This is a simple extension for IQueryable that allow the use of strongly typed specification objects. This can be useful when reusing and combining predicates in ORMs such as EnitityFramework.

Generally, you should inherit from Rh.Specifiable.Specification.ExpressionSpecification<T>. E.g.

public class BlogPostIsPublished : ExpressionSpecification<BlogPost>
{
    protected override Expression<Func<BlogPost, bool>> Expression { get; }

    public IsPublic()
    {
        Expression = b => b.VisibilityId == PostingVisibilityId.Public;
    }
}

Then you can use it directly in a Where call:

myDbContext.BlogPosts.Where(new BlogPostIsPublished());

There are also the AndSpecification and OrSpecification base classes which allow for strongly typed combinations of smaller specifications.

In order to use a specification inside an expression tree, e.g. in a Select call, you first need to call AsSpecifiable() on the source IQueryable<T>. That returns a VisitableQuery which visits the expression and replaces the Specifications with their corresponding Expression. You only need to do this once per queryable but it doesn't matter if you call it multiple times.

// AsSpecifiable enables specifications in the Select Expression tree.
myDbContext.BlogPosts.AsSpecifiable().Select(post => new BlogPostModel 
    { 
        Id = post.Id, 
        Content = post.Content, 
        Comments = post.Comments.AsQueryable().Where(new CommentIsPublic()) 
    });