Geek asks and answers

How to store Linq, where the condition is owned

I would like to know the most convenient way (if exists) for storing an entity condition in a parameter, in order to reuse it in every linq where the conditions are.

Suppose a Product object:

public class Product
{
    public bool IsDiscontinued { get; set; }

    public int UnitsInStock { get; set; }
}

I would like to add the IsOnSale property to the Product class, which contains logic to determine if a product is for sale or not. In this simple example, the logic could be:

IsOnSale = IsDiscontinued == false && UnitsInStock > 0

Then I would have to write a linq query of this type:

from p in context.Products
where p.IsOnSale == true
select p

The goal of the solution is that if in the future the logic will determine whether the product is being sold or not changing (for example, adding the IsBackOrderAllowed property), I do not need to edit linq queries everywhere, but just need to change the IsOnSale property.

here, , -, .

+3
4

, IQueryable, :

public IQueryable<Product> WhereOnSale(this IQueryable<Product> source)
{
    return source.Where(p => p.IsDiscontinued == false && p.UnitsInStock > 0);
}

:

from p in context.Products.WhereOnSale()
select p

, , :

Expression<Func<Product, bool>> isOnSale = 
(p => p.IsDiscontinued == false && p.UnitsInStock > 0);

:

context.Products.Where(isOnSale)

, , Entity Framework SQL, IL .

+1

- :

Func<Product, bool> isOnSaleFunc = 
                          (p => p.IsDiscontinued == false && p.UnitsInStock > 0);

:

context.Products.Where(isOnSaleFunc)

@DoctaJonez - ( ), Expression<Func<Product, bool>> Func<Product,bool>.

+2

, linq , (= ).

. Func, Linq :

public class Product
{
    ...

    public static Expression<Func<Product, bool>> IsOnSale
    {
        get
        {
            return p => (p.IsDiscontinued == false && p.UnitsInStock > 0);
        }
    }
}

:

var query = context.Products.Where(Product.IsOnSale);

.

+1

, .

EF, , http://huyrua.wordpress.com/2010/07/13/entity-framework-4-poco-repository-and-specification-pattern/.

An example of how to implement this is to make your request

from p in context.Products
where ProductSpecifications.IsOnSale.Predicate
select p

and use the following auxiliary and specification definition.

public static class ProductSpecifications{
     public static readonly Specification<Product> IsOnSale = new Specification<Product>(x => !x.IsDiscontinued && x.UnitsInStock > 0);  
}

public class Specification<TEntity> : ISpecification<TEntity>
{
    public Specification(Expression<Func<TEntity, bool>> predicate)
    {
        _predicate = predicate;
    }

    public bool IsSatisfiedBy(TEntity entity)
    {
        return _predicate.Compile().Invoke(entity);
    }

    private Expression<Func<TEntity, bool>> _predicate;

    public Expression<Func<TEntity,bool>> Predicate{ 
         get{ return _predicate; }
    }
}

You can do a lot more with this template, so I suggest exploring it!

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