在.NET System.Object.GetHashCode方法中，很多地方都使用了.NET基类库。特别是在快速查找集合中的项目或确定相等性时。是否有关于如何为我的自定义类实现GetHashCode覆盖的标准算法/最佳实践，因此我不会降低性能？
答案 0 :(得分：1483)
我通常会使用Josh Bloch的神话般的 Effective Java中的实现。它很快并且创建了一个非常好的哈希，不太可能导致冲突。选择两个不同的素数，例如17和23，并做：
public override int GetHashCode()
{
unchecked // Overflow is fine, just wrap
{
int hash = 17;
// Suitable nullity checks etc, of course :)
hash = hash * 23 + field1.GetHashCode();
hash = hash * 23 + field2.GetHashCode();
hash = hash * 23 + field3.GetHashCode();
return hash;
}
}
正如评论中所指出的，你可能会发现最好选择一个大的素数乘以。显然486187739是好的...虽然我看到的大多数例子都是小数字倾向于使用素数，但至少有类似的算法，其中经常使用非素数。例如，在稍后的FNV示例中，我使用的数字显然效果很好 - 但初始值不是素数。 （虽然乘法常量是素数。我不知道它有多重要。）
这比XOR哈希码的常见做法更好，主要有两个原因。假设我们有一个包含两个int字段的类型：
XorHash(x, x) == XorHash(y, y) == 0 for all x, y
XorHash(x, y) == XorHash(y, x) for all x, y
顺便说一下，早期的算法是C＃编译器当前用于匿名类型的算法。
This page提供了不少选项。我认为在大多数情况下，上述情况“足够好”并且非常容易记住并且正确。 FNV替代方法同样简单，但使用不同的常量和XOR而不是ADD作为组合操作。它看起来像某些，如下面的代码，但普通的FNV算法对单个字节进行操作，因此需要修改每个字节执行一次迭代，而不是每32位散列值。 FNV也是为可变长度的数据而设计的，而我们在这里使用它的方式总是针对相同数量的字段值。对这个答案的评论表明，这里的代码实际上并没有像上面的补充方法那样（在测试的案例中）。
// Note: Not quite FNV!
public override int GetHashCode()
{
unchecked // Overflow is fine, just wrap
{
int hash = (int) 2166136261;
// Suitable nullity checks etc, of course :)
hash = (hash * 16777619) ^ field1.GetHashCode();
hash = (hash * 16777619) ^ field2.GetHashCode();
hash = (hash * 16777619) ^ field3.GetHashCode();
return hash;
}
}
请注意，有一点需要注意的是，理想情况下，在将其添加到依赖于哈希码的集合之后，应该防止对等式敏感（因此对哈希码敏感）状态的更改。
您可以为不可变引用类型覆盖GetHashCode。通常，对于可变引用类型，只有在以下情况下才应覆盖GetHashCode：
- 您可以从不可变的字段计算哈希码;或
- 当对象包含在依赖于其哈希码的集合中时，您可以确保可变对象的哈希码不会更改。
答案 1 :(得分：360)
Microsoft已经提供了一个很好的通用HashCode生成器：只需将属性/字段值复制到匿名类型并散列它：
new { PropA, PropB, PropC, PropD }.GetHashCode();
这适用于任意数量的属性。它不使用拳击。它只是使用框架中已经实现的匿名类型算法。
正如@cactuaroid在评论中提到的，可以使用值元组。这节省了一些键击，更重要的是在堆栈上执行（没有垃圾）：
(PropA, PropB, PropC, PropD).GetHashCode();
（注意：使用匿名类型的原始技术似乎在堆上创建了一个对象，即垃圾，因为匿名类型是作为类实现的，尽管这可能会被编译器优化。对这些选项进行基准测试会很有趣。 ，但元组选项应该更优越。）
答案 2 :(得分：101)
这是我的哈希码助手 它的优点是它使用泛型类型参数，因此不会导致装箱：
public static class HashHelper
{
public static int GetHashCode<T1, T2>(T1 arg1, T2 arg2)
{
unchecked
{
return 31 * arg1.GetHashCode() + arg2.GetHashCode();
}
}
public static int GetHashCode<T1, T2, T3>(T1 arg1, T2 arg2, T3 arg3)
{
unchecked
{
int hash = arg1.GetHashCode();
hash = 31 * hash + arg2.GetHashCode();
return 31 * hash + arg3.GetHashCode();
}
}
public static int GetHashCode<T1, T2, T3, T4>(T1 arg1, T2 arg2, T3 arg3,
T4 arg4)
{
unchecked
{
int hash = arg1.GetHashCode();
hash = 31 * hash + arg2.GetHashCode();
hash = 31 * hash + arg3.GetHashCode();
return 31 * hash + arg4.GetHashCode();
}
}
public static int GetHashCode<T>(T[] list)
{
unchecked
{
int hash = 0;
foreach (var item in list)
{
hash = 31 * hash + item.GetHashCode();
}
return hash;
}
}
public static int GetHashCode<T>(IEnumerable<T> list)
{
unchecked
{
int hash = 0;
foreach (var item in list)
{
hash = 31 * hash + item.GetHashCode();
}
return hash;
}
}
/// <summary>
/// Gets a hashcode for a collection for that the order of items
/// does not matter.
