C# 如何實(shí)現(xiàn)一個基于值相等性比較的字典

更新時間：2021年02月01日 08:57:07 作者：WeihanLi

這篇文章主要介紹了C# 如何實(shí)現(xiàn)一個基于值相等性比較的字典，幫助大家更好的理解和使用c#，感興趣的朋友可以了解下

Intro

今天在項(xiàng)目里遇到一個需求，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(shí)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(xiàng)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(shí)現(xiàn)一個類似于 record 的類型，基于值去比較呢？于是就有了本文的探索

StringValueDictioanry

實(shí)現(xiàn)了一個基于值進(jìn)行比較的字典，實(shí)現(xiàn)代碼如下，實(shí)現(xiàn)的比較簡單，涉及到一些簡單的知識點(diǎn)，平時不怎么用已經(jīng)忘了怎么寫了，通過寫下面的代碼又學(xué)習(xí)了一下

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(shí)現(xiàn)，重寫了默認(rèn)的 Equals 和 GetHashCode，并重載了“==” 運(yùn)算符，并且實(shí)現(xiàn)了一個從 StringValueDictionary 到 Dictionary 的隱式轉(zhuǎn)換，來看下面的實(shí)現(xiàn)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;
 }
}

上述代碼實(shí)現(xiàn)的有點(diǎn)粗糙，可能會有一些問題，僅供參考

以上代碼基本實(shí)現(xiàn)了基于想要的值的相等性比較以及 Clone（復(fù)制、克?。┑哪繕?biāo)

實(shí)現(xiàn)相等性比較的時候，Equals 和 GetHashCode 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(shí)際在 HashSet 或者 Dictionary 里可能會出現(xiàn)重復(fù) key

重載運(yùn)算符的時候需要一個靜態(tài)方法，"==" 和 "!=" 是一對操作運(yùn)算符，如果要實(shí)現(xiàn)兩個都要實(shí)現(xiàn)，不能只實(shí)現(xiàn)其中一個

implicit 也算是一個特殊的運(yùn)算符，巧妙的使用隱式轉(zhuǎn)換可以大大簡化代碼的寫法，StackExchange.Redis 中就使用了 implicit 來實(shí)現(xiàn) RedisValue 和 string 等其他常用類型的隱式轉(zhuǎn)換