[英]Implementing Equals and GetHashCode - an easier way
我有一棵對象樹(DTO),其中一個對象引用了其他對象,依此類推:
class Person
{
public int Id { get; }
public Address Address { get; }
// Several other properties
}
public Address
{
public int Id { get; }
public Location Location { get; }
// Several other properties
}
這些對象可能非常復雜,並具有許多其他屬性。
在我的應用程序,一個Person
有相同的Id
可以在兩個存儲,本地存儲的應用程序,並從后端的到來。 我需要以特定的方式將在線Person
與本地Person
合並,因此,我需要首先知道在線Person
是否與本地存儲的相同(換句話說,如果應用未更新Local Person
)。 。
為了使用LINQ的Except,我知道我需要實現Equatable<T>
,而我看到的通常方式是這樣的:
class Person : IEquatable<Person>
{
public int Id { get; }
public Address Address { get; }
public override bool Equals(object obj)
{
return Equals(obj as Person);
}
public bool Equals(Person other)
{
return other != null &&
Id == other.Id &&
Address.Equals(other.Address);
}
public override int GetHashCode()
{
var hashCode = -306707981;
hashCode = hashCode * -1521134295 + Id.GetHashCode();
hashCode = hashCode * -1521134295 + (Address != null ? Address.GetHashCode() : 0);
return hashCode;
}
對我來說,這聽起來很復雜且難以維護,當屬性更改時,很容易忘記更新Equals
和GetHashCode
。 根據對象,它在計算上也可能會有點昂貴。
以下內容不是實現Equals
和GethashCode
的更簡單有效的方法嗎?
class Person : IEquatable<Person>
{
public int Id { get; }
public Address Address { get; private set; }
public DateTime UpdatedAt { get; private set; }
public void SetAdress(Address address)
{
Address = address;
UpdatedAt = DateTime.Now;
}
public override bool Equals(object obj)
{
return Equals(obj as Person);
}
public bool Equals(Person other)
{
return other != null &&
Id == other.Id &&
UpdatedAt.Ticks == other.UpdatedAt.Ticks;
}
public override int GetHashCode()
{
var hashCode = -306707981;
hashCode = hashCode * -1521134295 + Id.GetHashCode();
hashCode = hashCode * -1521134295 + UpdatedAt.Ticks.GetHashCode();
return hashCode;
}
}
我的想法是,只要對象更改,就會有一個時間戳記。 該時間戳記與對象一起保存。 我也在考慮將此字段用作存儲中的並發令牌。
由於解析DateTime可能是一個問題,因此與其使用時間,不如說我認為Guid也是替代DateTime的好選擇。 不會有太多的對象,因此Guid的唯一性不應該成為問題。
您認為這種方法有問題嗎?
就像我在上面說過的那樣,與讓Equals和GetHashCode遍歷所有屬性相比,我認為它更容易實現且運行起來更快。
更新 :我考慮得越多,我傾向於覺得在類上實現Equals
和GetHashCode
不是一個好方法。 我認為最好實現專門的IEqualityComparer<Person>
,以特定的方式比較Person
並將其傳遞給LINQ的方法。
這樣做的原因是,就像在評論和答案中一樣,可以以不同的方式使用Person
。
如果兩個對象具有相同的屬性但在不同的時間創建,則將給您帶來假的負相等;如果兩個對象的屬性具有不同的屬性卻又相繼創建,則會給您帶來假的正等式(時鍾不那么精確) 。
對於LINQ Except
,實際上是您需要實現的GetHashCode
,並且應該使用所有屬性的哈希碼。
理想情況下,它們也應該是不可變的(刪除私有設置器),以便一個對象在整個生命周期中具有相同的哈希碼。
您的GetHashCode
也應unchecked
。
另外,您可以將Except
與自定義比較器一起使用。
真正的惰性版本,用於使用值元組(不為此分配值)來實現GetHashCode
/ Equals
:
class Person : IEquatable<Person>
{
public int Id { get; }
public Address Address { get; }
public Person(int id, Address address) => (Id, Address) = (id, address);
public override bool Equals(object obj) => Equals(obj as Person);
public bool Equals(Person other) => other != null
&& (Id, Address).Equals((other.Id,other.Address));
public override int GetHashCode() => (Id, Address).GetHashCode();
}
以下是LinqPad草圖,您可以從這里開始。 它具有您可以用來根據需要定制的所有工具。 當然,這僅僅是一個概念,並不是所有方面都得到了詳盡闡述。
如您所見,有一個Include
屬性可以應用於您要包括在哈希中的后備字段。
void Main()
{
var o1 = new C { Interesting = "Whatever", NotSoInterresting = "Blah.." };
var o2 = new C { Interesting = "Whatever", NotSoInterresting = "Blah-blah.." };
(o1 == o2).Dump("o1 == o2"); // False
(o2 == o1).Dump("o2 == o1"); // False
var o3 = o1.Clone();
(o3 == o1).Dump("o3 == o1"); // True
(object.ReferenceEquals(o1, o3)).Dump("R(o3) == R(o2)"); // False
o3.NotSoInterresting = "Changed!";
(o1 == o3).Dump("o1 == C(o3)"); // True
o3.Interesting = "Changed!";
(o1 == o3).Dump("o1 == C(o3)"); // False
}
[AttributeUsage(AttributeTargets.Field)]
public class IncludeAttribute : Attribute { }
public static class ObjectExtensions
{
public static int GetHash(this object obj) => obj?.GetHashCode() ?? 1;
public static int CalculateHashFromFields(this object obj)
{
var fields = obj.GetType()
.GetFields(BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.DeclaredOnly /*or not*/)
.Where(f => f.CustomAttributes.Any(x => x.AttributeType.Equals(typeof(IncludeAttribute))));
var result = 1;
unchecked
{
