在C# LINQ分组时更新同一对象DocCount属性的极简方案

现有类定义 #

public class Contact
{
    public int Id { get; set; }
    public string Email { get; set; }
    public string DisplayName { get; set; }
    public int DocCount { get; set; }
    public string DocName { get; set; }
}

原始数据列表 #

var contactList = new List<Contact>()
{
    new Contact(){ Id = 1, Email = "abc@abc.com", DisplayName = "ABC", DocCount = 0, DocName = "testdoc1"},
    new Contact(){ Id = 1, Email = "abc@abc.com", DisplayName = "ABC", DocCount = 0, DocName = "testdoc2"},
    new Contact(){ Id = 1, Email = "abc@abc.com", DisplayName = "ABC", DocCount = 0, DocName = "testdoc3"},
    new Contact(){ Id = 2, Email = "xyz@abc.com", DisplayName = "XYZ", DocCount = 0, DocName = "testdoc1"},
    new Contact(){ Id = 2, Email = "xyz@abc.com", DisplayName = "XYZ", DocCount = 0, DocName = "testdoc2"},
    new Contact(){ Id = 2, Email = "xyz@abc.com", DisplayName = "XYZ", DocCount = 0, DocName = "testdoc3"},
    new Contact(){ Id = 3, Email = "mop@abc.com", DisplayName = "MOP", DocCount = 0, DocName = "testdoc1"},
    new Contact(){ Id = 3, Email = "mop@abc.com", DisplayName = "MOP", DocCount = 0, DocName = "testdoc2"},
};

极简实现（无需新建类） #

直接用LINQ分组后构造目标Contact对象：

var mergedContacts = contactList
    .GroupBy(c => new { c.Id, c.Email, c.DisplayName })
    .Select(g => new Contact
    {
        Id = g.Key.Id,
        Email = g.Key.Email,
        DisplayName = g.Key.DisplayName,
        DocCount = g.Count(),
        DocName = string.Join(" ", g.Select(c => c.DocName))
    })
    .ToList();

实现说明 #

• 用匿名类型作为GroupBy的分组键，确保Id、Email、DisplayName完全匹配的项被归为同一组
• 在Select中直接实例化Contact，将组内文档总数赋值给DocCount，用string.Join拼接所有文档名称到DocName
• 最终得到的mergedContacts完全符合需求的输出结构

内容的提问来源于stack exchange，提问作者Vijayashankar Aruchamy