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
截至https://github.com/dotnet/coreclr/pull/14863，有一种生成哈希代码的新方法非常简单！只要写
public override int GetHashCode()
=> HashCode.Combine(field1, field2, field3);
这将生成高质量的哈希代码，而无需担心实现细节。
其他回答
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。
与夜编码器的解决方案非常相似，只是如果你想提高素数更容易。
PS：这是你嘴里吐出一点东西的时候之一，因为你知道这可以用9个默认值重构成一个方法，但它会更慢，所以你闭上眼睛，试着忘掉它。
/// <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;
}
}
}
这是一个很好的例子：
/// <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());
}
}
下面是如何使用它：
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);
}
}
这是我的简单方法。我使用的是经典的生成器模式。它是类型安全的（无装箱/拆箱），并且与.NET 2.0兼容（无扩展方法等）。
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;
}
这里是实际的生成器类：
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);
}
}
}
如果您想从netstandard2.1中polyfill HashCode
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
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