使用 Fluent API 按属性.NET Core 3.1 对列表进行分组

Question

我需要根据列表属性对数据进行分组。下面的代码是我当前场景的模拟：

using System;
using System.Collections.Generic;
using System.Linq;
using static teste.StudentClass;

namespace teste
{
    class Program
    {
        public static void Main(string[] args)
        {
            List<Student> students = new List<Student>
            {
            new Student {FirstName = "Terry", LastName = "Adams", ID = 120,
                Year = GradeLevel.SecondYear,
                ExamScores = new List<int>{ 99, 82, 81, 79}},
            new Student {FirstName = "Fadi", LastName = "Fakhouri", ID = 116,
                Year = GradeLevel.ThirdYear,
                ExamScores = new List<int>{ 99, 86, 90, 94}},
            new Student {FirstName = "Hanying", LastName = "Feng", ID = 117,
                Year = GradeLevel.FirstYear,
                ExamScores = new List<int>{ 93, 92, 80, 87}},
            new Student {FirstName = "Cesar", LastName = "Garcia", ID = 114,
                Year = GradeLevel.FourthYear,
                ExamScores = new List<int>{ 97, 89, 85, 82}},
            new Student {FirstName = "Debra", LastName = "Garcia", ID = 115,
                Year = GradeLevel.ThirdYear,
                ExamScores = new List<int>{ 35, 72, 91, 70}},
            new Student {FirstName = "Hugo", LastName = "Garcia", ID = 118,
                Year = GradeLevel.SecondYear,
                ExamScores = new List<int>{ 92, 90, 83, 78}},
            new Student {FirstName = "Sven", LastName = "Mortensen", ID = 113,
                Year = GradeLevel.FirstYear,
                ExamScores = new List<int>{ 88, 94, 65, 91}},
            new Student {FirstName = "Claire", LastName = "O'Donnell", ID = 112,
                Year = GradeLevel.FourthYear,
                ExamScores = new List<int>{ 75, 84, 91, 39}},
            new Student {FirstName = "Svetlana", LastName = "Omelchenko", ID = 111,
                Year = GradeLevel.SecondYear,
                ExamScores = new List<int>{ 97, 92, 81, 60}},
            new Student {FirstName = "Lance", LastName = "Tucker", ID = 119,
                Year = GradeLevel.ThirdYear,
                ExamScores = new List<int>{ 68, 79, 88, 92}},
            new Student {FirstName = "Michael", LastName = "Tucker", ID = 122,
                Year = GradeLevel.FirstYear,
                ExamScores = new List<int>{ 94, 92, 91, 91}},
            new Student {FirstName = "Eugene", LastName = "Zabokritski", ID = 121,
                Year = GradeLevel.FourthYear,
                ExamScores = new List<int>{ 96, 85, 91, 60}}
        };
                students.ToList().ForEach(x =>
                {
                    Console.WriteLine($"\t{x.FirstName}, {x.LastName}, {x.Year}");
                });
        }
    }
    public class StudentClass
    {

        public enum GradeLevel { FirstYear = 1, SecondYear, ThirdYear, FourthYear };
        public class Student
        {
            public string FirstName { get; set; }
            public string LastName { get; set; }
            public int ID { get; set; }
            public GradeLevel Year;
            public List<int> ExamScores;
        }


    }

}

目前，我有以下结果：

Terry, Adams, SecondYear
Fadi, Fakhouri, ThirdYear
Hanying, Feng, FirstYear
Cesar, Garcia, FourthYear
Debra, Garcia, ThirdYear
Hugo, Garcia, SecondYear
Sven, Mortensen, FirstYear
Claire, O'Donnell, FourthYear
Svetlana, Omelchenko, SecondYear
Lance, Tucker, ThirdYear
Michael, Tucker, FirstYear
Eugene, Zabokritski, FourthYear

我必须更改返回值，以便数据按LatsName 属性分组。预期输出。如何使用 Fluent Api 做到这一点？预期输出（示例）：

{LastName:"Garcia": {FirstName:"Cesar", Year: "FourthYear"}, {FirstName:"Debra", Year: "ThirdYear"}, {FirstName:"Hugo", Year: "SecondYear"}},
{LastName:"Tucker": {FirstName:"Lance", Year: "ThirdYear"}, {FirstName:"Michael", Year: "FirstYear"}}
.... 

我正在尝试一个简单的 groupBy，但它在转换时返回错误消息：

students = students.GroupBy(x => x.LastName);

错误信息：

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<System.Linq.IGrouping<string, teste.StudentClass.Student>>' to 'System.Collections.Generic.List<teste.StudentClass.Student>'. An explicit conversion exists (are you missing a cast?)    

Answer 1

您的变量students 可能是List<Student> 类型，因此您尝试进行的转换是不可能的，因为GroupBy 返回IEnumerable<IGrouping<TKey, TSource>>。

这可能是您正在寻找的解决方案

public class StudentDto 
{
   public string FirstName { get; set; }
   public GradeLevel Year { get; set; }
}

public class StudentGroup
{
   public string LastName { get; set; }
   public IEnumerable<StudentDto> Students { get; set; }
}

现在你可以用这种方式进行分组

IEnumerable<StudentGroup> groupedData = students.GroupBy(x => x.LastName)
   .Select(x => new StudentGroup { 
      LastName = x.Key,
      Students = x.Select(student => new StudentDto 
      {
         FirstName = student.FirstName,
         Year = student.Year
       }).ToList()
   });

通过这种方式，您将按姓氏对所有学生进行分组。