foreach(var f in fields) result *= f.GetValue(obj).GetHash();
}
return result;
}
}
public partial class C
{
[Include]
private int id;
public int Id { get => id; private set { id = value; UpdateHash(); } }
[Include]
private string interesting;
public string Interesting { get => interesting; set { interesting = value; UpdateHash(); } }
public string NotSoInterresting { get; set; }
}
public partial class C: IEquatable<C>
{
public C Clone() => new C { Id = this.Id, Interesting = this.Interesting, NotSoInterresting = this.NotSoInterresting };
private static int _id = 1; // Some persistence is required instead
public C()
{
Id = _id++;
}
private int hash;
private void UpdateHash() => hash = this.CalculateHashFromFields();
public override bool Equals(object obj)
{
return Equals(obj as C);
}
public bool Equals(C other) => this.hash == other.hash;
public override int GetHashCode() => hash;
public static bool operator ==(C obj1, C obj2) => obj1.Equals(obj2);
public static bool operator !=(C obj1, C obj2) => !obj1.Equals(obj2);
}
[更新18.06.17]
更新后的版本:
void Main()
{
var o1 = new C { Interesting = "Whatever", NotSoInterresting = "Blah.." };
var o2 = new C { Interesting = "Whatever", NotSoInterresting = "Blah-blah.." };
(o1 == o2).Dump("o1 == o2"); // False
(o2 == o1).Dump("o2 == o1"); // False
var o3 = o1.Clone();
(o3 == o1).Dump("o3 == o1"); // True
(object.ReferenceEquals(o1, o3)).Dump("R(o3) == R(o2)"); // False
o3.NotSoInterresting = "Changed!";
(o1 == o3).Dump("o1 == C(o3)"); // True
o3.Interesting = "Changed!";
(o1 == o3).Dump("o1 == C(o3)"); // False
C o4 = null;
(null == o4).Dump("o4 == null"); // True
}
[AttributeUsage(AttributeTargets.Field)]
public class IncludeAttribute : Attribute { }
public static class ObjectExtensions
{
public static int GetHash(this object obj) => obj?.GetHashCode() ?? 1;
}
public abstract class EquatableBase : IEquatable<EquatableBase>
{
private static FieldInfo[] fields = null;
private void PrepareFields()
{
fields = this.GetType()
.GetFields(BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.DeclaredOnly /*or not*/)
.Where(f => f.CustomAttributes.Any(x => x.AttributeType.Equals(typeof(IncludeAttribute))))
.ToArray();
}
private int CalculateHashFromProperties()
{
if (fields == null) PrepareFields();
var result = 1;
unchecked
{
foreach (var f in fields) result ^= f.GetValue(this).GetHash();
}
return result;
}
private bool CheckDeepEqualityTo(EquatableBase other)
{
if (ReferenceEquals(other, null) || other.GetType() != GetType()) return false;
if (fields == null) PrepareFields();
var result = true;
for(int i = 0; i < fields.Length && result; i++)
{
var field = fields[i];
result &= field.GetValue(this).Equals(field.GetValue(other));
}
return result;
}
private int hash;
protected int UpdateHash() => hash = this.CalculateHashFromProperties();
protected void InvalidateHash() => hash = 0;
public override bool Equals(object obj) => Equals(obj as EquatableBase);
public bool Equals(EquatableBase other) => object.ReferenceEquals(this, other) || this.CheckDeepEqualityTo(other);
public override int GetHashCode() => hash == 0 ? UpdateHash() : hash;
public static bool operator ==(EquatableBase obj1, EquatableBase obj2) => ReferenceEquals(obj1, obj2) || obj1?.CheckDeepEqualityTo(obj2) == true;
public static bool operator !=(EquatableBase obj1, EquatableBase obj2) => !(obj1 == obj2);
}
public partial class C: EquatableBase
{
private static int _id = 1; // Some persistence is required instead
public C()
{
Id = _id++;
}
public C Clone() => new C { Id = this.Id, Interesting = this.Interesting, NotSoInterresting = this.NotSoInterresting };
[Include]
private int id;
public int Id { get => id; private set { id = value; InvalidateHash(); } }
[Include]
private string interesting;
public string Interesting { get => interesting; set { interesting = value; InvalidateHash(); } }
public string NotSoInterresting { get; set; }
}
仍然不能擺脫在setter中調用某些東西(當然仍然存在進行優化的地方),但是這些改進非常困難:
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