在IEqualityComparer中包装委托

Question

几个Linq.Enumerable函数需要一个IEqualityComparer<T>.是否有一个方便的包装类适应delegate(T,T)=>bool实现IEqualityComparer<T>？编写一个很容易(如果你忽略了定义正确的哈希码的问题),但我想知道是否有开箱即用的解决方案.

具体来说,我想对Dictionarys 进行集合操作,仅使用Keys来定义成员资格(同时根据不同的规则保留值).

Answer 1

关于的重要性 GetHashCode

其他人已经评论过这样一个事实:任何自定义IEqualityComparer<T>实现都应该包含一个GetHashCode方法 ; 但是没有人愿意在任何细节上解释原因.

这就是原因.您的问题特别提到了LINQ扩展方法; 几乎所有这些都依赖哈希码来正常工作,因为它们在内部利用哈希表来提高效率.

就拿Distinct,例如.如果所有使用的Equals方法都是一种方法,请考虑这种扩展方法的含义.如果您只有一个项目已按顺序扫描,您如何确定Equals？您枚举了您已查看的整个值集合并检查匹配项.这将导致Distinct使用最坏情况的O(N ²)算法而不是O(N )算法!

幸运的是,事实并非如此.Distinct不只是使用Equals; 它也用GetHashCode.事实上,如果没有提供适当的产品,它绝对不能正常工作IEqualityComparer<T>GetHashCode.下面是一个说明这一点的人为例子.

说我有以下类型:

class Value
{
    public string Name { get; private set; }
    public int Number { get; private set; }

    public Value(string name, int number)
    {
        Name = name;
        Number = number;
    }

    public override string ToString()
    {
        return string.Format("{0}: {1}", Name, Number);
    }
}

现在说我有一个List<Value>,我想找到所有具有不同名称的元素.这是Distinct使用自定义相等比较器的完美用例.因此,让我们使用Comparer<T>类从阿库的回答:

var comparer = new Comparer<Value>((x, y) => x.Name == y.Name);

现在,如果我们有一堆Value具有相同Name属性的元素,它们应该全部折叠成一个返回的值Distinct,对吧？让我们来看看...

var values = new List<Value>();

var random = new Random();
for (int i = 0; i < 10; ++i)
{
    values.Add("x", random.Next());
}

var distinct = values.Distinct(comparer);

foreach (Value x in distinct)
{
    Console.WriteLine(x);
}

输出:

x: 1346013431
x: 1388845717
x: 1576754134
x: 1104067189
x: 1144789201
x: 1862076501
x: 1573781440
x: 646797592
x: 655632802
x: 1206819377

嗯,这没用,是吗？

怎么样GroupBy？我们试试看:

var grouped = values.GroupBy(x => x, comparer);

foreach (IGrouping<Value> g in grouped)
{
    Console.WriteLine("[KEY: '{0}']", g);
    foreach (Value x in g)
    {
        Console.WriteLine(x);
    }
}

输出:

[KEY = 'x: 1346013431']
x: 1346013431
[KEY = 'x: 1388845717']
x: 1388845717
[KEY = 'x: 1576754134']
x: 1576754134
[KEY = 'x: 1104067189']
x: 1104067189
[KEY = 'x: 1144789201']
x: 1144789201
[KEY = 'x: 1862076501']
x: 1862076501
[KEY = 'x: 1573781440']
x: 1573781440
[KEY = 'x: 646797592']
x: 646797592
[KEY = 'x: 655632802']
x: 655632802
[KEY = 'x: 1206819377']
x: 1206819377

再说一遍:没有用.

如果你考虑一下,在内部Distinct使用HashSet<T>(或等效的),以及在内部GroupBy使用类似的东西是有意义的Dictionary<TKey, List<T>>.这可以解释为什么这些方法不起作用？我们试试这个:

var uniqueValues = new HashSet<Value>(values, comparer);

foreach (Value x in uniqueValues)
{
    Console.WriteLine(x);
}

输出:

x: 1346013431
x: 1388845717
x: 1576754134
x: 1104067189
x: 1144789201
x: 1862076501
x: 1573781440
x: 646797592
x: 655632802
x: 1206819377

是的......开始有意义吗？

希望从这些例子可以清楚地看出为什么GetHashCode在任何IEqualityComparer<T>实现中包含一个适当的实例是如此重要.

