Linq 检查集合中值的相等性，并将值分配给其他集合

GroupBy。

假设messages是 IEnumerable，以下内容应该可以满足您的需求：

var history = messages.GroupBy(m => new { m.Title, m.Content }, (group, data) => new HistoryModel()
{
    Title = group.Title,
    Content = group.Content,
    Files = data.Select(k => new File() 
                                { 
                                    FileId = k.Fileid, 
                                    FileName = k.FileName
                                }
                        ).ToList(),
});

这基本上是说按消息的Title和Content属性对消息进行分组。对于每个组，lambda 称为传入传递group，这是一种匿名类型，包含组的详细信息(即标题和内容(，data是包含组中每个MessageModel的IEnumerable<MessageModel>。由此我们可以构建一个 HistoryModel列表 .

操作后编辑问题

以上应该只给你 4 个元素 history .我用来测试它的完整列表是：

class MessageModel
{
    public string MessageId { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }
    public string Fileid { get; set; }
    public string FileName { get; set; }
    public MessageModel(string m, string t, string c, string f, string f2)
    {
        MessageId = m;
        Title = t;
        Content = c;
        Fileid = f;
        FileName = f2;
    }
}
class HistoryModel
{
    public string Title { get; set; }
    public string Content { get; set; }
    public List<File> Files { get; set; }
}
class File
{
    public string FileId { get; set; }
    public string FileName { get; set; }
}
class Program
{
    static void Main(string[] args)
    {
        List<MessageModel> messages = new List<MessageModel>();
        messages.Add(new MessageModel("100", "1st title", "1st content", "1", "User.pdf"));
        messages.Add(new MessageModel("100", "1st title", "1st content", "2", "Log.txt"));
        messages.Add(new MessageModel("100", "1st title", "1st content", "3", "manual.doc"));
        messages.Add(new MessageModel("101", "2nd title", "2nd content", "4", "dummy.txt"));
        messages.Add(new MessageModel("102", "3rd title", "3rd content", null, null));
        messages.Add(new MessageModel("103", "4th title", "4th content", null, null));
        var history = messages.GroupBy(m => new { m.Title, m.Content }, (group, data) => new HistoryModel()
        {
            Title = group.Title,
            Content = group.Content,
            Files = data.Select(k => new File()
            {
                FileId = k.Fileid,
                FileName = k.FileName
            }).ToList(),
        });
        foreach (var h in history)
        {
            Console.WriteLine(h.Title + " " + h.Content);
            foreach (var file in h.Files)
            {
                Console.WriteLine("'t" + file.FileId + " " + file.FileName);
            }
        }
        Console.ReadLine();
    }
}

以上生成输出：

第一

标题第一内容
    1 个用户.pdf
    2 日志.txt
    3 手动.doc
第二标题第二内容
    4个假人.txt
第三标题第三内容

第四标题第四内容

可以使用 Group by 子句对公共属性(MessageId、Title 和 Content(进行分组。然后使用它作为 ke，然后可以使用 Select 子句来构建 HistoryModel。

下面的完整示例(请注意 .Dump(( 来自 LinqPad(

void Main()
{
    var messages = new List<MessageModel>
    {
        new MessageModel(100,"1st title","1st content", 1,"User.pdf"),
        new MessageModel(100,"1st title","1st content",  2,"Log.txt"),
        new MessageModel(100,"1st title","1st content", 3,"manual.doc"),
        new MessageModel(101,"2nd title","2nd content", 4,"dummy.txt"),   
        new MessageModel(102,"3rd title","3rd content" ),
        new MessageModel(103,"3rd title","3rd content")
    };
    var groupedMessages = 
    messages.GroupBy(x=> new {x.MessageId, x.Title, x.Content})
            .Select(x=> new HistoryModel
                          {
                            MessageId =  x.Key.MessageId,
                            Title = x.Key.Title,
                            Content = x.Key.Content,
                            Files = new List<FileModel>(x.Where( f=>  f.FileId != null)
                                                         .Select( g => new FileModel(g.FileId,  g.FileName)))
                          });
    groupedMessages.Dump();
}
// Define other methods and classes here
public class MessageModel
{
    public int MessageId {get; set;}
    public string Title {get; set;}
    public string Content {get; set;}
    public int? FileId {get; set;}
    public string FileName {get; set;}
    public MessageModel(int id, string title, string content, int? fileId = null, string fileName= null)
    {
        MessageId = id;
        Title =  title;
        Content = content;
        FileId = fileId;
        FileName = fileName;
    }
}
public class HistoryModel
{
    public int MessageId {get; set;}
    public string Title {get; set;}
    public string Content {get; set;}
    public List<FileModel> Files{get; set;} = new List<FileModel>();
}
public class FileModel
{
    public int? FileId {get; set;}
    public string FileName {get; set;}
    public FileModel(int? id, string name)
    {
        FileId = id;
        FileName = name;
    }
}