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C# 实现一个基于值相等性比较的字典


C# 实现一个基于值相等性比较的字典

Intro

今天在项目里遇到一个需求,大概是这样的我要比较两个 JSON 字符串是不是相等,JSON 字符串其实是一个 Dictionary<string, string> 但是顺序可能不同,和上一篇 record 使用场景 中的第一个需求类似,前面我们介绍过使用 record 可以比较方便的解决,但是我们的项目是 .netcoreapp3.1 的,不能使用 record,如何比较方便的比较呢?我们能否自己实现一个类似于 record 的类型,基于值去比较呢?于是就有了本文的探索

StringValueDictioanry

实现了一个基于值进行比较的字典,实现代码如下,实现的比较简单,涉及到一些简单的知识点,平时不怎么用已经忘了怎么写了,通过写下面的代码又学习了一下

先来看测试代码吧,测试代码如下:

[Fact] public void EqualsTest() {     var abc = new { Id = 1, Name = "Tom" };     var dic1 = StringValueDictionary.FromObject(abc);     var dic2 = StringValueDictionary.FromObject(new Dictionary<string, object>()     {         {"Name", "Tom" },         {"Id", 1},     });      Assert.True(dic1 == dic2);     Assert.Equal(dic1, dic2); }  [Fact] public void DistinctTest() {     var abc = new { Id = 1, Name = "Tom" };     var dic1 = StringValueDictionary.FromObject(abc);     var dic2 = StringValueDictionary.FromObject(new Dictionary<string, object>()     {         {"Id", 1},         {"Name", "Tom" },     });     var set = new HashSet<StringValueDictionary>();     set.Add(dic1);     set.Add(dic2);      Assert.Single(set); }  [Fact] public void CloneTest() {     var dic1 = StringValueDictionary.FromObject(new Dictionary<string, object>()     {         {"Id", 1},         {"Name", "Tom" }     });     var dic2 = dic1.Clone();     Assert.False(ReferenceEquals(dic1, dic2));     Assert.True(dic1 == dic2); }  [Fact] public void ImplicitConvertTest() {     var abc = new { Id = 1, Name = "Tom" };     var stringValueDictionary = StringValueDictionary.FromObject(abc);     Dictionary<string, string> dictionary = stringValueDictionary;     Assert.Equal(stringValueDictionary.Count, dictionary.Count);      var dic2 = StringValueDictionary.FromObject(dictionary);      Assert.Equal(dic2, stringValueDictionary);     Assert.True(dic2 == stringValueDictionary); } 

从上面的代码可能大概能看出一些实现,重写了默认的 EqualsGetHashCode,并重载了“==” 运算符,并且实现了一个从 StringValueDictionaryDictionary 的隐式转换,来看下面的实现代码:

public sealed class StringValueDictionary : IEquatable<StringValueDictionary> {     private readonly Dictionary<string, string?> _dictionary = new();      private StringValueDictionary(IDictionary<string, string?> dictionary)     {         foreach (var pair in dictionary)         {             _dictionary[pair.Key] = pair.Value;         }     }      private StringValueDictionary(StringValueDictionary dictionary)     {         foreach (var key in dictionary.Keys)         {             _dictionary[key] = dictionary[key];         }     }      public static StringValueDictionary FromObject(object obj)     {         if (obj is null) throw new ArgumentNullException(nameof(obj));         if (obj is IDictionary<string, string?> dictionary)         {             return new StringValueDictionary(dictionary);         }         if (obj is IDictionary<string, object?> dictionary2)         {             return new StringValueDictionary(dictionary2.ToDictionary(p => p.Key, p => p.Value?.ToString()));         }         if (obj is StringValueDictionary dictionary3)         {             return new StringValueDictionary(dictionary3);         }         return new StringValueDictionary(obj.GetType().GetProperties()             .ToDictionary(p => p.Name, p => p.GetValue(obj)?.ToString()));     }      public static StringValueDictionary FromJson(string json)     {         Guard.NotNull(json, nameof(json));         var dic = json.JsonToObject<Dictionary<string, object?>>()             .ToDictionary(x => x.Key, x => x.Value?.ToString());         return new StringValueDictionary(dic);     }      public StringValueDictionary Clone() => new(this);      public int Count => _dictionary.Count;      public bool ContainsKey(string key) => _dictionary.ContainsKey(key) ? _dictionary.ContainsKey(key) : throw new ArgumentOutOfRangeException(nameof(key));      public string? this[string key] => _dictionary[key];      public Dictionary<string, string>.KeyCollection Keys => _dictionary.Keys!;      public bool Equals(StringValueDictionary? other)     {         if (other is null) return false;         if (other.Count != Count) return false;         foreach (var key in _dictionary.Keys)         {             if (!other.ContainsKey(key))             {                 return false;             }             if (_dictionary[key] != other[key])             {                 return false;             }         }         return true;     }      public override bool Equals(object obj)     {         return Equals(obj as StringValueDictionary);     }      public override int GetHashCode()     {         var stringBuilder = new StringBuilder();         foreach (var pair in _dictionary)         {             stringBuilder.Append($"{pair.Key}={pair.Value}_");         }         return stringBuilder.ToString().GetHashCode();     }      public static bool operator ==(StringValueDictionary? current, StringValueDictionary? other)     {         return current?.Equals(other) == true;     }      public static bool operator !=(StringValueDictionary? current, StringValueDictionary? other)     {         return current?.Equals(other) != true;     }      public static implicit operator Dictionary<string, string?>(StringValueDictionary dictionary)     {         return dictionary._dictionary;     } } 

More

上述代码实现的有点粗糙,可能会有一些问题,仅供参考

以上代码基本实现了基于想要的值的相等性比较以及 Clone(复制、克隆)的目标

实现相等性比较的时候,EqualsGetHashCode 方法也要重写,如果没有重写 GetHashCode,编译器也会给出警告,如果没有重写 GetHashCode 在实际在 HashSet 或者 Dictionary 里可能会出现重复 key

重载运算符的时候需要一个静态方法,”==” 和 “!=” 是一对操作运算符,如果要实现两个都要实现,不能只实现其中一个

implicit 也算是一个特殊的运算符,巧妙的使用隐式转换可以大大简化代码的写法,StackExchange.Redis 中就使用了 implicit 来实现 RedisValue 和 string 等其他常用类型的隐式转换

References

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