C# 如何实现一个基于值相等性比较的字典
作者：WeihanLi
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 Equals 和 GetHashCode，并重载了“==” 运算符，并且实现了一个从 StringValueDictionary 到 Dictionary 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，
Equals和GetHashCode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
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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