.net 将委托包装在IEqualityComparer中
几个Linq.Enumerable函数采用.net 将委托包装在IEqualityComparer中,.net,linq,delegates,.net,Linq,Delegates,几个Linq.Enumerable函数采用IEqualityComparer。是否有一个方便的包装器类可以调整委托(T,T)=>bool来实现IEqualityComparer?编写一个很容易(如果您忽略了定义正确哈希代码的问题),但我想知道是否有现成的解决方案 具体来说,我想在字典上执行set操作,只使用键定义成员身份(同时根据不同的规则保留值)。恐怕没有现成的包装器。然而,创建一个并不困难: class Comparer<T>: IEqualityComparer<T>
IEqualityComparer
。是否有一个方便的包装器类可以调整委托(T,T)=>bool
来实现IEqualityComparer
?编写一个很容易(如果您忽略了定义正确哈希代码的问题),但我想知道是否有现成的解决方案
具体来说,我想在
字典
上执行set操作,只使用键定义成员身份(同时根据不同的规则保留值)。恐怕没有现成的包装器。然而,创建一个并不困难:
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));
类比较器:IEqualityComparer
{
专用只读函数比较器;
公共比较器(函数比较器)
{
if(比较器==null)
抛出新的ArgumentNullException(“comparer”);
_比较器=比较器;
}
公共布尔等于(TX,TY)
{
返回比较器(x,y);
}
公共int GetHashCode(T obj)
{
返回obj.ToString().ToLower().GetHashCode();
}
}
...
Func f=(x,y)=>x==y;
var比较器=新比较器(f);
Console.WriteLine(comparer.Equals(1,1));
Console.WriteLine(comparer.Equals(1,2));
我不知道现有的类,但类似于:
public class MyComparer<T> : IEqualityComparer<T>
{
private Func<T, T, bool> _compare;
MyComparer(Func<T, T, bool> compare)
{
_compare = compare;
}
public bool Equals(T x, Ty)
{
return _compare(x, y);
}
public int GetHashCode(T obj)
{
return obj.GetHashCode();
}
}
公共类MyComparer:IEqualityComparer
{
私有函数比较;
MyComparer(功能比较)
{
_比较=比较;
}
公共布尔等于(tx,Ty)
{
返回_比较(x,y);
}
公共int GetHashCode(T obj)
{
返回obj.GetHashCode();
}
}
注意:我还没有编译并运行这个程序,所以可能有输入错误或其他错误。我将回答我自己的问题。要将字典视为集合,最简单的方法似乎是将集合操作应用于dict.Keys,然后将其转换回具有Enumerable.ToDictionary(…)的字典。
公共类FunctionEqualityComparer:IEqualityComparer
{
只读函数比较器;
只读函数散列;
公共函数相等比较程序(函数比较程序)
:这(比较器,t=>t.GetHashCode())
{
}
公共FuncEqualityComparer(Func比较器、Func哈希)
{
_比较器=比较器;
_散列=散列;
}
公共布尔等于(TX,TY)
{
返回比较器(x,y);
}
公共int GetHashCode(T obj)
{
返回散列(obj);
}
}
带扩展名:-
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 ) );
}
}
公共静态类SequenceExtensions
{
公共静态布尔序列相等(此IEnumerable first,IEnumerable second,Func comparer)
{
返回first.SequenceEqual(第二个,新的functionequalitycomparer(comparer));
}
public static bool SequenceEqual(此IEnumerable first,IEnumerable second,Func comparer,Func hash)
{
返回first.SequenceEqual(第二个,新的functequalitycomparer(comparer,hash));
}
}
当您想要自定义相等性检查时,99%的时间您感兴趣的是定义要比较的键,而不是比较本身
这可能是一个优雅的解决方案(来自Python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() ) );
var n = ItemList.Select(s => new { s.Vchr, s.Id, s.Ctr, s.Vendor, s.Description, s.Invoice }).ToList();
n.AddRange(OtherList.Select(s => new { s.Vchr, s.Id, s.Ctr, s.Vendor, s.Description, s.Invoice }).ToList(););
n = n.Distinct(x=>new{Vchr=x.Vchr,Id=x.Id}).ToList();
只有一个优化:
我们可以使用开箱即用的EqualityComparer进行值比较,而不是委派它
这也会使实现更干净,因为实际的比较逻辑现在保留在GetHashCode()和Equals()中,您可能已经重载了它们
代码如下:
public class MyComparer<T> : IEqualityComparer<T>
{
public bool Equals(T x, T y)
{
return EqualityComparer<T>.Default.Equals(x, y);
}
public int GetHashCode(T obj)
{
return obj.GetHashCode();
}
}
公共类MyComparer:IEqualityComparer
{
公共布尔等于(TX,TY)
{
返回EqualityComparer.Default.Equals(x,y);
}
公共int GetHashCode(T obj)
{
返回obj.GetHashCode();
}
}
不要忘记在对象上重载GetHashCode()和Equals()方法
这篇文章帮助了我:
Sushil通常,我会通过在答案上评论@Sam来解决这个问题(我对原始帖子做了一些编辑,在不改变行为的情况下将其清理干净。) 以下是我的riff of,对默认哈希策略进行了[IMNSHO]关键修复:-
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 );
}
}
类FuncEqualityComparer:IEqualityComparer
{
只读函数比较器;
只读函数散列;
公共函数相等比较程序(函数比较程序)
:this(comparer,t=>0)//NB不能假设关于例如t.GetHashCode()如何与comparer的行为交互的任何内容
{
}
公共FuncEqualityComparer(Func比较器、Func哈希)
{
_比较器=比较器;
_散列=散列;
}
公共布尔等于(TX,TY)
{
返回比较器(x,y);
}
公共int GetHashCode(T obj)
