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.

Just released another version of LINQ.Flickr. The release is out with several bug fixes, code optimization, new feature and overall mocking support. I have used Typemock for the unit test of the product. In coming posts, I will show how powerful mock can be in faking routine like upload photo. But, you can dig it right away, if you go by the release page and download a copy of the product. Truly speaking, testing was never fun for serviced API till mock engine is at my hand.

The release is not all about re-factoring and mocking , but now you can query, add , delete photo comment , query people and popular tags and do more that are mentioned in the release page.

Now, doing complex query is even more fun. lets take this.

var query = from photo in _context.Photos
where photo.Id == photoId && photo.PhotoSize == PhotoSize.Medium
select new FlickrPhoto
{
    Id = photo.Id,
    Description = photo.Description,
    Title = photo.Title,
    Url = photo.Url,
    User = photo.User,
    Comments = ((from comment in _context.Photos.Comments
                 where comment.PhotoId == photoId
                 select new PhotoComment
                 {
                     Author = comment.Author,
                     AuthorName = comment.AuthorName,
                     PermaLink = comment.PermaLink,
                     PhotoId = comment.PhotoId,
                     Text = comment.Text,
                 }).ToList<PhotoComment>()),
    Tags = photo.PhotoTags.Select(tag => tag.Title).ToArray<string>()
};

FlickrPhoto newPhoto = query.Single();

The query is about getting the photo info for a specific photo id and I have now combined it with tags, comments to make it more useful. Also, few posts back, I have mentioned about using a RestToCollecitonBuilder  to build an object on REST response,  it is in action in this release as well.

Finally, don't forget the link and it's www.codeplex.com/linqflickr . In the end, this project is not about querying Flickr and getting photo out of it, but it could be a great learning tool for Mocking, for building custom LINQ provider (with LinqExtender) and other things that you might need to know like, how to define a service endpoint by interface.