SpecPattern

Simple yet powerful implementation of specification pattern in C#.

Relying in IQueryable it covers the three requirements this patterns aims to solve

• Validation
• Querying
• Building

Example

// Check if project is either cool or popular but it should be trustable
var atLeastTrustableSpec = (coolProjectSpec | popularProjectSpec)
								   & trustableProjectSpec ;
atLeastTrustableSpec.IsSatisfiedBy(project);

// Use the same specs but now to query database with an ORM
var allGreatBuUnpopularProjects = Repository.Query<Project>()
								  .Where(greatButUnpopularSpec.Predicate);

Spec Pattern project consists of only five classes and in most of the cases you will only have to know about one.

Creating a new Specification

Simply inherit from Specification and override DefinePredicate method which should return the predicate that defines your specification:

public class Project
{
	public string Name{ get; set; }

	public int Features { get; set; }

	public int Followers { get; set; }

	public bool Stable { get; set; }
}

public class CoolProjectSpec : Specification<Project>
{
	protected override Expression<Func<Project, bool>> DefinePredicate()
	{
		return p => p.Features >= 5;
	}
}

Of course you can work with instance fields inside your predicate if needed (usually you will):

public class CoolProjectSpec : Specification<Project>
{
	private readonly int minFeaturesToBeCool;
	public CoolProjectSpec(int minFeaturesToBeCool)
	{
		this.minFeaturesToBeCool = minFeaturesToBeCool;
	}

	protected override Expression<Func<Project, bool>> DefinePredicate()
	{
		return p => p.Features >= minFeaturesToBeCool;
	}
}

Combining specifications

The main advantage of using specifications is the ability to combine them as if they were predicates, relying on C# operator overloading we can do this with a clean syntax:

Specification<Project> coolProjectSpec = new CoolProjectSpec(5);
Specification<Project> popularProjectSpec = new PopularProjectSpec();
Specification<Project> trustableProjectSpec = new TrustableProjectSpec();

var project = new Project { Features = 5, Followers = 250, Stable = true };

//Check if project rules
var projectRulesSpec = coolProjectSpec & popularProjectSpec 
						& trustableProjectSpec;
projectRulesSpec.IsSatisfiedBy(project);

// Check if project is cool and trustable but unpopular (i know that story)
var greatButUnpopularSpec = coolProjectSpec & trustableProjectSpec 
							& !popularProjectSpec;
greatButUnpopularSpec.IsSatisfiedBy(project);

// Check if project is either cool or popular but it should be trustable
var atLeastTrustableSpec = (coolProjectSpec | popularProjectSpec)
							& trustableProjectSpec ;
atLeastTrustableSpec.IsSatisfiedBy(project);

ORM integration

The best part is you dont have to do anything to integrate it with your ORM; if it supports IQueryable you are done:

// Use the same specs but now to query database with an ORM
var allGreatButUnpopularProjects = Repository.Query<Project>()
								.Where(greatButUnpopularSpec.Predicate);

Nuget Package

PM> Install-Package SpecPattern