System.Object.GetHashCode 方法
本文提供了此 API 参考文档的补充说明。
该方法GetHashCode为需要快速检查对象相等性的算法提供哈希代码。 哈希代码是一个数值，用于在基于哈希的集合中插入和标识对象，例如 Dictionary<TKey,TValue> 类、 Hashtable 类或派生自类的类型 DictionaryBase 。
注意
有关哈希表和一些其他哈希代码算法如何使用哈希代码的信息，请参阅 维基百科中的哈希函数 条目。
两个相等的对象返回相等的哈希代码。 但是，相反情况并非如此：相等哈希代码并不表示对象相等，因为不同的（不相等）对象可以具有相同的哈希代码。 此外，.NET 不保证方法的默认实现 GetHashCode ，此方法返回的值在 .NET 实现之间可能有所不同，例如不同版本的 .NET Framework 和 .NET Core，以及 32 位和 64 位平台等平台。 出于这些原因，请勿使用此方法的默认实现作为唯一的对象标识符进行哈希处理。 下面将产生两个后果：
- 不应假定相等哈希代码表示对象相等。
- 不应保留或使用创建它的应用程序域之外的哈希代码，因为同一对象可能会跨应用程序域、进程和平台进行哈希处理。
警告
哈希代码适用于基于哈希表的集合中的高效插入和查找。 哈希代码不是永久值。 出于此原因：
- 不要序列化哈希代码值或将它们存储在数据库中。
- 不要使用哈希代码作为键从键控集合中检索对象。
- 不要跨应用程序域或进程发送哈希代码。 在某些情况下，可以根据每个进程或每个应用程序域计算哈希代码。
- 如果需要加密强哈希，请不要使用哈希代码而不是加密哈希函数返回的值。 对于加密哈希，请使用派生自或System.Security.Cryptography.KeyedHashAlgorithm类的System.Security.Cryptography.HashAlgorithm类。
- 不要测试哈希代码的相等性，以确定两个对象是否相等。 （不相等对象可以具有相同的哈希代码。若要测试相等性，请调用 ReferenceEquals 或 Equals 方法。
方法 GetHashCode 可由派生类型重写。 如果未 GetHashCode 重写，则通过调用 Object.GetHashCode 基类的方法（基于对象的引用计算哈希代码）来计算引用类型的哈希代码;有关详细信息，请参阅 RuntimeHelpers.GetHashCode。 换句话说，该方法返回true的两个对象ReferenceEquals具有相同的哈希代码。 如果未重写 GetHashCode值类型， ValueType.GetHashCode 则基类的方法使用反射根据类型字段的值计算哈希代码。 换句话说，其字段具有相等值的值类型具有相等哈希代码。 有关重写 GetHashCode的详细信息，请参阅“继承者说明”部分。
警告
如果重写该方法 GetHashCode ，则还应重写 Equals，反之亦然。 如果重写 Equals 的方法在测试两个对象时返回 true 相等性，则 GetHashCode 重写的方法必须为这两个对象返回相同的值。
如果用作哈希表中键的对象不提供有用的实现，则可以通过向类构造函数的某个重载Hashtable提供IEqualityComparer实现GetHashCode来指定哈希代码提供程序。
Windows 运行时说明
在Windows 运行时中对类调用GetHashCode该方法时，它会为不重写GetHashCode的类提供默认行为。 这是 .NET 为Windows 运行时提供的支持的一部分（请参阅 Windows 应用商店应用的 .NET 支持和Windows 运行时）。 Windows 运行时中的类不继承Object，当前不实现GetHashCode类。 但是，在 C# 或 Visual Basic 代码中使用它们时，它们似乎具有ToStringEquals(Object)和GetHashCode方法，.NET Framework 为这些方法提供了默认行为。
注意
Windows 运行时用 C# 或 Visual Basic 编写的类可以替代该方法GetHashCode。
示例
计算数值哈希代码的最简单方法之一，其范围 Int32 与类型相同或更小，只需返回该值。 以下示例演示了结构的 Number 此类实现。
using System;
public struct Number
{
private int n;
public Number(int value)
{
n = value;
}
public int Value
{
get { return n; }
}
public override bool Equals(Object obj)
{
if (obj == null || ! (obj is Number))
return false;
else
return n == ((Number) obj).n;
}
public override int GetHashCode()
{
return n;
}
public override string ToString()
{
return n.ToString();
}
}
public class Example1
{
public static void Main()
{
Random rnd = new Random();
for (int ctr = 0; ctr <= 9; ctr++) {
int randomN = rnd.Next(Int32.MinValue, Int32.MaxValue);
Number n = new Number(randomN);
Console.WriteLine("n = {0,12}, hash code = {1,12}", n, n.GetHashCode());
}
}
}
// The example displays output like the following:
// n = -634398368, hash code = -634398368
// n = 2136747730, hash code = 2136747730
// n = -1973417279, hash code = -1973417279
// n = 1101478715, hash code = 1101478715
// n = 2078057429, hash code = 2078057429
// n = -334489950, hash code = -334489950
// n = -68958230, hash code = -68958230
// n = -379951485, hash code = -379951485
// n = -31553685, hash code = -31553685
// n = 2105429592, hash code = 2105429592
open System
[<Struct; CustomEquality; NoComparison>]
type Number(value: int) =
member _.Value = value
override _.Equals(obj) =
match obj with
| :? Number as n ->
