如何使用 linq 在单一方法中创建聚合函数？

Question

我有这样的课

public class Student
{

    public int Id { get; set; }
    public string Name { get; set; }
    public string Gender { get; set; }
    public GRADE Grade { get; set; }
    public string Nationality { get; set; }
}
public enum GRADE
{
    A = 0,
    B = 1,
    C = 2,
    D = 3,
    E = 4
}

var list = new List<Student>();
list.Add(new Student() { Id = 1, Name = "Prasad", Gender = "M", Nationality = "India", Grade = GRADE.A });
list.Add(new Student() { Id = 2, Name = "Raja", Gender = "M", Nationality = "India", Grade = GRADE.B });
list.Add(new Student() { Id = 3, Name = "Hindu", Gender = "F", Nationality = "India", Grade = GRADE.A });
list.Add(new Student() { Id = 4, Name = "Hamed", Gender = "M", Nationality = "India", Grade = GRADE.C });
list.Add(new Student() { Id = 5, Name = "Priya", Gender = "F", Nationality = "India", Grade = GRADE.D });
list.Add(new Student() { Id = 6, Name = "Meera", Gender = "F", Nationality = "India", Grade = GRADE.B });

我得到了这样的解决方案，对于每个表达式我想写一堆代码.. Sum、Avg、Count 等

Linq 表达式

//count
var c = (from x in list.GroupBy(k => k.Gender)
         select new
         {
             category = x.Key,
             Value = x.Count()
         }).ToList();

//sum
var s = (from x in list.GroupBy(k => k.Gender)
         select new
         {
             category = x.Key,
             Value = x.Sum(k => (int)k.Grade)
         }).ToList();

//avg
var a = (from x in list.GroupBy(k => k.Gender)
         select new
         {
             category = x.Key,
             Value = x.Average(k => (int)k.Grade)
         }).ToList();

我正在尝试基于聚合函数创建一个函数；它应该返回值，我试过找不到。

Answer 1

您遇到的一个问题是所有三个聚合都没有相同的返回类型，而且如果您使用函数，则返回类型必须是对象，因为您返回的是匿名类型。

我认为你想要的最接近的就是这个；

第一步：创建一个新类型；

public class AggregateValue<T>
{
    public string Category { get; set; }
    public T Value { get; set; }
}

第 2 步：创建一个返回此类型集合并接受 Func 作为参数的函数，该参数将计算您的不同聚合；

    IEnumerable<AggregateValue<T>> GetAggregateValues<T>(List<Student> students, Func<IEnumerable<Student>, T> aggregateFunction)
    {
        return (from x in students.GroupBy(k => k.Gender)
                 select new AggregateValue<T>
                 {
                     Category = x.Key,
                     Value = aggregateFunction(x)
                 }).ToList();
    }

你可以这样使用它；

        var list = new List<Student>();
        list.Add(new Student() { Id = 1, Name = "Prasad", Gender = "M", Nationality = "India", Grade = GRADE.A });
        list.Add(new Student() { Id = 2, Name = "Raja", Gender = "M", Nationality = "India", Grade = GRADE.B });
        list.Add(new Student() { Id = 3, Name = "Hindu", Gender = "F", Nationality = "India", Grade = GRADE.A });
        list.Add(new Student() { Id = 4, Name = "Hamed", Gender = "M", Nationality = "India", Grade = GRADE.C });
        list.Add(new Student() { Id = 5, Name = "Priya", Gender = "F", Nationality = "India", Grade = GRADE.D });
        list.Add(new Student() { Id = 6, Name = "Meera", Gender = "F", Nationality = "India", Grade = GRADE.B });

        var sumGrades = new Func<IEnumerable<Student>, int>(p => p.Sum(l => (int)l.Grade));
        var aveGrades = new Func<IEnumerable<Student>, double>(p => p.Average(k => (int)k.Grade));
        var count = new Func<IEnumerable<Student>, int>(p => p.Count());

        var c = GetAggregateValues(list, count);
        var s = GetAggregateValues(list, sumGrades);
        var a = GetAggregateValues(list, aveGrades);

Answer 2

您可以在一个语句中组合所有聚合：

var result = (from x in list.GroupBy(k => k.Gender)
    select new
    {
        category = x.Key,
        Count = x.Count(),
        Sum = x.Sum(k => (int)k.Grade),
        Average = x.Average(k => (int)k.Grade)
    }).ToList();