digraph hierarchy {
	            rankdir=LR;
                nodesep=0.2 // increases the separation between nodes
                
                node [color=Red,fontname=Courier,shape=box] //All nodes will this shape and colour
                edge [color=Blue, style=line] //All the lines look like this

                A->{Value1}
                A->{Value2}
                A->{Value3}
                B->{Value4}
                B->{Value5}
                C->{Value6}
}

digraph hierarchy {
            	rankdir=LR;
                nodesep=0.4 // increases the separation between nodes
	node[color=Red,fontname=Courier,shape=record]
    edge [color=Blue, style=line] //All the lines look like this
    
       subgraph cluster_level1{
                label ="Group 1";                  
	            struct1 [label="Value1|Value2|Value3"];
        }
        subgraph cluster_level2{
                label ="Group 2";                  
                struct2 [label="Value4|Value5"];
        }
        subgraph cluster_level3{
                label ="Group 3";                  
                struct3 [label="Value6"];
        }
        A->struct1
        B->struct2
        C->struct3
}

public static IEnumerable<TResult> GroupJoin<TOuter,TInner,TKey,TResult> (
        this IEnumerable<TOuter> outer,
        IEnumerable<TInner> inner,
        Func<TOuter,TKey> outerKeySelector, 
        Func<TInner,TKey> innerKeySelector, 
        Func<TOuter,IEnumerable<TInner>,TResult> resultSelector
);

public static IEnumerable<TResult> GroupJoin<TOuter,TInner,TKey,TResult> (
        this IEnumerable<TOuter> outer,
        IEnumerable<TInner> inner, 
        Func<TOuter,TKey> outerKeySelector,
        Func<TInner,TKey> innerKeySelector,    
        Func<TOuter,IEnumerable<TInner>,TResult> resultSelector, 
        IEqualityComparer<TKey> comparer
);

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

class Pet
{
     public string Name { get; set; }
     public Person Owner { get; set; }
}

static void Main(string[] args)
{
     var people = new List<Person> {
             new Person { Name = "王大明" },
             new Person { Name = "蔡阿高" },
             new Person { Name = "黃飛龍" }
     };

     var pets = new List<Pet> {
             new Pet { Name = "小白", Owner = people[1] },
             new Pet { Name = "小黑", Owner = people[1] },
             new Pet { Name = "小藍", Owner = people[2] },
             new Pet { Name = "小綠", Owner = people[0] }
     };

     var query = people.GroupJoin(
                 pets,
                 person => person.Name,
                 pet => pet.Owner.Name,
                 (person, petCollection) => 
                     new { 
                             OwnerName = person.Name, 
                             Pet = petCollection.Select(p => p.Name) 
                         }
             ).ToList();
             
     foreach(var q in query)
     {
          Console.WriteLine($"OwnerName : {q.OwnerName}");
          foreach(var petName in q.Pet)
          {
               Console.WriteLine($" ----- petName : {petName}");
          }
     }
     Console.WriteLine("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");
     Console.WriteLine("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");

     var sqlLikeQuery = (from person in people
                         join pet in pets on person.Name equals pet.Owner.Name into ps
                         select new { 
                            OwnerName = person.Name, 
                            Pet = ps.Select(p => p.Name) 
                         }
                        ).ToList();
     foreach (var q in sqlLikeQuery)
     {
          Console.WriteLine($"OwnerName : {q.OwnerName}");
          foreach (var petName in q.Pet)
          {
               Console.WriteLine($" ----- petName : {petName}");
          }
     }

     Console.ReadKey();
}

public static IEnumerable<TResult> MyGroupJoin<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector, IEqualityComparer<TKey> comparer = null)
{
     if (outer is null || inner is null || outerKeySelector is null || innerKeySelector is null || resultSelector is null)
     {
          throw new Exception("null");
     }
     return MyGroupJoinIterator(outer, inner, outerKeySelector, innerKeySelector, resultSelector, comparer);
}

static IEnumerable<TResult> MyGroupJoinIterator<TOuter, TInner, TKey, TResult>(IEnumerable<TOuter> outer, IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector, IEqualityComparer<TKey> comparer)
{
     Lookup<TKey, TInner> lookup = Lookup<TKey, TInner>.CreateForJoin(inner, innerKeySelector, comparer);
     foreach (var item in outer)
     {
          var group = lookup.GetGrouping(outerKeySelector(item), false);
          yield return resultSelector(item, group ?? new TInner[0] as IEnumerable<TInner>);
          //or use
          //yield return resultSelector(item, lookup[outerKeySelector(item)]);
     }
}