c# - How does one use a custom property in a LINQ-to-Entities query?

Question

I have a class Post which is an Entity Framework model. It contains a property like this:

public bool Showable {
  get {
    return this.Public && this.PublishedDate > DateTime.now
  }
}

I can use it in a query like this:

from p in db.Posts where p.Showable select p;

but when I have a property that uses it, like this

public IEnumerable<Post> ShowablePosts {
  get {
    return from p in db.Posts where p.Showable select p;
  }
}

then I can't do:

from p in ShowablePosts where p.Id > 42 select p;

It says:

The specified type member 'Showable' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are supported.

Answer 1

You can do this if you write the property as an Expression which is translatable to SQL.

Here's how to do it.

That's a bit complicated, because it's a general solution to a complicated problem.

The general idea is this: LINQ to Entities (like all LINQ providers) can't translate compiled code like your property into SQL at runtime. LINQ to Objects can execute compiled code, but it can't translate it. But they can translate an Expression<T>. So you could write:

public static Expression<Func<Post, bool>> WhereActive
{
    get
    {
        return p => p.Public && p.PublishedDate > DateTime.Now;
    }
}

Then you could write:

public IEnumerable<Post> ShowablePosts 
{
    get 
    {
        return db.Posts.Where(WhereActive);
    }
}

...and LINQ to Entities could translate that. The code in the post I link generalizes this idea.