/// So {1, 2, 3} and {3, 2, 1} will get same hash code.
/// </summary>
public static int GetHashCodeForOrderNoMatterCollection<T>(
IEnumerable<T> list)
{
unchecked
{
int hash = 0;
int count = 0;
foreach (var item in list)
{
hash += item.GetHashCode();
count++;
}
return 31 * hash + count.GetHashCode();
}
}
/// <summary>
/// Alternative way to get a hashcode is to use a fluent
/// interface like this:<br />
/// return 0.CombineHashCode(field1).CombineHashCode(field2).
/// CombineHashCode(field3);
/// </summary>
public static int CombineHashCode<T>(this int hashCode, T arg)
{
unchecked
{
return 31 * hashCode + arg.GetHashCode();
}
}
它还有扩展方法来提供流畅的界面，所以你可以像这样使用它：
public override int GetHashCode()
{
return HashHelper.GetHashCode(Manufacturer, PartN, Quantity);
}
或者像这样：
public override int GetHashCode()
{
return 0.CombineHashCode(Manufacturer)
.CombineHashCode(PartN)
.CombineHashCode(Quantity);
}
答案 3 :(得分：60)
我在Helper库中有一个Hashing类，我将它用于此目的。
/// <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，您只需将其用作：
public override int GetHashCode()
{
return Hashing.RSHash(_field1, _field2, _field3);
}
我没有评估其表现，所以欢迎任何反馈。
答案 4 :(得分：54)
这是使用Jon Skeet's implementation的帮助程序类。
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);
}
如果您想避免为System.Int32编写扩展方法：
public 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;
}
它仍然是通用的，它仍然避免任何堆分配，它的使用方式完全相同：
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);
}
在Martin发表评论后更新：
obj != null导致拳击，所以我切换到默认的比较器。
编辑（2018年5月）：
EqualityComparer<T>.Default getter现在是JIT内在的 - pull request由Stephen Toub在this blog post中提及。
答案 5 :(得分：29)
在大多数情况下，Equals（）比较多个字段，如果你的GetHash（）在一个字段或多个字段上进行哈希处理并不重要。你只需要确保计算哈希值非常便宜（没有分配，请）和快速（没有繁重的计算，当然没有数据库连接）并提供良好的分发
繁重的举动应该是Equals（）方法的一部分;哈希应该是一个非常便宜的操作，可以在尽可能少的项目上调用Equals（）。
最后一个提示：不要依赖GetHashCode（）在多个应用程序运行中保持稳定。许多.Net类型不保证其哈希码在重启后保持不变，因此您只应在内存数据结构中使用GetHashCode（）的值。
答案 6 :(得分：20)
直到最近，我的答案与Jon Skeet非常接近。但是，我最近启动了一个使用二次幂哈希表的项目，即哈希表，其中内部表的大小为8,16,32等。有一个很好的理由支持素数大小，但那里对于两种尺寸的尺寸也有一些优点。
它非常糟糕。经过一些实验和研究后，我开始用以下内容重新哈希：
public static int ReHash(int source)
{
unchecked
