C# LINQ into join（2）

最新推荐文章于 2025-07-18 07:59:49 发布

原创最新推荐文章于 2025-07-18 07:59:49 发布 · 468 阅读

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文章标签：

#C# #LINQ #into #join

本文深入探讨了使用C#的LINQ进行左外连接的原理及应用，通过具体示例对比了普通连接与左外连接的区别，展示了如何在查询中加入DefaultIfEmpty方法来处理空集合情况，为读者提供了理解和运用左外连接的重要指导。

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var list1 = new List<(string s1, string s2)>()
            {
                ("a","1"),
                ("b","2"),
                ("c","4"),
                ("e","1")
            };

            var list2 = new List<(string s1, string s2)>()
            {
                ("i","1"),
                ("j","2"),
                ("l","1"),
                ("m","2"),
                ("n","3")
            };

            var q = from r1 in list1
                    join r2 in list2
                    on r1.s2 equals r2.s2 into result
                    from t in result 
                    select t;

            foreach (var item in q)
            {
                Console.WriteLine(item);
                Console.WriteLine("------------");
            }

输出：

(i, 1)
------------
(l, 1)
------------
(j, 2)
------------
(m, 2)
------------
(i, 1)
------------
(l, 1)
------------

再来看将result改为result.DefaultIfEmpty()后的代码：

var list1 = new List<(string s1, string s2)>()
            {
                ("a","1"),
                ("b","2"),
                ("c","4"),
                ("e","1")
            };

            var list2 = new List<(string s1, string s2)>()
            {
                ("i","1"),
                ("j","2"),
                ("l","1"),
                ("m","2"),
                ("n","3")
            };

            var q = from r1 in list1
                    join r2 in list2
                    on r1.s2 equals r2.s2 into result
                    from t in result.DefaultIfEmpty()
                    select t;

            foreach (var item in q)
            {
                Console.WriteLine(item);
                Console.WriteLine("------------");
            }

输出：

(i, 1)
------------
(l, 1)
------------
(j, 2)
------------
(m, 2)
------------
(, )
------------
(i, 1)
------------
(l, 1)
------------

——————————————————————————————
再来看代码：

var list1 = new List<(string s1, string s2)>()
            {
                ("a","1"),
                ("b","2"),
                ("c","4"),
                ("e","1")
            };

            var list2 = new List<(string s1, string s2)>()
            {
                ("i","1"),
                ("j","2"),
                ("l","1"),
                ("m","2"),
                ("n","3")
            };

            var q = from r1 in list1
                    join r2 in list2
                    on r1.s2 equals r2.s2 into result
                    from t in result
                    select (s1:r1.s1,t:t);

            foreach (var item in q)
            {
                Console.WriteLine(item.s1);
                Console.WriteLine(item.t);
                Console.WriteLine("------------");
            }

输出：

a
(i, 1)
------------
a
(l, 1)
------------
b
(j, 2)
------------
b
(m, 2)
------------
e
(i, 1)
------------
e
(l, 1)
------------

再来看将result改为result.DefaultIfEmpty()后的代码：

var list1 = new List<(string s1, string s2)>()
            {
                ("a","1"),
                ("b","2"),
                ("c","4"),
                ("e","1")
            };

            var list2 = new List<(string s1, string s2)>()
            {
                ("i","1"),
                ("j","2"),
                ("l","1"),
                ("m","2"),
                ("n","3")
            };

            var q = from r1 in list1
                    join r2 in list2
                    on r1.s2 equals r2.s2 into result
                    from t in result.DefaultIfEmpty()
                    select (s1:r1.s1,t:t);

            foreach (var item in q)
            {
                Console.WriteLine(item.s1);
                Console.WriteLine(item.t);
                Console.WriteLine("------------");
            }

输出：

a
(i, 1)
------------
a
(l, 1)
------------
b
(j, 2)
------------
b
(m, 2)
------------
c
(, )
------------
e
(i, 1)
------------
e
(l, 1)
------------

这段代码就是左外连接