原始答案

扩展orip的答案:

这里可以做一些改进.

1. 首先,我会采取一个Func<T, TKey>而不是Func<T, object>; 这将防止在实际keyExtractor本身中装入值类型键.
2. 其次,我实际上添加了一个where TKey : IEquatable<TKey>约束; 这将阻止在Equals调用中装箱(object.Equals接受一个object参数;你需要一个IEquatable<TKey>实现来获取TKey参数而不用装箱).显然,这可能会造成太严格的限制,因此您可以创建没有约束的基类和带有它的派生类.

以下是生成的代码的外观:

public class KeyEqualityComparer<T, TKey> : IEqualityComparer<T>
{
    protected readonly Func<T, TKey> keyExtractor;

    public KeyEqualityComparer(Func<T, TKey> keyExtractor)
    {
        this.keyExtractor = keyExtractor;
    }

    public virtual bool Equals(T x, T y)
    {
        return this.keyExtractor(x).Equals(this.keyExtractor(y));
    }

    public int GetHashCode(T obj)
    {
        return this.keyExtractor(obj).GetHashCode();
    }
}

public class StrictKeyEqualityComparer<T, TKey> : KeyEqualityComparer<T, TKey>
    where TKey : IEquatable<TKey>
{
    public StrictKeyEqualityComparer(Func<T, TKey> keyExtractor)
        : base(keyExtractor)
    { }

    public override bool Equals(T x, T y)
    {
        // This will use the overload that accepts a TKey parameter
        // instead of an object parameter.
        return this.keyExtractor(x).Equals(this.keyExtractor(y));
    }
}

Answer 2

当您想要自定义相等性检查时,99%的时间您有兴趣定义要比较的键,而不是比较本身.

这可能是一个优雅的解决方案(Python的列表排序方法的概念).

用法:

var foo = new List<string> { "abc", "de", "DE" };

// case-insensitive distinct
var distinct = foo.Distinct(new KeyEqualityComparer<string>( x => x.ToLower() ) );

本KeyEqualityComparer类:

public class KeyEqualityComparer<T> : IEqualityComparer<T>
{
    private readonly Func<T, object> keyExtractor;

    public KeyEqualityComparer(Func<T,object> keyExtractor)
    {
        this.keyExtractor = keyExtractor;
    }

    public bool Equals(T x, T y)
    {
        return this.keyExtractor(x).Equals(this.keyExtractor(y));
    }

    public int GetHashCode(T obj)
    {
        return this.keyExtractor(obj).GetHashCode();
    }
}

Answer 3

我担心没有这样的包装盒开箱即用.然而,创建一个并不难:

class Comparer<T>: IEqualityComparer<T>
{
    private readonly Func<T, T, bool> _comparer;

    public Comparer(Func<T, T, bool> comparer)
    {
        if (comparer == null)
            throw new ArgumentNullException("comparer");

        _comparer = comparer;
    }

    public bool Equals(T x, T y)
    {
        return _comparer(x, y);
    }

    public int GetHashCode(T obj)
    {
        return obj.ToString().ToLower().GetHashCode();
    }
}

...

Func<int, int, bool> f = (x, y) => x == y;
var comparer = new Comparer<int>(f);
Console.WriteLine(comparer.Equals(1, 1));
Console.WriteLine(comparer.Equals(1, 2));

Answer 4

通常情况下,我会通过在答案上评论@Sam来解决这个问题(我已经对原始帖子进行了一些编辑,以便在不改变行为的情况下对其进行清理.)

以下是我对@Sam 的答案的重复,对[IMNSHO]关键修复默认的散列策略: -

class FuncEqualityComparer<T> : IEqualityComparer<T>
{
    readonly Func<T, T, bool> _comparer;
    readonly Func<T, int> _hash;

    public FuncEqualityComparer( Func<T, T, bool> comparer )
        : this( comparer, t => 0 ) // NB Cannot assume anything about how e.g., t.GetHashCode() interacts with the comparer's behavior
    {
    }

    public FuncEqualityComparer( Func<T, T, bool> comparer, Func<T, int> hash )
    {
        _comparer = comparer;
        _hash = hash;
    }

    public bool Equals( T x, T y )
    {
        return _comparer( x, y );
    }

    public int GetHashCode( T obj )
    {
        return _hash( obj );
    }
}

Answer 5

与丹涛的答案相同,但有一些改进:

1. 依赖EqualityComparer<>.Default于进行实际比较,以避免对struct已实现的值类型进行装箱IEquatable<>.

