.NET Core(C#)中IEqualityComparer<T>接口的对象的主要作用是实现接口来判断两个对象是否相等，以下介绍一下 IEqualityComparerin T接口的简单介绍和实现使用的方法，以及相关示例代码。

 

1､IEqualityComparer<in T>的的GetHashCode和Equals方法

IEqualityComparer<in T>是用来比较对象是否相等，需要实现接口的public bool Equals()和public int GetHashCode()方法， IEqualityComparer<T>接口的实现类主要用在Linq.Distinct<in T>()方法中。当进行比较的时候，先行运行GetHashCode()方法比较两个obj的HashCode(哈希值)，如果obj的HashCode(哈希值)相同，再执行Equals()方法来比较。如果HashCode不同，则不执行Equals()。

2､IEqualityComparer<in T>按口实现示例代码

1) 自定义Box对象添加到字典集合。如果Box对象的尺寸相同，则认为它们相等。

using System;
using System.Collections.Generic;
class Example
{static void Main(){BoxEqualityComparer boxEqC = new BoxEqualityComparer();var boxes = new Dictionary<Box, string>(boxEqC);var redBox = new Box(4, 3, 4);AddBox(boxes, redBox, "red");var blueBox = new Box(4, 3, 4);AddBox(boxes, blueBox, "blue");var greenBox = new Box(3, 4, 3);AddBox(boxes, greenBox, "green");Console.WriteLine();Console.WriteLine("The dictionary contains {0} Box objects.",boxes.Count);}private static void AddBox(Dictionary<Box, String> dict, Box box, String name){try {dict.Add(box, name);}catch (ArgumentException e) {Console.WriteLine("Unable to add {0}: {1}", box, e.Message);}}
}
public class Box
{public Box(int h,  int l, int w){this.Height = h;this.Length = l;this.Width = w;}public int Height { get; set; }public int Length { get; set; }public int Width { get; set; }public override String ToString(){return String.Format("({0}, {1}, {2})", Height, Length, Width);}
}
class BoxEqualityComparer : IEqualityComparer<Box>
{public bool Equals(Box b1, Box b2){if (b2 == null && b1 == null)return true;else if (b1 == null || b2 == null)return false;else if(b1.Height == b2.Height && b1.Length == b2.Length&& b1.Width == b2.Width)return true;elsereturn false;}public int GetHashCode(Box bx){int hCode = bx.Height ^ bx.Length ^ bx.Width;return hCode.GetHashCode();}
}

 输出如下:

Unable to add (4, 3, 4): An item with the same key has already been added.

The dictionary contains 2 Box objects.

 

相关文档：iequalitycomparer

2) 去除字典中key不同但value是相同对象的重复数据

public class CachedLookup<T, TKey>
{        private readonly ConcurrentDictionary<T, T> _hashSet;private readonly ConcurrentDictionary<TKey, List<T>> _lookup = new ConcurrentDictionary<TKey, List<T>>();public CachedLookup(ConcurrentDictionary<T, T> hashSet){_hashSet = hashSet;}   public CachedLookup(IEqualityComparer<T> equalityComparer = default){_hashSet = equalityComparer is null ? new ConcurrentDictionary<T, T>() : new ConcurrentDictionary<T, T>(equalityComparer);}public List<T> Get(TKey key) => _lookup.ContainsKey(key) ? _lookup[key] : null;public List<T> Get(TKey key, Func<TKey, List<T>> getData){if (_lookup.ContainsKey(key))return _lookup[key];var result = DedupeAndCache(getData(key));_lookup.TryAdd(key, result);return result;}public async ValueTask<List<T>> GetAsync(TKey key, Func<TKey, Task<List<T>>> getData){if (_lookup.ContainsKey(key))return _lookup[key];var result = DedupeAndCache(await getData(key));_lookup.TryAdd(key, result);return result;}public void Add(T value) => _hashSet.TryAdd(value, value);public List<T> AddOrUpdate(TKey key, List<T> data){            var deduped = DedupeAndCache(data);_lookup.AddOrUpdate(key, deduped, (k,l)=>deduped);return deduped;}private List<T> DedupeAndCache(IEnumerable<T> input) => input.Select(v => _hashSet.GetOrAdd(v,v)).ToList();
}

 使用示例：

public class ExampleUsage
{private readonly CachedLookup<LanguageInfoModel, (string frontendId, string languageId, string accessId)> _lookup = new CachedLookup<LanguageInfoModel, (string frontendId, string languageId, string accessId)>(new LanguageInfoModelComparer());public ValueTask<List<LanguageInfoModel>> GetLanguagesAsync(string frontendId, string languageId, string accessId){return _lookup.GetAsync((frontendId, languageId, accessId), GetLanguagesFromDB(k));}private async Task<List<LanguageInfoModel>> GetLanguagesFromDB((string frontendId, string languageId, string accessId) key) => throw new NotImplementedException();
}
public class LanguageInfoModel
{public string FrontendId { get; set; }public string LanguageId { get; set; }public string AccessId { get; set; }public string SomeOtherUniqueValue { get; set; }
}
public class LanguageInfoModelComparer : IEqualityComparer<LanguageInfoModel>
{public bool Equals(LanguageInfoModel x, LanguageInfoModel y){return (x?.FrontendId, x?.AccessId, x?.LanguageId, x?.SomeOtherUniqueValue).Equals((y?.FrontendId, y?.AccessId, y?.LanguageId, y?.SomeOtherUniqueValue));}public int GetHashCode(LanguageInfoModel obj) => (obj.FrontendId, obj.LanguageId, obj.AccessId, obj.SomeOtherUniqueValue).GetHashCode();
}

 相关文档：system.object.gethashcode