{
ulong c = 0xDEADBEEFDEADBEEF + (ulong)source;
ulong d = 0xE2ADBEEFDEADBEEF ^ c;
ulong a = d += c = c << 15 | c >> -15;
ulong b = a += d = d << 52 | d >> -52;
c ^= b += a = a << 26 | a >> -26;
d ^= c += b = b << 51 | b >> -51;
a ^= d += c = c << 28 | c >> -28;
b ^= a += d = d << 9 | d >> -9;
c ^= b += a = a << 47 | a >> -47;
d ^= c += b << 54 | b >> -54;
a ^= d += c << 32 | c >> 32;
a += d << 25 | d >> -25;
return (int)(a >> 1);
}
}
然后我的二次幂哈希表再也没用了。
这让我感到不安，因为上述情况不应该奏效。或者更确切地说，除非原始的GetHashCode()以一种非常特殊的方式变得贫穷，否则它不应该起作用。
重新混合哈希码无法改善一个很好的哈希码，因为唯一可能的效果就是我们引入了更多的冲突。
重新混合哈希码不能改善可怕的哈希码，因为唯一可能的效果是我们改变例如值53的大量碰撞到大量的值18,3487,291。
重新混合哈希码只能改进一个哈希码，该哈希码至少可以很好地避免整个范围内的绝对冲突（2 32 可能的值）但是在模数化时很难避免冲突在哈希表中实际使用。虽然两个幂表的简单模数使得这一点变得更加明显，但它对更常见的素数表也有负面影响，但这并不是那么明显（重新划分的额外工作将超过收益） ，但好处仍然存在）。
编辑：我也使用开放寻址，这也会增加对碰撞的敏感度，或许比它是二次幂的事实更多。
好吧，令人不安的是，.NET（或研究here）中的string.GetHashCode()实施可以通过这种方式改进多少（按照运行约20-30次的测试顺序）由于冲突更少而速度更快，并且更令我不安的是我自己的哈希码可以改进多少（远不止于此）。
我过去编写过的所有GetHashCode（）实现，实际上都是在这个网站上作为答案的基础，比我的要糟糕得多。大部分时间它对于大部分用途都“足够好”，但我想要更好的东西。
所以我将该项目放在一边（无论如何它都是一个宠物项目）并开始研究如何在.NET中快速生成一个良好，分布均匀的哈希代码。
最后我决定将SpookyHash移植到.NET。实际上，上面的代码是使用SpookyHash从32位输入产生32位输出的快速路径版本。
现在，SpookyHash很难记住一段代码。我的端口更是如此，因为我为了更好的速度而手动插入了大量的内容*。但这就是代码重用的目的。
然后我将那个项目放在一边，因为正如原始项目产生了如何产生更好的哈希代码的问题，因此该项目产生了如何产生更好的问题.NET memcpy。
然后我回来了，产生了很多重载，可以轻松地将所有原生类型（除了decimal†）提供给哈希码。
速度很快，鲍勃·詹金斯应该得到大部分功劳，因为我移植的原始代码更快，特别是在64位机器上，该算法针对‡进行了优化。
完整的代码可以在https://bitbucket.org/JonHanna/spookilysharp/src看到，但请注意上面的代码是它的简化版本。
然而，由于它现在已经写好了，人们可以更容易地使用它：
public override int GetHashCode()
{
var hash = new SpookyHash();
hash.Update(field1);
hash.Update(field2);
hash.Update(field3);
return hash.Final().GetHashCode();
}
它还需要种子值，因此如果您需要处理不受信任的输入并希望防范Hash DoS攻击，您可以根据正常运行时间或类似情况设置种子，并使攻击者无法预测结果：
private static long hashSeed0 = Environment.TickCount;
private static long hashSeed1 = DateTime.Now.Ticks;
public override int GetHashCode()
{
//produce different hashes ever time this application is restarted
//but remain consistent in each run, so attackers have a harder time
//DoSing the hash tables.