{
返回散列(obj);
}
}
关于GetHashCode
其他人已经评论过这样一个事实,即任何定制的IEqualityComparer
实现都应该包含GetHashCode
方法;但没有人愿意详细解释原因
原因如下。您的问题特别提到LINQ扩展方法;几乎所有这些都依赖于哈希代码来正常工作,因为它们在内部利用哈希表来提高效率
比如说。如果这个扩展方法使用的是<代码>等于方法,则考虑该扩展方法的含义。如果只有等于
,如何确定项目是否已按顺序扫描?枚举已查看的整个值集合,并检查是否匹配。这将导致使用最坏情况下的O(N2)算法而不是O(N)算法进行Distinct
幸运的是,情况并非如此Distinct
不只是使用等于
;它还使用GetHashCode
。事实上,如果没有提供适当的GetHashCode
的IEqualityComparer
,它绝对无法正常工作。下面是一个人为的例子来说明这一点
假设我有以下类型:
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
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);
}
}
var comparer = new Comparer<Value>((x, y) => x.Name == y.Name);
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);
}
x: 1346013431
x: 1388845717
x: 1576754134
x: 1104067189
x: 1144789201
x: 1862076501
x: 1573781440
x: 646797592
x: 655632802
x: 1206819377
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);
}
}
[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
var uniqueValues = new HashSet<Value>(values, comparer);
foreach (Value x in uniqueValues)
{
Console.WriteLine(x);
}
x: 1346013431
x: 1388845717
x: 1576754134
x: 1104067189
x: 1144789201
x: 1862076501
x: 1573781440
x: 646797592
x: 655632802
x: 1206819377
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));
}
}
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())
public static Dictionary<TKey, TValue> Distinct<TKey, TValue>(this IEnumerable<TValue> items, Func<TValue, TKey> selector)
{
Dictionary<TKey, TValue> result = null;
ICollection collection = items as ICollection;
if (collection != null)
result = new Dictionary<TKey, TValue>(collection.Count);
else
result = new Dictionary<TKey, TValue>();
foreach (TValue item in items)
result[selector(item)] = item;
return result;
}
Equality<Person>.CreateComparer(p => p.ID);
new EqualityComparer<Person, int>(p => p.ID);
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));
}
}
}
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);
class Person
{
public int ID { get; set; }
public string Name { get; set; }
public DateTime Birthday { get; set; }
}
public static class Comparer
{
public static IEqualityComparer<T> CreateComparerForElements<T>(this IEnumerable<T> enumerable, Func<T, object> keyExtractor)
{
return new KeyEqualityComparer<T>(keyExtractor);
}
}
var n = ItemList.Select(s => new { s.Vchr, s.Id, s.Ctr, s.Vendor, s.Description, s.Invoice }).ToList();
n.AddRange(OtherList.Select(s => new { s.Vchr, s.Id, s.Ctr, s.Vendor, s.Description, s.Invoice }).ToList(););
n = n.Distinct(x=>new{Vchr=x.Vchr,Id=x.Id}).ToList();
persons1.Union(persons2, person => person.LastName)
public class LambdaEqualityComparer<TSource, TComparable> : IEqualityComparer<TSource>
{
Func<TSource, TComparable> _keyGetter;
public LambdaEqualityComparer(Func<TSource, TComparable> keyGetter)
{
_keyGetter = keyGetter;
}
public bool Equals(TSource x, TSource y)
{
if (x == null || y == null) return (x == null && y == null);
return object.Equals(_keyGetter(x), _keyGetter(y));
}
public int GetHashCode(TSource obj)
{
if (obj == null) return int.MinValue;
var k = _keyGetter(obj);
if (k == null) return int.MaxValue;
return k.GetHashCode();
}
}
public static class LambdaEqualityComparer
{
// source1.Union(source2, lambda)
public static IEnumerable<TSource> Union<TSource, TComparable>(
this IEnumerable<TSource> source1,
IEnumerable<TSource> source2,
Func<TSource, TComparable> keySelector)
{
return source1.Union(source2,
new LambdaEqualityComparer<TSource, TComparable>(keySelector));
}
}