Ang3lFir3 – Life as a Code Poet

July 28, 2008

Specification Pattern and Lambdas

While working on my current project I decided that I had a need to use the Specification Pattern . After finding a clean implementation by Jeff Perrin I set to work creating the specifications that I needed. I quickly realized that we were going to end up having a ton of specifications and sometimes they would only apply to very special cases. Other times they would be very broad cases and they needed to be even more composable than even the fluid interface implemented in Jeff’s implementation wasn’t going to be enough. It after all still required me to create a concrete implementation for each specification, no matter how minuscule it might be.

This is the part where I thought to my self that since i was really only creating implementations for a single method that I could just write a LambdaSpecification and be able to use this for all the special cases I had.

Below is the LambdaSpecification Code:


using System;
using System.Linq.Expressions;

namespace Specification
{
    public class LambdaSpecification<T> : Specification<T>
    {
        private Func<T,bool> _expression;

        public LambdaSpecification(Func<T,bool> expression)
        {
             if(expression ==null) throw new ArgumentNullException(“expression”);
              _expression = expression;
        }

        public override bool IsSatisfiedBy(T obj)
        {
            return _expression(obj);
        }
    }
}

And the Tests:


[Test]
public void LambdaSpecification_CanExecuteSimpleLambda()
{
    var p = new Person() {Name = "Eric"};
    var spec = new LambdaSpecification<Person>(x => x.Name == "Eric");

        Assert.IsTrue(spec.IsSatisfiedBy(p));
}

[Test]
public void LambdaSpecification_CanExecuteComplexLambda()
{
    var p = new Person() {Name = “Eric”, Age = 28};
    var spec = new LambdaSpecification<Person>(x => x.Name == “Eric” &&
                                                                        new IsLegalDrinkingAgeSpecification().IsSatisfiedBy(x));

    Assert.IsTrue(spec.IsSatisfiedBy(p));
}

//Might Not be needed but I wanted to be sure
[Test]
public void LambdaSpecification_CanExecuteLambda_AndUseAndSpecification()
{
    var p = new Person() {Name = “Eric”, Age = 28};
    var spec = new LambdaSpecification<Person>(x => x.Name == “Eric” );

    Assert.IsTrue(spec.And(new IsLegalDrinkingAgeSpecification()).IsSatisfiedBy(p));
}

Comments are welcome and encouraged, especially if you see a reason why I shouldn’t be doing this. Or, if you have any ways to make this better, I would love to hear them. This is the first time I have ever used a lambda as a parameter in my own code and so far i am liking it.

Thanks to Jeff Perrin again for his post on creating a clean implementation of the specification pattern.

**EDIT: Thanks to Greg Beech for his input. I’ve updated the code to reflect his suggestions.

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