C# 如何實現一個基於值相等性比較的字典

Intro

今天在項目裡遇到一個需求,大概是這樣的我要比較兩個 JSON 字符串是不是相等,JSON 字符串其實是一個 Dictionary<string, string> 但是順序可能不同,和上一篇 record 使用場景 中的第一個需求類似,前面我們介紹過使用 record 可以比較方便的解決,但是我們的項目是 .netcoreapp3.1 的,不能使用 record,如何比較方便的比較呢?我們能否自己實現一個類似於 record 的類型,基於值去比較呢?於是就有瞭本文的探索

StringValueDictioanry

實現瞭一個基於值進行比較的字典,實現代碼如下,實現的比較簡單,涉及到一些簡單的知識點,平時不怎麼用已經忘瞭怎麼寫瞭,通過寫下面的代碼又學習瞭一下

先來看測試代碼吧,測試代碼如下:

[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);
}

從上面的代碼可能大概能看出一些實現,重寫瞭默認的 EqualsGetHashCode,並重載瞭“==” 運算符,並且實現瞭一個從 StringValueDictionary Dictionary 的隱式轉換,來看下面的實現代碼:

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

上述代碼實現的有點粗糙,可能會有一些問題,僅供參考

以上代碼基本實現瞭基於想要的值的相等性比較以及 Clone(復制、克隆)的目標

實現相等性比較的時候,EqualsGetHashCode 方法也要重寫,如果沒有重寫 GetHashCode,編譯器也會給出警告,如果沒有重寫 GetHashCode 在實際在 HashSet 或者 Dictionary 裡可能會出現重復 key

重載運算符的時候需要一個靜態方法,”==” 和 “!=” 是一對操作運算符,如果要實現兩個都要實現,不能隻實現其中一個

implicit 也算是一個特殊的運算符,巧妙的使用隱式轉換可以大大簡化代碼的寫法,StackExchange.Redis 中就使用瞭 implicit 來實現 RedisValuestring 等其他常用類型的隱式轉換

References

https://github.com/WeihanLi/WeihanLi.Common/blob/dev/src/WeihanLi.Common/Models/StringValueDictionary.cs
https://github.com/WeihanLi/WeihanLi.Common/blob/dev/test/WeihanLi.Common.Test/ModelsTest/StringValueDictionaryTest.cs

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