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使用linq联表或者连接两个对象集合查询时一般使用的是join关键字，返回结果中包含两个表或两个对象集合中连接字段相等的数据记录，如果要实现sql语句中的左连接效果，并没有现成的left join关键字，此时可以使用DefaultIfEmpty 实现左连接（也可以GroupJoin和SelectMany，具体请百度或咨询大模型）。
首先定义两个集合对象，如下所示：
List<Customer> lstCustomer = new List<Customer>();
lstCustomer.Add(new Customer { Id = 1, Name = "张三", Sex = "男" });
lstCustomer.Add(new Customer { Id = 2, Name = "李四", Sex = "男" });
lstCustomer.Add(new Customer { Id = 3, Name = "王五", Sex = "男" });
lstCustomer.Add(new Customer { Id = 4, Name = "赵六", Sex = "男" });
lstCustomer.Add(new Customer { Id = 5, Name = "钱多多", Sex = "男" });
lstCustomer.Add(new Customer { Id = 6, Name = "李雷", Sex = "男" });
lstCustomer.Add(new Customer { Id = 7, Name = "韩梅梅", Sex = "女" });List<Contact> lstContact = new List<Contact>();
lstContact.Add(new Contact { Id = 2, Address = "北京", Email = "123@qq.com", Telephone = "123456789" });
lstContact.Add(new Contact { Id = 4, Address = "上海", Email = "456@qq.com", Telephone = "123456789" });
lstContact.Add(new Contact { Id = 6, Address = "天津", Email = "789@qq.com", Telephone = "123456789" });
lstContact.Add(new Contact { Id = 8, Address = "重庆", Email = "101112@qq.com", Telephone = "123456789" });
lstContact.Add(new Contact { Id = 10, Address = "伦敦", Email = "121314@qq.com", Telephone = "123456789" });
首先是join关键字的效果，如下所示，join仅返回两个集合中id相同的数据记录。
var result = from g in lstCustomerjoin h in lstContact on g.Id equals h.Idselect new{ID = g.Id,Name = g.Name,Address = h.Address,Email = h.Email};
然后就是左连接使用示例，如下所示：
var result = from g in lstCustomerjoin h in lstContact on g.Id equals h.Id into LeftJoinGroupfrom k in LeftJoinGroup.DefaultIfEmpty()select new{ID = g.Id,Name = g.Name,Address = k?.Address,Email = k?.Email};
参考文献：
[1]https://www.cnblogs.com/keeplearningandsharing/p/16620651.html
[2]https://cloud.tencent.com/developer/article/2414328