asarnaout/FluentSpec

Basic Implementation of the Specification Pattern

A simple library providing an implementation for the Specification Pattern and supporting Composite Specifications through Fluent APIs.

Specification Pattern:

Specifications are value objects that are capable of verifying whether an argument fits specific criteria. Specifications are mainly used in:

• Verifying Business Rules
• Querying Data
• Generating Data

Specifications are best used in C# to encapsulate complex LINQ expressions that are well hidden and duplicated across the codebase. Using Specifications in conjunction with Repositories allows for a simpler Repository interface.

Note that Specifications should be only used when necessary and abusing this pattern would highly increase the code complexity.

Usage:

To define a Specification, simply declare a class and extend the Specification<T> type. The extended class should feed the Expression Tree representing the specification to the constructor.

public class PremiumCustomerSpecification : Specification<Customer> 
{
    public PremiumCustomerSpecification () : base (x => DateTime.Now.Year - x.MemberSince.Year > 10) 
    { 
    }
}

To minimize the number of created objects, a Flyweight Factory SpecificationFactory is provided to serve the required Specifications:

using static FluentSpec.SpecificationFactory;

public class SpecificationDemo 
{
  public void Test()
  {
    var spec = Default<PremiumCustomerSpecification>();
  }
}

Note that clients are allowed to bypass the SpecificationFactory and directly create new instances of their Specifications in code.

Use the IsSatisfied method to verify whether an argument meets the criterion specified in ExpressionTree

var spec = Default<PremiumCustomerSpecification>();
var customer = new Customer();
var isSatisfied = spec.IsSatisfied(customer);

Chain Specifications to produce more complex Specifications

var spec = Not(Default<AdultCustomerSpecification>()).And(Default<ValidCustomerNameSpecification>())    
                             .Or(Default<PremiumCustomerSpecification>());

Note that Specifications are immutable Value Objects, and thus any chaining operation would result in a new Specification object being spawned.