你可以用这个

var result = data.GroupBy(item => item.Id)
    .Select(group => new
    {
        Id = group.Key,
        Count = group.Count(),
        TotalValue = group.Sum(item => item.Value)
    })
    .ToList();

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首先，您需要使用GroupBy，然后创建一个新的对象，该对象接受每个组并计算（Count和Sum）需要从它们中获得的值：
DotNetFiddle Example的

using System;
using System.Collections.Generic;
using System.Linq;
                    
public class Program
{
    public static void Main()
    {
        var source = new List<(int Id, int Value)>
        {
            (1, 5),
            (2, 3),
            (3, -2),
            (1, 27),
            (2, 0),
            (3, 1),
            (1, -4),
        };
        
        var computed = source
         .GroupBy(x => x.Id)
         .Select(x => new {Id = x.Key, Count = x.Count(), TotalValue = x.Sum(s => s.Value)})
         .ToList();
        
        foreach(var value in computed)
        {
            Console.WriteLine($"{value.Id} {value.Count} {value.TotalValue}");
        }
    }
}

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输出量：
1328
二二三
32 - 1

让我用一个元组来表示一个对象

var x = new List<(int Id, int Value)>();
            
var y = x.GroupBy(x => x.Id).Select(_ => new { Id = _.Key, Count = _.Count(), TotalValue = _.Sum(i => i.Value) });

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