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();
}