由多个字段组成的C＃与对象列表 [英] Group by multiple fields C# with object List

问题描述

我有以下数据结构：

List<List<RecordItem>> combinedList = new List<List<RecordItem>>()
{
    new List<RecordItem>()
    {
        new RecordItem() { fieldName = "StudentId", value = "S1" },
        new RecordItem() { fieldName = "Maths", value = "90" },
        new RecordItem() { fieldName = "Term", value = "1" },
    },
    new List<RecordItem>()
    {
        new RecordItem() { fieldName = "StudentId", value = "S1" },
        new RecordItem() { fieldName = "Science", value = "70" },
        new RecordItem() { fieldName = "Term", value = "1" },
    }
}; 

我需要使用StudentId和Term进行分组，我可以按一个字段进行分组 - > studentId （学分：Enigmativity），但由多个字段（studentId和term）组合起来。
第一个问题是如何使用LINQ取另一个字段（术语）？其次，如何完成此代码的分组。

I need to group it using StudentId and Term, I was able to group by one field -> studentId (Credits: Enigmativity) but unbale to group by multiple fields (studentId and term). The first issue is how to use LINQ to take another field (term)? and secondly, how to complete this code for the grouping.

List<List<RecordItem>> groupedList =
(
    from records in combinedList
    from student in records.Take(1) 
    from term   in records.Take(?) // how do I get the term which is the 4th element???
    let StudentId = student.value
    let Term = term.value
    let Subjects = records.Skip(1)
    group Subjects by new
    {
       StudentId,Term   
    into gss
    select new []
    {
        new RecordItem() { fieldName = "StudentId", value = gss.Key.StudentId },
        new RecordItem() { fieldName = "Term", value = gss.Key.Term },
    }.Concat(gss.SelectMany(x => x)).ToList()
).ToList();

推荐答案

我真的不明白这里发生了什么你正在选择什么，但这是语法正确的代码：

I can't really understand what is going on here and what you are selecting, but this is syntactically correct code:

static void Main(string[] args)
        {
            List<List<RecordItem>> combinedList = new List<List<RecordItem>>()
            {
                new List<RecordItem>()
                {
                    new RecordItem() { fieldName = "StudentId", value = "S1" },
                    new RecordItem() { fieldName = "Maths", value = "90" },
                    new RecordItem() { fieldName = "Term", value = "1" },
                },
                new List<RecordItem>()
                {
                    new RecordItem() { fieldName = "StudentId", value = "S1" },
                    new RecordItem() { fieldName = "Science", value = "70" },
                    new RecordItem() { fieldName = "Term", value = "1" },
                }
            };

            List<List<RecordItem>> groupedList =
            (
                from records in combinedList
                from student in records.Take(1)
                let StudentId = student.value
                let Term = records.ToArray().ElementAt(3).value
                let Subjects = records.Skip(1)
                group Subjects by new
                {
                    StudentId,
                    Term
                }
                    into gss
                    select new[]
                {
                    new RecordItem() { fieldName = "StudentId", value = gss.Key.StudentId },
                    new RecordItem() { fieldName = "Term", value = gss.Key.Term },
                }.Concat(gss.SelectMany(x => x)).ToList()
            ).ToList();
        }

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