var hash = new SpookyHash(hashSeed0, hashSeed1);
hash.Update(field1);
hash.Update(field2);
hash.Update(field3);
return hash.Final().GetHashCode();
}
*令人惊讶的是，手动内联返回(x << n) | (x >> -n)改进的旋转方法。我本可以肯定的是，抖动会为我编写内容，但是分析显示不然。
†decimal虽然来自C＃，但它不是.NET的本机。它的问题在于它自己的GetHashCode()将精度视为重要，而它自己的Equals()却没有。两者都是有效的选择，但不是那样的混合。在实现你自己的版本时，你需要选择做一个或另一个，但我不知道你想要的是什么。
‡通过比较。如果在字符串上使用，64位上的SpookyHash比32位上的string.GetHashCode()快得多，这比64位上的string.GetHashCode()略快，这比32位上的SpookyHash要快得多，尽管仍然很快足以成为一个合理的选择。
答案 7 :(得分：13)
这是一个很好的：
/// <summary>
/// Helper class for generating hash codes suitable
/// for use in hashing algorithms and data structures like a hash table.
/// </summary>
public static class HashCodeHelper
{
private static int GetHashCodeInternal(int key1, int key2)
{
unchecked
{
var num = 0x7e53a269;
num = (-1521134295 * num) + key1;
num += (num << 10);
num ^= (num >> 6);
num = ((-1521134295 * num) + key2);
num += (num << 10);
num ^= (num >> 6);
return num;
}
}
/// <summary>
/// Returns a hash code for the specified objects
/// </summary>
/// <param name="arr">An array of objects used for generating the
/// hash code.</param>
/// <returns>
/// A hash code, suitable for use in hashing algorithms and data
/// structures like a hash table.
/// </returns>
public static int GetHashCode(params object[] arr)
{
int hash = 0;
foreach (var item in arr)
hash = GetHashCodeInternal(hash, item.GetHashCode());
return hash;
}
/// <summary>
/// Returns a hash code for the specified objects
/// </summary>
/// <param name="obj1">The first object.</param>
/// <param name="obj2">The second object.</param>
/// <param name="obj3">The third object.</param>
/// <param name="obj4">The fourth object.</param>
/// <returns>
/// A hash code, suitable for use in hashing algorithms and
/// data structures like a hash table.
/// </returns>
public static int GetHashCode<T1, T2, T3, T4>(T1 obj1, T2 obj2, T3 obj3,
T4 obj4)
{
return GetHashCode(obj1, GetHashCode(obj2, obj3, obj4));
}
/// <summary>
/// Returns a hash code for the specified objects
/// </summary>
/// <param name="obj1">The first object.</param>
/// <param name="obj2">The second object.</param>
/// <param name="obj3">The third object.</param>
/// <returns>
/// A hash code, suitable for use in hashing algorithms and data
/// structures like a hash table.
/// </returns>
public static int GetHashCode<T1, T2, T3>(T1 obj1, T2 obj2, T3 obj3)
{
return GetHashCode(obj1, GetHashCode(obj2, obj3));
}
/// <summary>
/// Returns a hash code for the specified objects
/// </summary>
/// <param name="obj1">The first object.</param>
/// <param name="obj2">The second object.</param>
/// <returns>
/// A hash code, suitable for use in hashing algorithms and data
/// structures like a hash table.
