使用列表類型的 class 的屬性以外的值創建一個新列表

Question

我有一個Customers具有Id和Name屬性的客戶。 創建Customers列表后，我想要一個沒有重復名稱的新customer列表，即 Customers.Name 必須是唯一的。 我現在的output：

Gustavo
Gabriel
Mariza
Sandro
Gustavo
Gabriel
Lilian
Sandro

預計 Output：

Gustavo
Gabriel
Mariza
Sandro
Lilian

我試試這個，但我一直在犯與類型轉換相關的錯誤：

using System;
using System.Collections.Generic;
using System.Linq;

namespace SelectDistinct
{
    public class Program
    {
        static void Main(string[] args)
        {
            List<Customers> customers = new List<Customers>();
            customers.Add(new Customers(0, "Gustavo"));
            customers.Add(new Customers(0, "Gabriel"));
            customers.Add(new Customers(0, "Mariza"));
            customers.Add(new Customers(0, "Sandro"));
            customers.Add(new Customers(0, "Gustavo"));
            customers.Add(new Customers(0, "Gabriel"));
            customers.Add(new Customers(0, "Lilian"));
            customers.Add(new Customers(0, "Sandro"));

            customers = customers.Select(x => x.Name.Distinct());

            customers.ToList().ForEach(x =>
            {
                Console.WriteLine(x.ToString());
            });
        }

        public class Customers
        {
            public int Id { get; set; }
            public string Name { get; set; }

            public Customers(int id, string name)
            {
                Id = id;
                Name = name;
            }
        }
    }
}

誰能幫忙？

Answer 1

使用GroupBy給你一個預期的結果

var result = customers
    .GroupBy(c => c.Name)
    .Select(g => g.First())
    .ToList();
result.ForEach(c => Console.WriteLine(c.Name));

Output 將是以下

Gustavo
Gabriel
Mariza
Sandro
Lilian

Answer 2

您發布的代碼中有很多錯誤。

我注意到的第一件事是.Distinct()的錯誤放置。 您已將它放在 select 內和Name變量之后，這是不合邏輯的。

第二件事是，您正在嘗試將類型為IEnumerable<char>的customers.Select(x => x.Name.Distinct())分配給類型為List<Customers>的customers變量。

現在，要實現您要查找的內容，您必須正確使用Distinct擴展並將結果分配給正確的變量。

這是您的完整工作代碼：

static void Main(string[] args)
{
    List<Customers> customers = new List<Customers>();
    customers.Add(new Customers(0, "Gustavo"));
    customers.Add(new Customers(0, "Gabriel"));
    customers.Add(new Customers(0, "Mariza"));
    customers.Add(new Customers(0, "Sandro"));
    customers.Add(new Customers(0, "Gustavo"));
    customers.Add(new Customers(0, "Gabriel"));
    customers.Add(new Customers(0, "Lilian"));
    customers.Add(new Customers(0, "Sandro"));

    IEnumerable<string> distinctNames = customers.Select(x=>x.Name).Distinct();

    distinctNames.ToList().ForEach(x =>
    {
        Console.WriteLine(x.ToString());
    });

    Console.ReadKey();
}

public class Customers
{
    public int Id { get; set; }
    public string Name { get; set; }

    public Customers(int id, string name)
    {
        Id = id;
        Name = name;
    }
}

這是 output：

Answer 3

Distinct方法使用默認的相等比較器。
您需要重載Equals和GetHashCode方法才能正常工作。

public class Customers
{
    // your code here

    public override string ToString() => Id + " " + Name;

    public override bool Equals(object obj)
    {
        return obj is Customers customers &&
                Id == customers.Id &&
                Name == customers.Name;
    }

    public override int GetHashCode()
    {
        int hashCode = -1919740922;
        hashCode = hashCode * -1521134295 + Id.GetHashCode();
        hashCode = hashCode * -1521134295 + EqualityComparer<string>.Default.GetHashCode(Name);
        return hashCode;
    }
}