c#groupbyorderbythenby嘗試顯式指定型別引數

我想使用 Grade 進行分組，并根據每個組中的元素數按降序排序，然后使用 Number 屬性按升序對每個組元素進行排序。我試過了，但它不起作用。你能幫我解決我做錯了什么嗎？

以下是錯誤內容：
無法從用法中推斷方法 'System.Linq.Enumerable.OrderBy<TSource,TKey>(System.Collections.Generic.IEnumerable, System.Func<TSource,TKey>)' 的型別引數。嘗試明確指定型別引數。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp6
{
    internal class Program
    {
        static void Main(string[] args)
        {
            List<Test> list = new List<Test>
            {
                new Test { Grade = "C", Name = "apple1", Number = 10 },
                new Test { Grade = "C", Name = "apple2", Number = 90 },
                new Test { Grade = "C", Name = "apple3", Number = 30 },
                new Test { Grade = "B", Name = "orange1", Number = 12 },
                new Test { Grade = "B", Name = "orange2", Number = 31 },
                new Test { Grade = "A", Name = "pineapple1", Number = 63 },
                new Test { Grade = "A", Name = "pineapple2", Number = 13 },
                new Test { Grade = "A", Name = "pineapple3", Number = 23 },
                new Test { Grade = "A", Name = "pineapple4", Number = 93 },
                new Test { Grade = "A", Name = "pineapple5", Number = 463 },
                new Test { Grade = "A", Name = "pineapple6", Number = 14 },
                new Test { Grade = "D", Name = "watermelon", Number = 74 }
            };

            //

            //CS0411
            //The type arguments for method cannot be inferred from the usage.
            //Try specifying the type arguments explicitly.
            var aaa = list.GroupBy(x => x.Grade, x => (x.Name, x.Number))
                .Where(g => g.Count() >= 1)
                .OrderByDescending(g => g.Count())
                .ThenBy(g => g.Select(a => a.Number).ToList().Sort((x, y) => x.CompareTo(y)))
                .ToList();

            //expected answers
            //{ Grade = "A", Name = "pineapple2", Number = 13 }
            //{ Grade = "A", Name = "pineapple6", Number = 14 }
            //{ Grade = "A", Name = "pineapple3", Number = 23 }
            //{ Grade = "A", Name = "pineapple1", Number = 63 }
            //{ Grade = "A", Name = "pineapple4", Number = 93 }
            //{ Grade = "A", Name = "pineapple5", Number = 463 }
            //{ Grade = "C", Name = "apple1", Number = 10 }
            //{ Grade = "C", Name = "apple3", Number = 30 }
            //{ Grade = "C", Name = "apple2", Number = 90 }
            //{ Grade = "B", Name = "orange1", Number = 12 }
            //{ Grade = "B", Name = "orange2", Number = 31 }
            //{ Grade = "D", Name = "watermelon", Number = 74 }
        }
    }
    class Test
    {
        public string Grade { get; set; }
        public string Name { get; set; }
        public int Number { get; set; }
    }
}

uj5u.com熱心網友回復：

我想使用 Grade 進行分組，并根據每個組中的元素數按降序排序，然后使用 Number 屬性按升序對每個組元素進行排序。

那不是ThenBy目的。為相同的序列ThenBy提供二級排序。這并不意味著在序列中的專案內執行排序。（此外，你的 lambda 運算式的最后一部分is ，它回傳... 而編譯器期望一個回傳“你想要執行二級排序的東西”的委托。）ThenByList<T>.Sortvoid

“外部序列”最終是一個組序列。如果要轉換這些組中的每一個，則需要使用Select. 所以我懷疑你想要：

var aaa = list.GroupBy(x => x.Grade, x => (x.Name, x.Number))
    // Order the groups by descending size of group
    .OrderByDescending(g => g.Count())
    // Order the elements of each individual group by number
    .Select(group => group.OrderBy(a => a.Number).ToList())
    .ToList();

注意：我洗掉了該Where子句，因為它GroupBy永遠不會產生任何空組。

或者，假設回傳的組的元素GroupBy與原始序列中的元素相同，您可以先按數字對原始序列進行排序：

var aaa = list
    .OrderBy(x => x.Number)
    .GroupBy(x => x.Grade, x => (x.Name, x.Number))
    .OrderByDescending(g => g.Count())
    .ToList();

這樣，您仍然會得到一系列IGrouping元素，而不僅僅是一系列串列......這意味著您仍然可以訪問每個組的鍵。

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