.NET Core(C#) IEqualityComparer<in T>接口的使用方法及示例代码

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;
        else
            return 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