n.Value = value
| _ -> false
override _.GetHashCode() =
value
override _.ToString() =
string value
let rnd = Random()
for _ = 0 to 9 do
let randomN = rnd.Next(Int32.MinValue, Int32.MaxValue)
let n = Number randomN
printfn $"n = {n,12}, hash code = {n.GetHashCode(),12}"
// The example displays output like the following:
// n = -634398368, hash code = -634398368
// n = 2136747730, hash code = 2136747730
// n = -1973417279, hash code = -1973417279
// n = 1101478715, hash code = 1101478715
// n = 2078057429, hash code = 2078057429
// n = -334489950, hash code = -334489950
// n = -68958230, hash code = -68958230
// n = -379951485, hash code = -379951485
// n = -31553685, hash code = -31553685
// n = 2105429592, hash code = 2105429592
Public Structure Number
Private n As Integer
Public Sub New(value As Integer)
n = value
End Sub
Public ReadOnly Property Value As Integer
Get
Return n
End Get
End Property
Public Overrides Function Equals(obj As Object) As Boolean
If obj Is Nothing OrElse Not TypeOf obj Is Number Then
Return False
Else
Return n = CType(obj, Number).n
End If
End Function
Public Overrides Function GetHashCode() As Integer
Return n
End Function
Public Overrides Function ToString() As String
Return n.ToString()
End Function
End Structure
Module Example1
Public Sub Main()
Dim rnd As New Random()
For ctr As Integer = 0 To 9
Dim randomN As Integer = rnd.Next(Int32.MinValue, Int32.MaxValue)
Dim n As New Number(randomN)
Console.WriteLine("n = {0,12}, hash code = {1,12}", n, n.GetHashCode())
Next
End Sub
End Module
' The example displays output like the following:
' n = -634398368, hash code = -634398368
' n = 2136747730, hash code = 2136747730
' n = -1973417279, hash code = -1973417279
' n = 1101478715, hash code = 1101478715
' n = 2078057429, hash code = 2078057429
' n = -334489950, hash code = -334489950
' n = -68958230, hash code = -68958230
' n = -379951485, hash code = -379951485
' n = -31553685, hash code = -31553685
' n = 2105429592, hash code = 2105429592
通常，类型具有多个数据字段，这些字段可以参与生成哈希代码。 生成哈希代码的一种方法是使用这些 XOR (eXclusive OR) 操作合并这些字段，如以下示例所示。
using System;
// A type that represents a 2-D point.
public struct Point2
{
private int x;
private int y;
public Point2(int x, int y)
{
this.x = x;
this.y = y;
}
public override bool Equals(Object obj)
{
if (! (obj is Point2)) return false;
Point2 p = (Point2) obj;
return x == p.x & y == p.y;
}
public override int GetHashCode()
{
return x ^ y;
}
}
public class Example3
{
public static void Main()
{
Point2 pt = new Point2(5, 8);
Console.WriteLine(pt.GetHashCode());
pt = new Point2(8, 5);
Console.WriteLine(pt.GetHashCode());
}
}
// The example displays the following output:
// 13
// 13
// A type that represents a 2-D point.
[<Struct; CustomEquality; NoComparison>]
type Point(x: int, y: int) =
member _.X = x
member _.Y = y
override _.Equals(obj) =
match obj with
| :? Point as p ->
x = p.X && y = p.Y
| _ ->
false
override _.GetHashCode() =
x ^^^ y
let pt = Point(5, 8)
printfn $"{pt.GetHashCode()}"
let pt2 = Point(8, 5)
printfn $"{pt.GetHashCode()}"
// The example displays the following output:
// 13
// 13
' A type that represents a 2-D point.
