如何使用 Linq 获取没有重复的记录

Question

嗨，有人可以帮忙吗，我有一个包含我的促销代码的列表，在列表中我想只返回出现一次的促销代码，即没有重复，请参阅下面来自 JSON 的数据，我想返回促销代码 A123 和 B500 并将它们存储在另一个列表中。

[
   {
    "PromCode": "A123",
    "Priority": 1,
    "offer": "Win a Free Cap",
    "StartDte": "2020-08-11T00:16:23.184Z",
    "endDte": "2020-09-10T17:16:23.184Z",
  }, 
  {
    "PromCode": "A100",
    "Priority": 1,
    "offer": "Win a perfume",
    "StartDte": "2020-08-11T00:16:23.184Z",
    "endDte": "2020-09-10T17:16:23.184Z",
  },
{
    "PromCode": "A100",
    "Priority": 2,
    "offer": "Win a Phone pouch",
    "StartDte": "2020-09-11T00:16:23.184Z",
    "endDte": "2020-10-10T17:16:23.184Z",
  },
 {
    "PromCode": "B500",
    "Priority": 1,
    "offer": "Win a free router",
    "StartDte": "2020-08-11T00:16:23.184Z",
    "endDte": "2020-09-10T17:16:23.184Z",
  },

]

我有一个包含所有这些促销代码的列表，如下所示

var existingProms = await _Repo.GetAllPromCodes(promCodeList);

我试图得到像这样在列表中出现一次的那些

var appearOnce = existingProms.Count(x => existingBnplIndicators.Contains(x.PromCode)).ToList()<2;
var appearOnce = existingProms.where(x=> x.PromCode.Count()).ToList()<2;

但这没有用，返回了 0 个结果，有人可以告诉我如何让我的两个 Proms A123，B500 进入我的出现一次。谢谢

Answer 1

您可以使用GroupBy 获取按促销代码分组的所有结果。然后，根据每个组必须仅在Count() == 1 时显示的项目数过滤结果。

可能是这样的，

public class Class1
{
    public string PromCode { get; set; }
    public int Priority { get; set; }
    public string offer { get; set; }
    public DateTime StartDte { get; set; }
    public DateTime endDte { get; set; }
}

var obj = JsonConvert.DeserializeObject<List<Class1>>(json);
var singlesOnly = obj.GroupBy(x => x.PromCode).Where(y => y.Count() == 1);

Answer 2

您应该仅通过 promcode 隐式比较所需的对象。看看 Equals 和 GetHashCode() 是如何工作的。

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

namespace test
{
    public class TestObj : IEquatable<TestObj>
    {
        public string Promocode { get; set; }
        public int Priority { get; set; }
        public string offer { get; set; }
        public DateTime StartDate { get; set; }
        public DateTime endDate { get; set; }
        public override int GetHashCode() => this.Promocode.GetHashCode();
        public bool Equals(TestObj other) => Promocode.Equals(other.Promocode);
    }
    class Program
    {
        static void Main(string[] args)
        {
            var a = new TestObj()
            {
                Promocode = "1",
                Priority = 1,
                offer = "1",
                StartDate = new DateTime(1, 2, 3),
                endDate = new DateTime(1, 2, 3)
            };

            var b = new TestObj()
            {
                Promocode = "1",
                Priority = 1,
                offer = "1",
                StartDate = new DateTime(1, 2, 3),
                endDate = new DateTime(1, 2, 3)
            };
            var c = new TestObj()
            {
                Promocode = "2",
                Priority = 1,
                offer = "1",
                StartDate = new DateTime(1, 2, 3),
                endDate = new DateTime(1, 2, 3)
            };

            var list = new List<TestObj>()
            {
                a,
                b,
                c
            };

            var uniqueOnly = list.Distinct();

            foreach (var item in uniqueOnly)
            {
                Console.WriteLine(item.Promocode);
            }
        }
    }
}

Answer 3

首先定义类项-

        public string PromCode { get; set; }
        public int Priority { get; set; }
        public string offer { get; set; }
        public DateTime StartDte { get; set; }
        public DateTime endDte { get; set; }

然后取出列表中的多个项目并使用 distinct() 函数删除重复值-

static void Main(string[] args)
    {
        List<TestClass> list = new List<TestClass>()
        {
            new TestClass(){PromCode="A123",Priority=1,offer="Win a Free 
            Cap",StartDte=new DateTime(2020-08-11),endDte=new DateTime(2020-09-10)},

            new TestClass(){PromCode="A100",Priority=1,offer="Win a 
            perfume",StartDte=new DateTime(2020-08-11),endDte=new DateTime(2020-09- 
            10)},

            new TestClass(){PromCode="A100",Priority=2,offer="Win a Phone 
            pouch",StartDte=new DateTime(2020-09-11),endDte=new DateTime(2020-10-10)},

            new TestClass(){PromCode="B500",Priority=1,offer="Win a free 
            router",StartDte=new DateTime(2020-08-11),endDte=new DateTime(2020-09-10)}
        };

        var finalList = list.Select(b => b.PromCode).Distinct().ToList();
        foreach(var item in finalList)
        {
            Console.WriteLine(item + "");
        }
        Console.Read();
        }
    }