Question

I am currently struggling to assemble a LINQ query on my objects collections : Persons, Cars

Each person can have multiple cars.

I want to select all PERSONS in persons and all group all cars owned by this person. The query I have written so far is:

from c in persons,d in cars 
    where c.id = d.ownerID 
    group by c.Firstname into MG = tolist() 

but it only returns persons who have cars. If a person does not have car, he is not in list. I cannot make up with the right logic.

Answer 1

try:

List<person> plist = new List<person>();
        plist.Add(new person(1, "a"));
        plist.Add(new person(2, "b"));
        plist.Add(new person(3, "c"));
        plist.Add(new person(4, "d"));

        List<cars> clist = new List<cars>();
        clist.Add(new cars(1, "c1"));
        clist.Add(new cars(1, "c2"));
        clist.Add(new cars(1, "c5"));
        clist.Add(new cars(2, "c1"));
        clist.Add(new cars(3, "c1"));
        clist.Add(new cars(3, "c5"));
        clist.Add(new cars(3, "c3"));
        clist.Add(new cars(3, "c2"));
        clist.Add(new cars(4, "c2"));
        clist.Add(new cars(4, "c5"));


        var result = from p in plist
                join c in clist on p.id equals c.ownerID into k
                from s in k.DefaultIfEmpty()
                select new { p.firstName , carName = (s == null ? String.Empty :s.name)};

string sss = "";
 foreach (var v in result)
 {
      sss+= ( v.firstName + " : " + v.carName +  " >> "+"\n");
 }
 textBox1.Text = sss;

and classes are :

class person
{
    public int id;
    public string firstName;

    public person(int id1, string name)
    {
        id = id1;
        firstName = name;
    }
}

class cars
{
    public int ownerID;
    public string name;

   public cars(int id,string name1)
    {
        ownerID = id;
        name = name1;
    }
}