Question

I have the below

public class Person
    { 
        // Properties
        public string Name { get; set; }
        public string Area { get; set; }
        public string Gender { get; set; }
        public int Age { get; set; }

        public List<Person> GetPersonData()
        {
            List<Person> personLst = new List<Person>  {
           new Person { Name="Shashidhar Niketani", Age=20, Gender="Male" , Area = "Assam"},
           new Person { Name="Ahmed Ali Khan", Age=25 ,Gender="Male", Area = "Assam" },
           new Person { Name="S. Mirja", Age=20, Gender="Female", Area = "Assam"},
           new Person { Name="Neru Kumar", Age=18, Gender="Female", Area = "Colombo"},
           new Person { Name="Chidam P", Age=19, Gender="Male", Area = "Colombo"},
           new Person { Name="H Kontala", Age=19, Gender="Male", Area = "Bombay"},
           new Person { Name="Priya Pankhraj", Age=23, Gender="Female", Area = "North Punjab"},
             new Person { Name="Ambla", Age=20, Gender="Female", Area = "Madras"},
           new Person { Name="H Kontala", Age=25, Gender="Male", Area = "Bombay"},
           new Person { Name="Sirisha Chalukuri", Age=30, Gender="Female", Area = "Bombay"}};
            return personLst;
        }

        public override string ToString()
        {
            return string.Format("Patient Name: =  {0} , Age:= {1}, Gender:= {2}, Area: = {3}", Name, Age, Gender, Area);
        }

    }

    public class Hobbies
    {
        public string Owner { get; set; }
        public string HobbyName { get; set; }

        public List<Hobbies> GetHobbies()
        {
            List<Hobbies> hobbyList = new List<Hobbies>  {
           new Hobbies { Owner="Sirisha Chalukuri", HobbyName = "Singing"},
           new Hobbies { Owner="Priya Pankhraj", HobbyName = "Cooking" },
           new Hobbies { Owner="S. Mirja", HobbyName="Playing"},
           new Hobbies { Owner="Neru Kumar", HobbyName="Programing"}};
           return hobbyList;
        }
    }

I am trying to perfom a left join

//fetch those records

Person p = new Person();
var personSource = p.GetPersonData();

Hobbies h = new Hobbies();
var hobbySource = h.GetHobbies();

//Left outer join

var res8 = (from person in personSource                       
                        join hobby in hobbySource
                        on p.Name equals hobby.Owner into temp
                        from hobby in temp.DefaultIfEmpty()
                        select new
                            {
                                PersonName = p.Name,
                                PersonAge = p.Age,
                                Gender = p.Gender,
                                LivesIn = p.Area,
                                Hobby = (hobby == null) ? "N/A" : hobby.HobbyName
                            });

But I am not able to get the correct result…All the records are coming as null/non matching records..There are atleast 4 matching records and the rest will be non-matching records…

I am rather looking for like given these two datasource, the result that we will expect if we perform a left join on PersonName and HobbyOwner.

Help needed

Answer 1

When you reference hobby.HobbyName in your select projection, hobby is null when there are no matching hobbies for a person (DefaultIfEmpty returns null for an empty hobby collection). You need to check if hobby is null, and if so assign the Hobby property in the select projection a default value. For example:

select new
{
  PersonName = p.Name,
  PersonAge = p.Age,
  Gender = p.Gender,
  LivesIn = p.Area,
  Hobby = (hobby == null) ? "N/A" : hobby.HobbyName
});

Also, don’t know if it was a typo, but in your query you use p when you should be using person:

var res8 = (from person in personSource                       
        join hobby in hobbySource
        on person.Name equals hobby.Owner into temp
        from hobby in temp.DefaultIfEmpty()
        select new
        {
         PersonName = person.Name,
         PersonAge = person.Age,
         Gender = person.Gender,
         LivesIn = person.Area,
         Hobby = (hobby == null) ? "N/A" : hobby.HobbyName
         });