在.NET中，GetHashCode方法在整个.NET基类库的许多地方都使用。正确执行它对于在集合中或确定相等时快速查找项目尤为重要。
对于如何为自定义类实现GetHashCode，是否有标准算法或最佳实践，以便不会降低性能？
当前回答
使用System.HashCode
如果使用的是.NET Standard 2.1或更高版本，则可以使用System.HashCode结构。在早期的框架中，它可以从Microsoft.Bcl.HashCode包中获得。有两种使用方法：
HashCode.Combine
Combine方法可用于创建哈希代码，最多可提供八个对象。
public override int GetHashCode() => HashCode.Combine(this.object1, this.object2);
HashCode.添加
Add方法帮助您处理集合：
public override int GetHashCode()
{
var hashCode = new HashCode();
hashCode.Add(this.object1);
foreach (var item in this.collection)
{
hashCode.Add(item);
}
return hashCode.ToHashCode();
}
GetHashCode变得简单
System.HashCode的替代品，超级容易使用，但速度仍然很快。您可以阅读完整的博客文章“GetHashCode Made Easy”以了解更多详细信息和评论。
用法示例
public class SuperHero
{
public int Age { get; set; }
public string Name { get; set; }
public List<string> Powers { get; set; }
public override int GetHashCode() =>
HashCode.Of(this.Name).And(this.Age).AndEach(this.Powers);
}
实施
public struct HashCode : IEquatable<HashCode>
{
private const int EmptyCollectionPrimeNumber = 19;
private readonly int value;
private HashCode(int value) => this.value = value;
public static implicit operator int(HashCode hashCode) => hashCode.value;
public static bool operator ==(HashCode left, HashCode right) => left.Equals(right);
public static bool operator !=(HashCode left, HashCode right) => !(left == right);
public static HashCode Of<T>(T item) => new HashCode(GetHashCode(item));
public static HashCode OfEach<T>(IEnumerable<T> items) =>
items == null ? new HashCode(0) : new HashCode(GetHashCode(items, 0));
public HashCode And<T>(T item) =>
new HashCode(CombineHashCodes(this.value, GetHashCode(item)));
public HashCode AndEach<T>(IEnumerable<T> items)
{
if (items == null)
{
return new HashCode(this.value);
}
return new HashCode(GetHashCode(items, this.value));
}
public bool Equals(HashCode other) => this.value.Equals(other.value);
public override bool Equals(object obj)
{
if (obj is HashCode)
{
return this.Equals((HashCode)obj);
}
return false;
}
public override int GetHashCode() => this.value.GetHashCode();
private static int CombineHashCodes(int h1, int h2)
{
unchecked
{
// Code copied from System.Tuple a good way to combine hashes.
return ((h1 << 5) + h1) ^ h2;
}
}
private static int GetHashCode<T>(T item) => item?.GetHashCode() ?? 0;
private static int GetHashCode<T>(IEnumerable<T> items, int startHashCode)
{
var temp = startHashCode;
var enumerator = items.GetEnumerator();
if (enumerator.MoveNext())
{
temp = CombineHashCodes(temp, GetHashCode(enumerator.Current));
while (enumerator.MoveNext())
{
temp = CombineHashCodes(temp, GetHashCode(enumerator.Current));
}
}
else
{
temp = CombineHashCodes(temp, EmptyCollectionPrimeNumber);
}
return temp;
}
}
什么是好算法？
表演
计算哈希码的算法需要很快。简单的算法通常会更快。不分配额外内存的内存也会减少垃圾收集的需求，这反过来也会提高性能。
具体来说，在C#哈希函数中，您经常使用unchecked关键字来停止溢出检查以提高性能。
确定性
哈希算法需要是确定性的，即给定相同的输入，它必须始终产生相同的输出。
减少碰撞
计算哈希代码的算法需要将哈希冲突保持在最小值。哈希冲突是在两个不同对象上对GetHashCode的两次调用产生相同哈希代码时发生的情况。请注意，碰撞是允许的（有些人认为不允许），但应将其保持在最低限度。
许多哈希函数包含像17或23这样的幻数。这些是特殊的素数，与使用非素数相比，由于其数学财产有助于减少散列冲突。
哈希一致性
一个好的哈希函数应该在其输出范围内尽可能均匀地映射期望的输入，即，它应该基于均匀分布的输入输出广泛的哈希。它应该具有哈希一致性。
阻止的DoS
在.NETCore中，每次重新启动应用程序时，都会得到不同的哈希代码。这是防止拒绝服务攻击（DoS）的安全功能。对于.NET Framework，应通过添加以下App.config文件来启用此功能：
<?xml version ="1.0"?>
<configuration>
<runtime>
<UseRandomizedStringHashAlgorithm enabled="1" />
</runtime>
</configuration>
由于此特性，哈希代码不应在创建它们的应用程序域之外使用，也不应将其用作集合中的关键字段，也不应该持久化。
请在此处阅读更多信息。
加密安全？
算法不必是加密哈希函数。这意味着它不必满足以下条件：
