Question

Lets say that I have a particular way of deciding whether some strings “match”, like this:

public bool stringsMatch(string searchFor, string searchIn)
{
  if (string.IsNullOrEmpty(searchFor))
  {
    return true;
  }

  return searchIn != null &&
    (searchIn.Trim().ToLower().StartsWith(searchFor.Trim().ToLower()) ||
     searchIn.Contains(" " + searchFor));
}

I would like to pull matches out of a database using Linq To Entities and this helper. However, when I try this:

IQueryable<Blah> blahs = query.Where(b => stringsMatch(searchText, b.Name);

I get “LINQ to Entities does not recognize the method…”

If I re-write the code as:

IQueryable<Blah> blahs = query.Where(b =>
      string.IsNullOrEmpty(searchText) ||
      (b.Name != null &&
        (b.Name.Trim().ToLower().StartsWith(searchText.Trim().ToLower()) ||
         b.Name.Contains(" " + searchText)));

Which is logically equivalent, then things work fine. The problem is that the code isn’t as readable, and I have to re-write it for each different entity I want to match.

As far as I can tell from questions like this one, what I want to do is impossible at the moment, but I’m hoping that I’m missing something, am I?

Answer 1

Using a freely available library called LINQKit (as mentioned by @Eranga) this task becomes reasonable. Using LINQKit the code I have now looks like:

protected Expression<Func<T, bool>> stringsMatch(string searchFor, Expression<Func<T, string>> searchIn)
{
  if (string.IsNullOrEmpty(searchFor))
  {
    return e => true;
  }

  return
    e =>
    (searchIn.Invoke(e) != null &&
      (searchIn.Invoke(e).Trim().ToLower().StartsWith(searchFor.Trim().ToLower()) ||
       searchIn.Invoke(e).Contains(" " + searchFor)));
}

And needs to be called like this (note the AsExpandable() call)

IQueryable<Blah> blahs = query().AsExpandable().Where(StringsMatch(searchText, b => b.Name));

The magic parts are the searchIn.Invoke(e) calls and the use of AsExpandable() which adds a wrapper layer that allows them to work.

The AsExpandable() bit is explained in detail by the original author here.

Note that I’m still a bit hazy on some of the details of expressions, so please add a comment/edit this answer if it can be made better/shorter/clearer.