/// </returns>
public static int GetHashCode<T1, T2>(T1 obj1, T2 obj2)
{
return GetHashCodeInternal(obj1.GetHashCode(), obj2.GetHashCode());
}
}
以下是如何使用它：
private struct Key
{
private Type _type;
private string _field;
public Type Type { get { return _type; } }
public string Field { get { return _field; } }
public Key(Type type, string field)
{
_type = type;
_field = field;
}
public override int GetHashCode()
{
return HashCodeHelper.GetHashCode(_field, _type);
}
public override bool Equals(object obj)
{
if (!(obj is Key))
return false;
var tf = (Key)obj;
return tf._field.Equals(_field) && tf._type.Equals(_type);
}
}
答案 8 :(得分：9)
这是the algorithm posted above by Jon Skeet的另一个流畅的实现，但不包括分配或装箱操作：
public static class Hash
{
public const int Base = 17;
public static int HashObject(this int hash, object obj)
{
unchecked { return hash * 23 + (obj == null ? 0 : obj.GetHashCode()); }
}
public static int HashValue<T>(this int hash, T value)
where T : struct
{
unchecked { return hash * 23 + value.GetHashCode(); }
}
}
用法：
public class MyType<T>
{
public string Name { get; set; }
public string Description { get; set; }
public int Value { get; set; }
public IEnumerable<T> Children { get; set; }
public override int GetHashCode()
{
return Hash.Base
.HashObject(this.Name)
.HashObject(this.Description)
.HashValue(this.Value)
.HashObject(this.Children);
}
}
由于泛型类型约束，编译器将确保不使用类调用HashValue。但是HashObject没有编译器支持，因为添加泛型参数也会增加装箱操作。
答案 9 :(得分：8)
这是我的简单方法。我正在使用经典的构建器模式。它是类型安全的（没有装箱/拆箱），也适用于.NET 2.0（没有扩展方法等）。
它的用法如下：
public override int GetHashCode()
{
HashBuilder b = new HashBuilder();
b.AddItems(this.member1, this.member2, this.member3);
return b.Result;
}
这是真正的建设者类：
internal class HashBuilder
{
private const int Prime1 = 17;
private const int Prime2 = 23;
private int result = Prime1;
public HashBuilder()
{
}
public HashBuilder(int startHash)
{
this.result = startHash;
}
public int Result
{
get
{
return this.result;
}
}
public void AddItem<T>(T item)
{
unchecked
{
this.result = this.result * Prime2 + item.GetHashCode();
}
}
public void AddItems<T1, T2>(T1 item1, T2 item2)
{
this.AddItem(item1);
this.AddItem(item2);
}
public void AddItems<T1, T2, T3>(T1 item1, T2 item2, T3 item3)
{
this.AddItem(item1);
this.AddItem(item2);
this.AddItem(item3);
}
public void AddItems<T1, T2, T3, T4>(T1 item1, T2 item2, T3 item3,
T4 item4)
{
this.AddItem(item1);
this.AddItem(item2);
this.AddItem(item3);
this.AddItem(item4);
}
public void AddItems<T1, T2, T3, T4, T5>(T1 item1, T2 item2, T3 item3,
T4 item4, T5 item5)
{
this.AddItem(item1);
this.AddItem(item2);
this.AddItem(item3);
this.AddItem(item4);
this.AddItem(item5);
}
public void AddItems<T>(params T[] items)
{
foreach (T item in items)
{
this.AddItem(item);
}
}
}
答案 10 :(得分：7)
从https://github.com/dotnet/coreclr/pull/14863开始，有一种新方法可以生成超级简单的哈希码！只需写下
public override int GetHashCode()
=> HashCode.Combine(field1, field2, field3);
这将生成高质量的哈希码，而无需担心实现细节。
答案 11 :(得分：4)
如果使用的是.NET Core 2.1或更高版本，则可以使用System.HashCode结构。有两种使用方法：
Call body.BeginInsert(bodyNavigator, True)
方法可用于创建哈希码，最多可分配8个对象。
Combine public override int GetHashCode() => HashCode.Combine(this.object1, this.object2);
方法可帮助您处理集合：
Add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。
public override int GetHashCode()
{
var hashCode = new HashCode();
hashCode.Add(this.object1);
foreach (var item in this.collection)
{
hashCode.Add(item);
}
return hashCode.ToHashCode();
}
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);
}
答案 12 :(得分：4)
ReSharper个用户可以使用ReSharper -> Edit -> Generate Code -> Equality Members生成GetHashCode，Equals和其他人。
// ReSharper's GetHashCode looks like this
public override int GetHashCode() {
unchecked {
int hashCode = Id;
hashCode = (hashCode * 397) ^ IntMember;
hashCode = (hashCode * 397) ^ OtherIntMember;
hashCode = (hashCode * 397) ^ (RefMember != null ? RefMember.GetHashCode() : 0);
// ...
return hashCode;
}
}
答案 13 :(得分：3)
我的大部分工作都是通过数据库连接完成的，这意味着我的所有类都具有数据库中的唯一标识符。我总是使用数据库中的ID来生成哈希码。
// Unique ID from database
private int _id;
...