生成生成给定哈希值的消息是不可行的。找到具有相同哈希值的两个不同消息是不可行的。对消息进行一次小的更改应该会对哈希值进行广泛的更改，以使新的哈希值看起来与旧的哈希值不相关（雪崩效应）。
其他回答
可以尝试采用C++Boost库的方法。类似于：
class HashUtil
{
public static int HashCombine(int seed, int other)
{
unchecked
{
return other + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
}
}
然后：
class MyClass
{
private string _field1;
private int _field2;
private AnotherClass _field3;
private YetAnotherClass _field4;
public override int GetHashCode()
{
int result = HashUtil.HashCombine(_field1.GetHashCode(), _field2);
result = HashUtil.HashCombine(result, _field3.GetHashCode());
return HashUtil.HashCombine(result, _field4.GetHashCode());
}
}
这是我使用JonSkeet实现的助手类。
public static class HashCode
{
public const int Start = 17;
public static int Hash<T>(this int hash, T obj)
{
var h = EqualityComparer<T>.Default.GetHashCode(obj);
return unchecked((hash * 31) + h);
}
}
用法：
public override int GetHashCode()
{
return HashCode.Start
.Hash(_field1)
.Hash(_field2)
.Hash(_field3);
}
如果要避免为System.Int32编写扩展方法：
public readonly struct HashCode
{
private readonly int _value;
public HashCode(int value) => _value = value;
public static HashCode Start { get; } = new HashCode(17);
public static implicit operator int(HashCode hash) => hash._value;
public HashCode Hash<T>(T obj)
{
var h = EqualityComparer<T>.Default.GetHashCode(obj);
return unchecked(new HashCode((_value * 31) + h));
}
public override int GetHashCode() => _value;
}
它仍然避免了任何堆分配，使用方式完全相同：
public override int GetHashCode()
{
// This time `HashCode.Start` is not an `Int32`, it's a `HashCode` instance.
// And the result is implicitly converted to `Int32`.
return HashCode.Start
.Hash(_field1)
.Hash(_field2)
.Hash(_field3);
}
编辑（2018年5月）：EqualityComparer＜T＞。默认getter现在是JIT内在的-Stephen Toub在这篇博文中提到了pull请求。
我在Helper库中有一个Hashing类，用于此目的。
/// <summary>
/// This is a simple hashing function from Robert Sedgwicks Hashing in C book.
/// Also, some simple optimizations to the algorithm in order to speed up
/// its hashing process have been added. from: www.partow.net
/// </summary>
/// <param name="input">array of objects, parameters combination that you need
/// to get a unique hash code for them</param>
/// <returns>Hash code</returns>
public static int RSHash(params object[] input)
{
const int b = 378551;
int a = 63689;
int hash = 0;
// If it overflows then just wrap around
unchecked
{
for (int i = 0; i < input.Length; i++)
{
if (input[i] != null)
{
hash = hash * a + input[i].GetHashCode();
a = a * b;
}
}
}
return hash;
}
然后，只需将其用作：
public override int GetHashCode()
{
return Hashing.RSHash(_field1, _field2, _field3);
}
我没有评估它的表现，所以欢迎任何反馈。
我的大部分工作都是通过数据库连接完成的，这意味着我的类都具有来自数据库的唯一标识符。我总是使用数据库中的ID来生成哈希代码。
// Unique ID from database
private int _id;
...
{
return _id.GetHashCode();
}
这是一个实现Josh Bloch实现的静态助手类；并且提供了显式重载来“防止”装箱，并且还专门为长原语实现哈希。
您可以传递与equals实现匹配的字符串比较。
因为Hash输出始终是int，所以您可以只链接Hash调用。
using System;
using System.Collections;
using System.Collections.Generic;
using System.Reflection;
using System.Runtime.CompilerServices;
namespace Sc.Util.System
{
/// <summary>
/// Static methods that allow easy implementation of hashCode. Example usage:
/// <code>
/// public override int GetHashCode()
/// => HashCodeHelper.Seed
/// .Hash(primitiveField)
/// .Hsh(objectField)
/// .Hash(iEnumerableField);
/// </code>
/// </summary>
public static class HashCodeHelper
{
/// <summary>
/// An initial value for a hashCode, to which is added contributions from fields.