Public Structure Point3
Private x As Integer
Private y As Integer
Public Sub New(x As Integer, y As Integer)
Me.x = x
Me.y = y
End Sub
Public Overrides Function Equals(obj As Object) As Boolean
If Not TypeOf obj Is Point3 Then Return False
Dim p As Point3 = CType(obj, Point3)
Return x = p.x And y = p.y
End Function
Public Overrides Function GetHashCode() As Integer
Return x Xor y
End Function
End Structure
Public Module Example3
Public Sub Main()
Dim pt As New Point3(5, 8)
Console.WriteLine(pt.GetHashCode())
pt = New Point3(8, 5)
Console.WriteLine(pt.GetHashCode())
End Sub
End Module
上一个示例为 （n1， n2） 和 （n2， n1） 返回相同的哈希代码，因此可能会生成比所需的冲突更多的冲突。 许多解决方案都可用，以便这些情况下的哈希代码不相同。 一个是返回反映每个字段顺序的对象哈希代码 Tuple 。 以下示例演示了使用 Tuple<T1,T2> 类的可能实现。 但是，请注意，实例化 Tuple 对象的性能开销可能会显著影响将大量对象存储在哈希表中的应用程序的整体性能。
using System;
public struct Point3
{
private int x;
private int y;
public Point3(int x, int y)
{
this.x = x;
this.y = y;
}
public override bool Equals(Object obj)
{
if (obj is Point3)
{
Point3 p = (Point3) obj;
return x == p.x & y == p.y;
}
else
{
return false;
}
}
public override int GetHashCode()
{
return Tuple.Create(x, y).GetHashCode();
}
}
public class Example
{
public static void Main()
{
Point3 pt = new Point3(5, 8);
Console.WriteLine(pt.GetHashCode());
pt = new Point3(8, 5);
Console.WriteLine(pt.GetHashCode());
}
}
// The example displays the following output:
// 173
// 269
[<Struct; CustomEquality; NoComparison>]
type Point(x: int, y: int) =
member _.X = x
member _.Y = y
override _.Equals(obj) =
match obj with
| :? Point as p ->
x = p.X && y = p.Y
| _ ->
false
override _.GetHashCode() =
(x, y).GetHashCode()
let pt = Point(5, 8)
printfn $"{pt.GetHashCode()}"
let pt2 = Point(8, 5)
printfn $"{pt2.GetHashCode()}"
// The example displays the following output:
// 173
// 269
Public Structure Point
Private x As Integer
Private y As Integer
Public Sub New(x As Integer, y As Integer)
Me.x = x
Me.y = y
End Sub
Public Overrides Function Equals(obj As Object) As Boolean
If Not TypeOf obj Is Point Then Return False
Dim p As Point = CType(obj, Point)
Return x = p.x And y = p.y
End Function
Public Overrides Function GetHashCode() As Integer
Return Tuple.Create(x, y).GetHashCode()
End Function
End Structure
Public Module Example
Public Sub Main()
Dim pt As New Point(5, 8)
Console.WriteLine(pt.GetHashCode())
pt = New Point(8, 5)
Console.WriteLine(pt.GetHashCode())
End Sub
End Module
' The example displays the following output:
' 173
' 269
第二种替代解决方案涉及通过将连续字段的哈希代码左移两位或更多位来加权各个哈希代码。 最佳情况下，超出位 31 的位应环绕而不是取消卡。 由于 C# 和 Visual Basic 中的左移运算符卡位，因此这需要创建左移和换行方法，如下所示：
public int ShiftAndWrap(int value, int positions)
{
positions = positions & 0x1F;
// Save the existing bit pattern, but interpret it as an unsigned integer.
uint number = BitConverter.ToUInt32(BitConverter.GetBytes(value), 0);
// Preserve the bits to be discarded.
uint wrapped = number >> (32 - positions);
// Shift and wrap the discarded bits.
return BitConverter.ToInt32(BitConverter.GetBytes((number << positions) | wrapped), 0);
}
let shiftAndWrap (value: int) positions =
let positions = positions &&& 0x1F
// Save the existing bit pattern, but interpret it as an unsigned integer.
let number = BitConverter.ToUInt32(BitConverter.GetBytes value, 0)
// Preserve the bits to be discarded.
let wrapped = number >>> (32 - positions)
// Shift and wrap the discarded bits.
