1.左连接使用DefaultIfEmpty()；
2.分组时候判断newper.FirstOrDefault() == null ? null: newper.ToList()这个经常出错误，如果不判断，会出现空引用的错误
class Program
{
class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
} class Pet
{
public string Name { get; set; }
public Person Owner { get; set; }
}
static void Main(string[] args)
{
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 = from person in people
join pet in pets on person equals pet.Owner into gj
from subpet in gj.DefaultIfEmpty()
group subpet by person.FirstName into newper
select new { newper.Key, Persons = newper.FirstOrDefault() == null ? null: newper.ToList() }; foreach (var ownerAndPet in query)
{
if (ownerAndPet.Persons != null)
{
foreach (var p in ownerAndPet.Persons)
{
Console.WriteLine(string.Format("{0} is owned by {1}", ownerAndPet.Key, p.Name));
} }
else
{
Console.WriteLine(string.Format("{0} is owned by {1}", ownerAndPet.Key, "没有值"));
}
} // Keep the console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
//output
//Magnus is owned by Daisy
//Terry is owned by Barley
//Terry is owned by Boots
//Terry is owned by Blue Moon
//Charlotte is owned by Whiskers
//Arlene is owned by 没有值