/// Using a non-zero value decreases collisions of hashCode values.
/// </summary>
public const int Seed = 23;
private const int oddPrimeNumber = 37;
/// <summary>
/// Rotates the seed against a prime number.
/// </summary>
/// <param name="aSeed">The hash's first term.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static int rotateFirstTerm(int aSeed)
{
unchecked {
return HashCodeHelper.oddPrimeNumber * aSeed;
}
}
/// <summary>
/// Contributes a boolean to the developing HashCode seed.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="aBoolean">The value to contribute.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Hash(this int aSeed, bool aBoolean)
{
unchecked {
return HashCodeHelper.rotateFirstTerm(aSeed)
+ (aBoolean
? 1
: 0);
}
}
/// <summary>
/// Contributes a char to the developing HashCode seed.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="aChar">The value to contribute.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Hash(this int aSeed, char aChar)
{
unchecked {
return HashCodeHelper.rotateFirstTerm(aSeed)
+ aChar;
}
}
/// <summary>
/// Contributes an int to the developing HashCode seed.
/// Note that byte and short are handled by this method, through implicit conversion.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="aInt">The value to contribute.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Hash(this int aSeed, int aInt)
{
unchecked {
return HashCodeHelper.rotateFirstTerm(aSeed)
+ aInt;
}
}
/// <summary>
/// Contributes a long to the developing HashCode seed.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="aLong">The value to contribute.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Hash(this int aSeed, long aLong)
{
unchecked {
return HashCodeHelper.rotateFirstTerm(aSeed)
+ (int)(aLong ^ (aLong >> 32));
}
}
/// <summary>
/// Contributes a float to the developing HashCode seed.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="aFloat">The value to contribute.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Hash(this int aSeed, float aFloat)
{
unchecked {
return HashCodeHelper.rotateFirstTerm(aSeed)
+ Convert.ToInt32(aFloat);
}
}
/// <summary>
/// Contributes a double to the developing HashCode seed.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="aDouble">The value to contribute.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Hash(this int aSeed, double aDouble)
=> aSeed.Hash(Convert.ToInt64(aDouble));
/// <summary>
/// Contributes a string to the developing HashCode seed.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="aString">The value to contribute.</param>
/// <param name="stringComparison">Optional comparison that creates the hash.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Hash(
this int aSeed,
string aString,
StringComparison stringComparison = StringComparison.Ordinal)
{
if (aString == null)
return aSeed.Hash(0);
switch (stringComparison) {
case StringComparison.CurrentCulture :
return StringComparer.CurrentCulture.GetHashCode(aString);
case StringComparison.CurrentCultureIgnoreCase :
return StringComparer.CurrentCultureIgnoreCase.GetHashCode(aString);
case StringComparison.InvariantCulture :
return StringComparer.InvariantCulture.GetHashCode(aString);
case StringComparison.InvariantCultureIgnoreCase :
return StringComparer.InvariantCultureIgnoreCase.GetHashCode(aString);
case StringComparison.OrdinalIgnoreCase :
return StringComparer.OrdinalIgnoreCase.GetHashCode(aString);
default :
return StringComparer.Ordinal.GetHashCode(aString);
}
}
/// <summary>
/// Contributes a possibly-null array to the developing HashCode seed.
/// Each element may be a primitive, a reference, or a possibly-null array.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="aArray">CAN be null.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Hash(this int aSeed, IEnumerable aArray)
{
if (aArray == null)
return aSeed.Hash(0);
int countPlusOne = 1; // So it differs from null
foreach (object item in aArray) {
++countPlusOne;
if (item is IEnumerable arrayItem) {
if (!object.ReferenceEquals(aArray, arrayItem))
aSeed = aSeed.Hash(arrayItem); // recursive call!
} else
aSeed = aSeed.Hash(item);
}
return aSeed.Hash(countPlusOne);
}
/// <summary>
/// Contributes a possibly-null array to the developing HashCode seed.
