accepts ISpecification<T>.
public IQueryable<T> Get(ISpecification<T> specification) {
return Set.Where(specification.Predicate);
}