C# Linq full outer join on repetitive values

Question

I have two IQueryable collections having this kind of type

public class Property  
{  
   public string Name {get; set;}  
}

Collection 1, with the following Name values:

A  
A  
A  
B  

Collection 2, with the following Name values:

A  
B  
B

What I would like to get is a third collection having Name values from Collections 1 and 2 matched, and if there is no match, than null (empty), so as follows:

Result Collection:  

A     A
A     null  
A     null  
B     B  
null  B

How is it possible to achieve this with C#, LINQ?

Answer 1

Seems to be a lot of interest for this question so I have attempted to come up with a more generalised solution. I have taken inspiration from this link https://www.codeproject.com/Articles/488643/LinQ-Extended-Joins.

I've created a fullouterjoin extension method which does what the op asks for. Not sure if fullouterjoin is the right name though.

I have used my extension method to solve the ops problem.

using System;
using System.Collections.Generic;
using System.Linq;


namespace Testing
{


    public class Property
    {
        public string Name { get; set; }
    }

    public class JoinedProperty
    {
        public Property Name1 { get; set; }
        public Property Name2 { get; set; }

        public override string ToString()
        {
            return (Name1 == null ? "" : Name1.Name)
                + (Name2 == null ? "" : Name2.Name);
        }  
    }

    class Program
    {
        static void Main(string[] args)
        {
            var list1 = new List<Property>
        {
            new Property{ Name = "A" },
            new Property{ Name = "A" },
            new Property{ Name = "A" },
            new Property{ Name = "B" }
        };

            var list2 = new List<Property>
        {
            new Property{ Name = "A" },
            new Property{ Name = "B" },
            new Property{ Name = "B" }
        };



            var result = list1.FullOuterJoin(
                list2,
                p1 => p1.Name,
                p2 => p2.Name,
                (p1, p2) => new JoinedProperty
                {
                    Name1 = p1,
                    Name2 = p2
                }).ToList();


            foreach (var res in result)
            {
                Console.WriteLine(res.ToString());
            }
            Console.ReadLine();

        }

    }

    public static class MyExtensions
    {



        public static IEnumerable<TResult>
            FullOuterJoin<TSource, TInner, TKey, TResult>(this IEnumerable<TSource> source,
                                IEnumerable<TInner> inner,
                                Func<TSource, TKey> pk,
                                Func<TInner, TKey> fk,
                                Func<TSource, TInner, TResult> result)
            where TSource : class where TInner : class
        {

            var fullList = source.Select(s => new Tuple<TSource, TInner>(s, null))
                .Concat(inner.Select(i => new Tuple<TSource, TInner>(null, i)));


            var joinedList = new List<Tuple<TSource, TInner>>();

            foreach (var item in fullList)
            {
                var matchingItem = joinedList.FirstOrDefault
                    (
                        i => matches(i, item, pk, fk)
                    );

                if(matchingItem != null)
                {
                    joinedList.Remove(matchingItem);
                    joinedList.Add(combinedMatchingItems(item, matchingItem));
                }
                else
                {
                    joinedList.Add(item);
                }
            }
            return joinedList.Select(jl => result(jl.Item1, jl.Item2)).ToList();

        }

        private static Tuple<TSource, TInner> combinedMatchingItems<TSource, TInner>(Tuple<TSource, TInner> item1, Tuple<TSource, TInner> item2)
            where TSource : class
            where TInner : class
        {
            if(item1.Item1 == null && item2.Item2 == null && item1.Item2 != null && item2.Item1 !=null)
            {
                return new Tuple<TSource, TInner>(item2.Item1, item1.Item2);
            }

            if(item1.Item2 == null && item2.Item1 == null && item1.Item1 != null && item2.Item2 != null)
            {
                return new Tuple<TSource, TInner>(item1.Item1, item2.Item2);
            }

            throw new InvalidOperationException("2 items cannot be combined");
        }

        public static bool matches<TSource, TInner, TKey>(Tuple<TSource, TInner> item1, Tuple<TSource, TInner> item2, Func<TSource, TKey> pk, Func<TInner, TKey> fk)
            where TSource : class
            where TInner : class
        {          

            if (item1.Item1 != null && item1.Item2 == null && item2.Item2 != null && item2.Item1 == null && pk(item1.Item1).Equals(fk(item2.Item2)))
            {
                return true;
            }

            if (item1.Item2 != null && item1.Item1 == null && item2.Item1 != null && item2.Item2 == null && fk(item1.Item2).Equals(pk(item2.Item1)))
            {
                return true;
            }

            return false;

        }

    }
}