{
return _id.GetHashCode();
}
答案 14 :(得分：3)
与nightcoder的解决方案非常相似，只是如果你愿意，它更容易提升素数。
PS：这是你在嘴里呕吐的时间之一，知道这可以被重构成一种方法，有9个默认值但速度会慢一点，所以你只是闭上眼睛试着忘记它/// <summary>
/// Try not to look at the source code. It works. Just rely on it.
/// </summary>
public static class HashHelper
{
private const int PrimeOne = 17;
private const int PrimeTwo = 23;
public static int GetHashCode<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9, T10 arg10)
{
unchecked
{
int hash = PrimeOne;
hash = hash * PrimeTwo + arg1.GetHashCode();
hash = hash * PrimeTwo + arg2.GetHashCode();
hash = hash * PrimeTwo + arg3.GetHashCode();
hash = hash * PrimeTwo + arg4.GetHashCode();
hash = hash * PrimeTwo + arg5.GetHashCode();
hash = hash * PrimeTwo + arg6.GetHashCode();
hash = hash * PrimeTwo + arg7.GetHashCode();
hash = hash * PrimeTwo + arg8.GetHashCode();
hash = hash * PrimeTwo + arg9.GetHashCode();
hash = hash * PrimeTwo + arg10.GetHashCode();
return hash;
}
}
public static int GetHashCode<T1, T2, T3, T4, T5, T6, T7, T8, T9>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9)
{
unchecked
{
int hash = PrimeOne;
hash = hash * PrimeTwo + arg1.GetHashCode();
hash = hash * PrimeTwo + arg2.GetHashCode();
hash = hash * PrimeTwo + arg3.GetHashCode();
hash = hash * PrimeTwo + arg4.GetHashCode();
hash = hash * PrimeTwo + arg5.GetHashCode();
hash = hash * PrimeTwo + arg6.GetHashCode();
hash = hash * PrimeTwo + arg7.GetHashCode();
hash = hash * PrimeTwo + arg8.GetHashCode();
hash = hash * PrimeTwo + arg9.GetHashCode();
return hash;
}
}
public static int GetHashCode<T1, T2, T3, T4, T5, T6, T7, T8>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8)
{
unchecked
{
int hash = PrimeOne;
hash = hash * PrimeTwo + arg1.GetHashCode();
hash = hash * PrimeTwo + arg2.GetHashCode();
hash = hash * PrimeTwo + arg3.GetHashCode();
hash = hash * PrimeTwo + arg4.GetHashCode();
hash = hash * PrimeTwo + arg5.GetHashCode();
hash = hash * PrimeTwo + arg6.GetHashCode();
hash = hash * PrimeTwo + arg7.GetHashCode();
hash = hash * PrimeTwo + arg8.GetHashCode();
return hash;
}
}
public static int GetHashCode<T1, T2, T3, T4, T5, T6, T7>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7)
{
unchecked
{
int hash = PrimeOne;
hash = hash * PrimeTwo + arg1.GetHashCode();
hash = hash * PrimeTwo + arg2.GetHashCode();
hash = hash * PrimeTwo + arg3.GetHashCode();
hash = hash * PrimeTwo + arg4.GetHashCode();
hash = hash * PrimeTwo + arg5.GetHashCode();
hash = hash * PrimeTwo + arg6.GetHashCode();
hash = hash * PrimeTwo + arg7.GetHashCode();
return hash;
}
}
public static int GetHashCode<T1, T2, T3, T4, T5, T6>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6)
{
unchecked
{
int hash = PrimeOne;
hash = hash * PrimeTwo + arg1.GetHashCode();
hash = hash * PrimeTwo + arg2.GetHashCode();
hash = hash * PrimeTwo + arg3.GetHashCode();
hash = hash * PrimeTwo + arg4.GetHashCode();
