C# Linq 左外联
2021/9/26 14:10:54
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C# Linq 左外联
假设我们有一个Student类,具有姓名、年龄、班级Id三个属性
还有一个Class类,具有Id,班级名两个属性
现要查找学生姓名、年龄和其所在班级名,因此需要做一个Student表左联Class表的操作
方式一:
var results = from s in students join c in classes on s.ClassId equals c.Id into scs from sc in scs.DefaultIfEmpty() select new { Name = s.Name, Age = s.Age, ClassName = sc?.ClassName ?? string.Empty };
方式二:
var results = students.GroupJoin(classes, s => s.ClassId, c => c.Id, (s, cs) => new { Name = s.Name, Age = s.Age, ClassName = cs.ElementAtOrDefault(0) is null ? string.Empty : cs.ElementAt(0).ClassName });
完整代码演示:
using System; using System.Collections.Generic; using System.Linq; public class Student { public string Name { get; set; } public int Age { get; set; } public int ClassId { get; set; } } public class Class { public int Id { get; set; } public string ClassName { get; set; } } public class Program { public static void Main() { List<Student> students = new List<Student>() { new Student(){Name = "B", Age = 18, ClassId = 1}, new Student(){Name = "C", Age = 19, ClassId = 2}, new Student(){Name = "A", Age = 20, ClassId = 3}, new Student(){Name = "D", Age = 20, ClassId = 4}, new Student(){Name = "E", Age = 20, ClassId = 5}, }; List<Class> classes = new List<Class>() { new Class(){ Id = 1, ClassName = "1班"}, new Class(){ Id = 2, ClassName = "2班"}, new Class(){ Id = 3, ClassName = "3班"}, }; //方式一: //var results = from s in students // join c in classes on s.ClassId equals c.Id into scs // from sc in scs.DefaultIfEmpty() // select new { Name = s.Name, Age = s.Age, ClassName = sc?.ClassName ?? string.Empty }; //方式二: var results = students.GroupJoin(classes, s => s.ClassId, c => c.Id, (s, cs) => new { Name = s.Name, Age = s.Age, ClassName = cs.ElementAtOrDefault(0) is null ? string.Empty : cs.ElementAt(0).ClassName }); foreach (var result in results) { Console.WriteLine($"{result.Name} {result.Age} {result.ClassName}"); } } }
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