左外连接错误

Question

I'm trying to get this left outer join working, but I seem to be encountering some issues. I have taken the example code from the MSDN left join article . This example is in LINQ syntax, but I want mine in extension method syntax, so I have also referenced this SO question.

Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };

Pet barley = new Pet { Name = "Barley", Owner = terry };
Pet boots = new Pet { Name = "Boots", Owner = terry };
Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte };
Pet bluemoon = new Pet { Name = "Blue Moon", Owner = terry };
Pet daisy = new Pet { Name = "Daisy", Owner = magnus };

// Create two lists.
List<Person> people = new List<Person> { magnus, terry, charlotte, arlene };
List<Pet> pets = new List<Pet> { barley, boots, whiskers, bluemoon, daisy };

var query = people
            .GroupJoin(pets,
            p1 => p1.FirstName,
            p2 => p2.Owner.FirstName,
            (p1, p2) => new {p1,p2})
            .SelectMany(x => x.p2.DefaultIfEmpty(),
             (x, y) => new { FirstName = x.p1.FirstName, PetName = y.Name });

        foreach (var v in query)
        {
            Console.WriteLine("{0,-15}{1}", v.FirstName + ":", v.PetName);
        }

My query is basically identical to the code I referenced, but I am getting this error:

NullReferenceException
Object reference not set to an instance of an object. 

This should be extremely simple. What am I doing wrong?

Answer 1

You're missing that part of sample query:

PetName = (subpet == null ? String.Empty : subpet.Name)

On your query it's just PetName = y.Name , so whenever there is no corresponding row in pets list you're getting NullReferrenceException because y is null .

Should be:

var query = people
            .GroupJoin(pets,
            p1 => p1.FirstName,
            p2 => p2.Owner.FirstName,
            (p1, p2) => new { p1, p2 })
            .SelectMany(x => x.p2.DefaultIfEmpty(),
                (x, y) => new { FirstName = x.p1.FirstName, PetName = (y == null ? String.Empty : y.Name) });

Or you can use DefaultIdEmpty(TSource) method overload:

var def = new Pet { Name = string.Empty };

var query = people
            .GroupJoin(pets,
            p1 => p1.FirstName,
            p2 => p2.Owner.FirstName,
            (p1, p2) => new { p1, p2 })
            .SelectMany(x => x.p2.DefaultIfEmpty(def),
                (x, y) => new { FirstName = x.p1.FirstName, PetName = y.Name });