C# LINQ如何按release_number分组获取每组最大year值

现有列表包含如下结构的条目数据：

{ surname = surname_1, title = title_4, release_number = 111, year = 2021 }
{ surname = surname_1, title = title_1, release_number = 111, year = 2020 }
{ surname = surname_2, title = title_5, release_number = 222, year = 2021 }
{ surname = surname_2, title = title_2, release_number = 222, year = 2020 }
{ surname = surname_3, title = title_8, release_number = 222, year = 2022 }

需要使用C# LINQ实现：获取每个release_number对应的最大year值，期望输出结果为：

{ReleaseNumber = 111, Year = 2021}
{ReleaseNumber = 222, Year = 2022}

首先定义和数据结构匹配的实体类，假设数据源为List<Record>类型，变量名为records：

public class Record
{
    public string Surname { get; set; }
    public string Title { get; set; }
    public int ReleaseNumber { get; set; }
    public int Year { get; set; }
}

核心实现逻辑是按release_number字段分组，对每个分组取year字段的最大值，有两种常见写法：

• 方法语法（日常开发最常用）

var result = records
    .GroupBy(item => item.ReleaseNumber)
    .Select(group => new 
    {
        ReleaseNumber = group.Key,
        Year = group.Max(i => i.Year)
    })
    .ToList();

• 查询表达式写法

var result = from item in records
             group item by item.ReleaseNumber into releaseGroup
             select new 
             {
                 ReleaseNumber = releaseGroup.Key,
                 Year = releaseGroup.Max(i => i.Year)
             };

逻辑说明 #

• GroupBy会把ReleaseNumber相同的条目归为同一组，每组的Key属性就是对应的ReleaseNumber值
• 对每个分组调用Max方法，传入取Year字段的选择器，就能得到当前分组下的最大年份
• 上述代码输出的是匿名类型集合，属性和值完全匹配预期结果，如果需要强类型返回，直接把匿名类型替换为自定义的输出DTO类即可

内容的提问来源于stack exchange，提问作者Yura Solokha