如何在LINQ的表达式树中创建连接？

var query = from item1 in source1
            join item2 in source2 on item1.Key equals item2.Key
            select new { item1, item2 };

var query = from item1 in source1
            join item2 in source2 on item1.Key equals item2.Key into g
            from item2 in g.DefaultIfEmpty()
            select new { item1, item2 };

var query = source1.Join(source2, item1 => item1.Key, item2 => item2.Key, (item1, item2) => new { item1, item2 });

var query = source1.GroupJoin(source2, item1 => item1.Key, item2 => item2.Key, (item1, g) => new { item1, item2 = g.DefaultIfEmpty() })
                   .SelectMany(x => x.item2.DefaultIfEmpty(), (x, item2) => new { x.item1, item2 });