/// You must provide the hash function for each element.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="aArray">CAN be null.</param>
/// <param name="hashElement">Required: yields the hash for each element
/// in <paramref name="aArray"/>.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Hash<T>(this int aSeed, IEnumerable<T> aArray, Func<T, int> hashElement)
{
if (aArray == null)
return aSeed.Hash(0);
int countPlusOne = 1; // So it differs from null
foreach (T item in aArray) {
++countPlusOne;
aSeed = aSeed.Hash(hashElement(item));
}
return aSeed.Hash(countPlusOne);
}
/// <summary>
/// Contributes a possibly-null object to the developing HashCode seed.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="aObject">CAN be null.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Hash(this int aSeed, object aObject)
{
switch (aObject) {
case null :
return aSeed.Hash(0);
case bool b :
return aSeed.Hash(b);
case char c :
return aSeed.Hash(c);
case int i :
return aSeed.Hash(i);
case long l :
return aSeed.Hash(l);
case float f :
return aSeed.Hash(f);
case double d :
return aSeed.Hash(d);
case string s :
return aSeed.Hash(s);
case IEnumerable iEnumerable :
return aSeed.Hash(iEnumerable);
}
return aSeed.Hash(aObject.GetHashCode());
}
/// <summary>
/// This utility method uses reflection to iterate all specified properties that are readable
/// on the given object, excluding any property names given in the params arguments, and
/// generates a hashcode.
/// </summary>
/// <param name="aSeed">The developing hash code, or the seed: if you have no seed, use
/// the <see cref="Seed"/>.</param>
/// <param name="aObject">CAN be null.</param>
/// <param name="propertySelector"><see cref="BindingFlags"/> to select the properties to hash.</param>
/// <param name="ignorePropertyNames">Optional.</param>
/// <returns>A hash from the properties contributed to <c>aSeed</c>.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int HashAllProperties(
this int aSeed,
object aObject,
BindingFlags propertySelector
= BindingFlags.Instance
| BindingFlags.Public
| BindingFlags.GetProperty,
params string[] ignorePropertyNames)
{
if (aObject == null)
return aSeed.Hash(0);
if ((ignorePropertyNames != null)
&& (ignorePropertyNames.Length != 0)) {
foreach (PropertyInfo propertyInfo in aObject.GetType()
.GetProperties(propertySelector)) {
if (!propertyInfo.CanRead
|| (Array.IndexOf(ignorePropertyNames, propertyInfo.Name) >= 0))
continue;
aSeed = aSeed.Hash(propertyInfo.GetValue(aObject));
}
} else {
foreach (PropertyInfo propertyInfo in aObject.GetType()
.GetProperties(propertySelector)) {
if (propertyInfo.CanRead)
aSeed = aSeed.Hash(propertyInfo.GetValue(aObject));
}
}
return aSeed;
}
/// <summary>
/// NOTICE: this method is provided to contribute a <see cref="KeyValuePair{TKey,TValue}"/> to
/// the developing HashCode seed; by hashing the key and the value independently. HOWEVER,
/// this method has a different name since it will not be automatically invoked by
/// <see cref="Hash(int,object)"/>, <see cref="Hash(int,IEnumerable)"/>,
/// or <see cref="HashAllProperties"/> --- you MUST NOT mix this method with those unless
/// you are sure that no KeyValuePair instances will be passed to those methods; or otherwise
/// the generated hash code will not be consistent. This method itself ALSO will not invoke
/// this method on the Key or Value here if that itself is a KeyValuePair.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="keyValuePair">The value to contribute.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int HashKeyAndValue<TKey, TValue>(this int aSeed, KeyValuePair<TKey, TValue> keyValuePair)
=> aSeed.Hash(keyValuePair.Key)
.Hash(keyValuePair.Value);
/// <summary>
/// NOTICE: this method is provided to contribute a collection of <see cref="KeyValuePair{TKey,TValue}"/>
/// to the developing HashCode seed; by hashing the key and the value independently. HOWEVER,
/// this method has a different name since it will not be automatically invoked by
/// <see cref="Hash(int,object)"/>, <see cref="Hash(int,IEnumerable)"/>,
/// or <see cref="HashAllProperties"/> --- you MUST NOT mix this method with those unless
/// you are sure that no KeyValuePair instances will be passed to those methods; or otherwise
/// the generated hash code will not be consistent. This method itself ALSO will not invoke
/// this method on a Key or Value here if that itself is a KeyValuePair or an Enumerable of
/// KeyValuePair.
/// </summary>
/// <param name="aSeed">The developing HashCode value or seed.</param>
/// <param name="keyValuePairs">The values to contribute.</param>
/// <returns>The new hash code.</returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int HashKeysAndValues<TKey, TValue>(
this int aSeed,
IEnumerable<KeyValuePair<TKey, TValue>> keyValuePairs)
{
if (keyValuePairs == null)
return aSeed.Hash(null);
foreach (KeyValuePair<TKey, TValue> keyValuePair in keyValuePairs) {
aSeed = aSeed.HashKeyAndValue(keyValuePair);
}
return aSeed;
}
}
}
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