BitConverter.ToInt32(BitConverter.GetBytes((number <<< positions) ||| wrapped), 0)
Public Function ShiftAndWrap(value As Integer, positions As Integer) As Integer
positions = positions And &h1F
' Save the existing bit pattern, but interpret it as an unsigned integer.
Dim number As UInteger = BitConverter.ToUInt32(BitConverter.GetBytes(value), 0)
' Preserve the bits to be discarded.
Dim wrapped AS UInteger = number >> (32 - positions)
' Shift and wrap the discarded bits.
Return BitConverter.ToInt32(BitConverter.GetBytes((number << positions) Or wrapped), 0)
End Function
以下示例随后使用此 shift-and-wrap 方法计算前面示例中使用的结构的 Point 哈希代码。
using System;
public struct Point
{
private int x;
private int y;
public Point(int x, int y)
{
this.x = x;
this.y = y;
}
public override bool Equals(Object obj)
{
if (!(obj is Point)) return false;
Point p = (Point) obj;
return x == p.x & y == p.y;
}
public override int GetHashCode()
{
return ShiftAndWrap(x.GetHashCode(), 2) ^ y.GetHashCode();
}
private int ShiftAndWrap(int value, int positions)
{
positions = positions & 0x1F;
// Save the existing bit pattern, but interpret it as an unsigned integer.
uint number = BitConverter.ToUInt32(BitConverter.GetBytes(value), 0);
// Preserve the bits to be discarded.
uint wrapped = number >> (32 - positions);
// Shift and wrap the discarded bits.
return BitConverter.ToInt32(BitConverter.GetBytes((number << positions) | wrapped), 0);
}
}
public class Example2
{
public static void Main()
{
Point pt = new Point(5, 8);
Console.WriteLine(pt.GetHashCode());
pt = new Point(8, 5);
Console.WriteLine(pt.GetHashCode());
}
}
// The example displays the following output:
// 28
// 37
open System
[<Struct; CustomEquality; NoComparison>]
type Point(x: int, y: int) =
member _.X = x
member _.Y = y
override _.Equals(obj) =
match obj with
| :? Point as p ->
x = p.X && y = p.Y
| _ ->
false
override this.GetHashCode() =
this.ShiftAndWrap(x.GetHashCode(), 2) ^^^ y.GetHashCode()
member _.ShiftAndWrap(value, positions) =
let positions = positions &&& 0x1F
// Save the existing bit pattern, but interpret it as an unsigned integer.
let number = BitConverter.ToUInt32(BitConverter.GetBytes value, 0)
// Preserve the bits to be discarded.
let wrapped = number >>> (32 - positions)
// Shift and wrap the discarded bits.
BitConverter.ToInt32(BitConverter.GetBytes((number <<< positions) ||| wrapped), 0)
let pt = Point(5, 8)
printfn $"{pt.GetHashCode()}"
let pt2 = Point(8, 5)
printfn $"{pt2.GetHashCode()}"
// The example displays the following output:
// 28
// 37
Public Structure Point5
Private x As Integer
Private y As Integer
Public Sub New(x As Integer, y As Integer)
Me.x = x
Me.y = y
End Sub
Public Overrides Function Equals(obj As Object) As Boolean
If Not TypeOf obj Is Point5 Then Return False
Dim p As Point5 = CType(obj, Point5)
Return x = p.x And y = p.y
End Function
Public Overrides Function GetHashCode() As Integer
Return ShiftAndWrap(x.GetHashCode(), 2) Xor y.GetHashCode()
End Function
Private Function ShiftAndWrap(value As Integer, positions As Integer) As Integer
positions = positions And &H1F
' Save the existing bit pattern, but interpret it as an unsigned integer.
Dim number As UInteger = BitConverter.ToUInt32(BitConverter.GetBytes(value), 0)
' Preserve the bits to be discarded.
Dim wrapped As UInteger = number >> (32 - positions)
' Shift and wrap the discarded bits.
Return BitConverter.ToInt32(BitConverter.GetBytes((number << positions) Or wrapped), 0)
End Function
End Structure
Module Example2
Public Sub Main()
Dim pt As New Point5(5, 8)
Console.WriteLine(pt.GetHashCode())
pt = New Point5(8, 5)
Console.WriteLine(pt.GetHashCode())
End Sub
End Module
' The example displays the following output:
' 28
' 37
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