hash = hash * PrimeTwo + arg5.GetHashCode();
hash = hash * PrimeTwo + arg6.GetHashCode();
return hash;
}
}
public static int GetHashCode<T1, T2, T3, T4, T5>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5)
{
unchecked
{
int hash = PrimeOne;
hash = hash * PrimeTwo + arg1.GetHashCode();
hash = hash * PrimeTwo + arg2.GetHashCode();
hash = hash * PrimeTwo + arg3.GetHashCode();
hash = hash * PrimeTwo + arg4.GetHashCode();
hash = hash * PrimeTwo + arg5.GetHashCode();
return hash;
}
}
public static int GetHashCode<T1, T2, T3, T4>(T1 arg1, T2 arg2, T3 arg3, T4 arg4)
{
unchecked
{
int hash = PrimeOne;
hash = hash * PrimeTwo + arg1.GetHashCode();
hash = hash * PrimeTwo + arg2.GetHashCode();
hash = hash * PrimeTwo + arg3.GetHashCode();
hash = hash * PrimeTwo + arg4.GetHashCode();
return hash;
}
}
public static int GetHashCode<T1, T2, T3>(T1 arg1, T2 arg2, T3 arg3)
{
unchecked
{
int hash = PrimeOne;
hash = hash * PrimeTwo + arg1.GetHashCode();
hash = hash * PrimeTwo + arg2.GetHashCode();
hash = hash * PrimeTwo + arg3.GetHashCode();
return hash;
}
}
public static int GetHashCode<T1, T2>(T1 arg1, T2 arg2)
{
unchecked
{
int hash = PrimeOne;
hash = hash * PrimeTwo + arg1.GetHashCode();
hash = hash * PrimeTwo + arg2.GetHashCode();
return hash;
}
}
}
答案 15 :(得分：2)
如果我们不超过8个属性（希望如此），这是另一种选择。
ValueTuple是一个结构，似乎有一个可靠的GetHashCode实现。
这意味着我们可以简单地这样做：
// Yay, no allocations and no custom implementations!
public override int GetHashCode() => (this.PropA, this.PropB).GetHashCode();
让我们看看.NET Core目前对ValueTuple GetHashCode的实现。
这是ValueTuple：
internal static int CombineHashCodes(int h1, int h2)
{
return HashHelpers.Combine(HashHelpers.Combine(HashHelpers.RandomSeed, h1), h2);
}
internal static int CombineHashCodes(int h1, int h2, int h3)
{
return HashHelpers.Combine(CombineHashCodes(h1, h2), h3);
}
这是来自HashHelper：
public static readonly int RandomSeed = Guid.NewGuid().GetHashCode();
public static int Combine(int h1, int h2)
{
unchecked
{
// RyuJIT optimizes this to use the ROL instruction
// Related GitHub pull request: dotnet/coreclr#1830
uint rol5 = ((uint)h1 << 5) | ((uint)h1 >> 27);
return ((int)rol5 + h1) ^ h2;
}
}
英文：
了解有关此ROL-5哈希码算法属性的更多信息会很高兴。
令人遗憾的是，为我们自己的ValueTuple推迟GetHashCode可能不会像我们希望的那样快。相关讨论中的This comment说明直接调用HashHelpers.Combine更具效果。另一方面，那个是内部的，所以我们必须复制代码，牺牲我们在这里获得的大部分内容。此外，我们要负责记住随机种子的Combine。如果我们跳过这一步，我不知道后果是什么。
答案 16 :(得分：1)
我使用选定的实现作为上面的答案遇到浮点数和小数的问题。
此测试失败（浮动;哈希是相同的，即使我将2个值切换为负值）：
var obj1 = new { A = 100m, B = 100m, C = 100m, D = 100m};
var obj2 = new { A = 100m, B = 100m, C = -100m, D = -100m};
var hash1 = ComputeHash(obj1.A, obj1.B, obj1.C, obj1.D);
var hash2 = ComputeHash(obj2.A, obj2.B, obj2.C, obj2.D);
Assert.IsFalse(hash1 == hash2, string.Format("Hashcode values should be different hash1:{0} hash2:{1}",hash1,hash2));
但是这个测试通过了（带有整数）：
var obj1 = new { A = 100m, B = 100m, C = 100, D = 100};
