C# 实现FULL JOIN 效果

参考：https://dotnettutorials.net/lesson/full-outer-join-in-linq/
思路就是先left join 再right join，最后union

namespace Test
{
    internal class Program
    {
        static void Main(string[] args)
        {
            // 创建测试数据
            var listA = new List<ItemA>
        {
            new ItemA { Key = 1, Value = "A1" },
            new ItemA { Key = 2, Value = "A2" },
            new ItemA { Key = 3, Value = "A3" }
        };
 
            var listB = new List<ItemB>
        {
            new ItemB { Key = 1, Value2 = "B1" },
            new ItemB { Key = 2, Value2 = "B2" },
            // Comment or uncomment the following line to test with a null entry
            new ItemB { Key = 4, Value2 = "B4" }
        };
 
            // 执行 full join
            var leftJoinQuery = from itemA in listA
                                join itemB in listB
                                on itemA.Key equals itemB.Key into gj
                                from subItemB in gj.DefaultIfEmpty()
                                select new { Key = itemA.Key, ValueA = itemA.Value, ValueB = (subItemB == null) ? default : subItemB.Value2 };
 
            var rightJoinQuery = from itemB in listB
                                join itemA in listA
                                on itemB.Key equals itemA.Key into gj
                                from subItemA in gj.DefaultIfEmpty()
                                select new { Key = itemB.Key, ValueA =(subItemA == null)?default: subItemA.Value, ValueB = itemB.Value2 };
            var fullJoinQuery= leftJoinQuery.Union(rightJoinQuery);
            // 输出结果
            foreach (var result in fullJoinQuery)
            {
                Console.WriteLine($"Key: {result.Key}, ValueA: {result.ValueA}, Value2: {result.ValueB}");
            }
        }
    }
 
 
    // 测试模型
    class ItemA
    {
        public int Key { get; set; }
        public string Value { get; set; }
    }
 
    class ItemB
    {
        public int Key { get; set; }
        public string Value2 { get; set; }
    }
}