2. 自EqualityComparer<>.Default使用以来它不会爆炸null.Equals(something).

3. 提供静态包装器IEqualityComparer<>,它将有一个静态方法来创建比较器实例 - 简化调用.相比

   Equality<Person>.CreateComparer(p => p.ID);
   

   同

   new EqualityComparer<Person, int>(p => p.ID);
   

4. 添加了指定IEqualityComparer<>密钥的重载.

班级:

public static class Equality<T>
{
    public static IEqualityComparer<T> CreateComparer<V>(Func<T, V> keySelector)
    {
        return CreateComparer(keySelector, null);
    }

    public static IEqualityComparer<T> CreateComparer<V>(Func<T, V> keySelector, 
                                                         IEqualityComparer<V> comparer)
    {
        return new KeyEqualityComparer<V>(keySelector, comparer);
    }

    class KeyEqualityComparer<V> : IEqualityComparer<T>
    {
        readonly Func<T, V> keySelector;
        readonly IEqualityComparer<V> comparer;

        public KeyEqualityComparer(Func<T, V> keySelector, 
                                   IEqualityComparer<V> comparer)
        {
            if (keySelector == null)
                throw new ArgumentNullException("keySelector");

            this.keySelector = keySelector;
            this.comparer = comparer ?? EqualityComparer<V>.Default;
        }

        public bool Equals(T x, T y)
        {
            return comparer.Equals(keySelector(x), keySelector(y));
        }

        public int GetHashCode(T obj)
        {
            return comparer.GetHashCode(keySelector(obj));
        }
    }
}

你可以像这样使用它:

var comparer1 = Equality<Person>.CreateComparer(p => p.ID);
var comparer2 = Equality<Person>.CreateComparer(p => p.Name);
var comparer3 = Equality<Person>.CreateComparer(p => p.Birthday.Year);
var comparer4 = Equality<Person>.CreateComparer(p => p.Name, StringComparer.CurrentCultureIgnoreCase);

人是一个简单的类:

class Person
{
    public int ID { get; set; }
    public string Name { get; set; }
    public DateTime Birthday { get; set; }
}

Answer 6

public class FuncEqualityComparer<T> : IEqualityComparer<T>
{
    readonly Func<T, T, bool> _comparer;
    readonly Func<T, int> _hash;

    public FuncEqualityComparer( Func<T, T, bool> comparer )
        : this( comparer, t => t.GetHashCode())
    {
    }

    public FuncEqualityComparer( Func<T, T, bool> comparer, Func<T, int> hash )
    {
        _comparer = comparer;
        _hash = hash;
    }

    public bool Equals( T x, T y )
    {
        return _comparer( x, y );
    }

    public int GetHashCode( T obj )
    {
        return _hash( obj );
    }
}

随着扩展: -

public static class SequenceExtensions
{
    public static bool SequenceEqual<T>( this IEnumerable<T> first, IEnumerable<T> second, Func<T, T, bool> comparer )
    {
        return first.SequenceEqual( second, new FuncEqualityComparer<T>( comparer ) );
    }

    public static bool SequenceEqual<T>( this IEnumerable<T> first, IEnumerable<T> second, Func<T, T, bool> comparer, Func<T, int> hash )
    {
        return first.SequenceEqual( second, new FuncEqualityComparer<T>( comparer, hash ) );
    }
}

Answer 7

orip的答案很棒.

这里有一个小扩展方法,使它更容易:

public static IEnumerable<T> Distinct<T>(this IEnumerable<T> list, Func<T, object>    keyExtractor)
{
    return list.Distinct(new KeyEqualityComparer<T>(keyExtractor));
}
var distinct = foo.Distinct(x => x.ToLower())