var obj2 = new { A = 100m, B = 100m, C = -100, D = -100};
var hash1 = ComputeHash(obj1.A, obj1.B, obj1.C, obj1.D);
var hash2 = ComputeHash(obj2.A, obj2.B, obj2.C, obj2.D);
Assert.IsFalse(hash1 == hash2, string.Format("Hashcode values should be different hash1:{0} hash2:{1}",hash1,hash2));
我将我的实现改为不使用GetHashCode用于原始类型，它似乎更好用
private static int InternalComputeHash(params object[] obj)
{
unchecked
{
var result = (int)SEED_VALUE_PRIME;
for (uint i = 0; i < obj.Length; i++)
{
var currval = result;
var nextval = DetermineNextValue(obj[i]);
result = (result * MULTIPLIER_VALUE_PRIME) + nextval;
}
return result;
}
}
private static int DetermineNextValue(object value)
{
unchecked
{
int hashCode;
if (value is short
|| value is int
|| value is byte
|| value is sbyte
|| value is uint
|| value is ushort
|| value is ulong
|| value is long
|| value is float
|| value is double
|| value is decimal)
{
return Convert.ToInt32(value);
}
else
{
return value != null ? value.GetHashCode() : 0;
}
}
}
答案 17 :(得分：1)
这是一个静态助手类，用于实现Josh Bloch的实现；并提供显式的重载以“防止”装箱，并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。
由于散列输出始终是一个int，因此可以只链接散列调用。
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;
}
}
}
答案 18 :(得分：0)
微软领导多种哈希方法......
//for classes that contain a single int value
return this.value;
//for classes that contain multiple int value
return x ^ y;
//for classes that contain single number bigger than int
return ((int)value ^ (int)(value >> 32));
//for classes that contain class instance fields which inherit from object
return obj1.GetHashCode();
//for classes that contain multiple class instance fields which inherit from object
return obj1.GetHashCode() ^ obj2.GetHashCode() ^ obj3.GetHashCode();
我可以猜测，对于多个大的int你可以使用它：
int a=((int)value1 ^ (int)(value1 >> 32));
int b=((int)value2 ^ (int)(value2 >> 32));
int c=((int)value3 ^ (int)(value3 >> 32));
return a ^ b ^ c;
多类型相同：所有使用int首先转换为GetHashCode()
然后int值将被xor'ed，结果就是你的哈希。
对于那些使用哈希作为ID的人（我的意思是一个唯一值），哈希自然限于多个数字，我认为哈希算法至少是5个字节，至少是MD5。
您可以将多个值转换为散列值，其中一些值相同，因此请勿将其用作标识符。 （也许有一天我会使用你的组件）
答案 19 :(得分：0)
如果要从HashCode填充netstandard2.1
public static class HashCode
{
public static int Combine(params object[] instances)
{
int hash = 17;
foreach (var i in instances)
{
hash = unchecked((hash * 31) + (i?.GetHashCode() ?? 0));
}
return hash;
}
}
注意：如果与struct一起使用，它将由于装箱而分配内存
答案 20 :(得分：0)
可以尝试采用 C++ Boost 库中的方法。像这样：
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());
}
}
答案 21 :(得分：0)
我想将我的最新发现添加到我经常回来的这个线程中。
我当前的视觉工作室/项目设置提供了自动将元组重构为结构的功能。这将生成一个 GetHashCode 函数，如下所示：
public override int GetHashCode()
{
int hashCode = -2088324004;
hashCode = hashCode * -1521134295 + AuftragGesperrt.GetHashCode();
hashCode = hashCode * -1521134295 + Auftrag_gesperrt_von.GetHashCode();
hashCode = hashCode * -1521134295 + Auftrag_gesperrt_am.GetHashCode();
return hashCode;
}