Linq反对右外部联接

Question

I have written following linq query but I am not getting expected result.My Expected result is all the matching record bettween lst and list and all the non matching record from list for example.

I want following result

a,b,c,d,e,f

public class Com : IEqualityComparer<DuplicateData>
    {
        public bool Equals(DuplicateData x, DuplicateData y)
        {
            return x.address.Equals(y.address);
        }

        public int GetHashCode(DuplicateData obj)
        {
            return obj.address.GetHashCode();

        }
    }

static void Run ()
{
    List<string> lst = new List<string>();
    lst.Add("a");
    lst.Add("b");
    lst.Add("c");
    lst.Add("p");

    List<DuplicateData> list = new List<DuplicateData>()
    {
        new DuplicateData{address="a"},
        new DuplicateData{address="a"},
        new DuplicateData{address="a"},
        new DuplicateData{address="b"},
        new DuplicateData{address="b"},
        new DuplicateData{address="c"},
        new DuplicateData{address="d"},
        new DuplicateData{address="e"},
        new DuplicateData{address="f"},
    };

    var dup = list.Distinct(new Com());
    var RightJoin = from x in dup
                    join y in lst
                    on x.address equals y
                    into right
                    from z in right
                    select new
                    {
                        UniqueAddress = z,

                    };

}

Answer 1

Try it like this:

var RightJoin = from x in dup
                join y in lst
                on x.address equals y
                into right
                from z in right.DefaultIfEmpty(x.address)
                select new
                {
                    UniqueAddress = z,
                };

result is (a,b,c,d,e,f)

Working sample: http://ideone.com/MOIhZH

---

Explanation

To make a left/right join in linq you have to use DefaultIfEmpty method, that will yield (default) result when there is no match (the joined result is empty). However, default value for string is null so you have to provide default value from the "left side" collection to see it in the result set.

---

Alternative approach

This is probably more convenient approach. Instead of selecting from z and providing the default value, you will select from x.address - the left side of the join.

var RightJoin = from x in dup
                join y in lst
                on x.address equals y
                into right
                from z in right.DefaultIfEmpty()
                select new
                {
                    UniqueAddress = x.address,
                };

Answer 2

This should work:

var res = from x in list
          join y in lst on x.address equals y into right 
          from xy in right.DefaultIfEmpty()
          select new { UniqueAddress = xy };

result:

{ UniqueAddress = a },
{ UniqueAddress = a },
{ UniqueAddress = a },
{ UniqueAddress = b },
{ UniqueAddress = b },
{ UniqueAddress = c },
{ UniqueAddress = null },
{ UniqueAddress = null },
{ UniqueAddress = null }