Specifications
Structr.Specifications package provides .NET implementation of Specification pattern with set of simple operations with them.
dotnet add package Structr.Specifications
Main class of package -
Specification<T> - should be inherited by your own classes in order to use its functionality and extensions.Example of basic usage:
// Some model.
public class Book
{
public string Title { get; set; }
public string Author { get; set; }
public int Year { get; set; }
public int Price { get; set; }
}
// Specification matching only books written after some year.
public class BookYearIsGreaterThanSpec : Specification<Book>
{
public int Year { get; }
public BookYearIsGreaterThanSpec(int year)
=> Year = year;
// The only method to be overridden - ToExpression().
// It should return expression that gives needed condition.
public override Expression<Func<Book, bool>> ToExpression()
{
return x => x.Year > Year;
}
}
// Specification matching only books which have price less than some value.
public class BookPriceIsLessThanSpec : Specification<Book>
{
public int Price { get; }
public BookPriceIsLessThanSpec(int price)
=> Price = price;
public override Expression<Func<Book, bool>> ToExpression()
{
return x => x.Price < Price;
}
}
Use specifications to filter models collection:
// Books collection.
var books = new List<Book>
{
new Book { Title = "Don Quixote", Author = "Miguel de Cervantes", Year = 1605, Price = 16 },
new Book { Title = "The Brothers Karamazov", Author = "Fyodor Dostoevsky", Year = 1879, Price = 13 },
new Book { Title = "The Lord of the Rings", Author = "J.R.R. Tolkien", Year = 1954, Price = 32 },
new Book { Title = "One Hundred Years of Solitude", Author = "Gabriel Garcia Marquez", Year = 1967, Price = 21 },
};
// Create specification instances.
var newBookSpec = new BookYearIsGreaterThanSpec(1950);
var cheapBookSpec = new BookPriceIsLessThanSpec(25);
// Then filter books collection.
IEnumerable<Book> newBooks = books
.Where(x => newBookSpec.IsSatisfiedBy(x)); // Gives "The Lord of the Rings" and "One Hundred Years of Solitude".
IEnumerable<Book> cheapBooks = books
.Where(x => cheapBookSpec.IsSatisfiedBy(x)); // Gives "Don Quixote", "The Brothers Karamazov" and "One Hundred Years of Solitude".
// You also can combine two specifications.
// For example: Cheap and "new" books.
Specification<Book> cheapAndNewBooksSpec = cheapBookSpec.And(newBookSpec);
IEnumerable<Book> cheapAndNewBooks = books
.Where(x => cheapAndNewBooksSpec.IsSatisfiedBy(x)); // Gives only "One Hundred Years of Solitude".
// For example: Cheap and "old" books.
Specification<Book> cheapAndOldBooksSpec = cheapBookSpec.AndNot(newBookSpec);
IEnumerable<Book> cheapAndOldBooks = books
.Where(x => cheapAndOldBooksSpec.IsSatisfiedBy(x)); // Gives only "Don Quixote" and "The Brothers Karamazov".
The whole list of
Specification<T> extensions is provided below:Method name | Description |
|---|---|
And<T> | Creates specification which will be satisfied only when both specifications will be satisfied by provided instance of type T. |
Or<T> | Creates specification which will be satisfied when at least one two specifications will be satisfied by provided instance of type T. |
Not<T> | Creates specification which will be satisfied when given specification won't be satisfied by provided instance of type T. |
AndNot<T> | Creates specification which will be satisfied when first specification will AND second will not be satisfied by provided instance of T. |
OrNot<T> | Creates specification which will be satisfied when first specification will OR second will not be satisfied by provided instance of T. |
Additionally two predefined specifications are available:
Name | Description |
|---|---|
AnySpecification<T> | Specification to which all objects of T will match. |
NoneSpecification<T> | Specification to which none of objects of T will match. |
Structr.Specifications may be helpful with Entity Framework 6 or Entity Framework Core. Use
ToExpression() method with Where() for filtering entities in DbContext:// Create specification instances.
var newBookSpec = new BookYearIsGreaterThanSpec(1950);
var cheapBookSpec = new BookPriceIsLessThanSpec(25);
// Then filter books entities.
List<Book> newBooks = dbContext.Books
.Where(x => newBookSpec.ToExpression(x))
.ToList(); // Gives "The Lord of the Rings" and "One Hundred Years of Solitude".
List<Book> cheapBooks = books
.Where(x => cheapBookSpec.ToExpression(x))
.ToList(); // Gives "Don Quixote", "The Brothers Karamazov" and "One Hundred Years of Solitude".
Last modified 10mo ago