Entity Framework - joining nulls - NullReferenceException was unhandled by user code

Question

I am writing an asp.net mvc application to learn Entity Framework with and am running into an issue that I do not know how to handle. For simplicity lets take the following table structure:

Movie  
ID (int, not null, auto increment)  
Name (varchar)  
GenreID (int)

and

Genre  
ID (int, not null, auto increment)  
Name (varchar)

Movie.GenreID is a FK reference to Genre.ID

I have brought across all of the tables using the visual designer in VS 2008 and tried the following Linq query:

IEnumerable<Movie> movieList = from f in dataContext.MovieSet.Include("Genre").ToList();

I can output the data in a view using:

<%= Html.Encode( movieList.Genre.Name ) %>

Everything works just fine until I have an item in the Movie table with a null GenreID. Is there something I can do to this query to make it still be able to output (just leave it blank when applicable) or am I doing something horribly wrong?

Answer 1

The problem is that movieList.Genre is null, and you can't access the Name property of a null object.

You can solve this by writing <%= Html.Encode(movieList.Genre == null ? String.Empty : movieList.Genre.Name) %>.

If you don't want your views to be so verbose, you could add a GenreName property to the Movie entity class and move the null check there.

Answer 2

This is probably much more readable than .Include("")

from f in dataContext.MovieSet
select new
    {
       Name = f.Name,
       Genre = f.Genre // This effectively performs a join.
       ...
    }

You can also check for the problem that way:

from f in dataContext.MovieSet
select new
    {
       Name = f.Name,
       GenreName = f.Genre == null ? "" : f.Genre.Name
       ...
    }

This gives you more flexibility, for instance:

from f in dataContext.Genres
select new
    {
        Name = f.Name
        Movies = from movie in f.Movies
                 where movie.Duration > 240
                 select movied
    }