要实现对象的相等比较,需要实现IEquatable<T>,或单独写一个类实现IEqualityComparer<T>接口。
像List<T>的Contains这样的函数,如果我们自己定义的对象不实现IEquatable<T>接口,这个函数会默认调用object的Equels来比较对象,得出非预期的结果。
先自定义一个类:
public class DaichoKey
{
public int ID { get; set; }
public int SubID { get; set; }
}
List<DaichoKey> lst = new List<DaichoKey>() {
new DaichoKey(){ID = 1,SubID =2},
new DaichoKey(){ID = 1,SubID = 3}
};
var newItem = new DaichoKey() { ID = 1, SubID = 2 };
bool isContains = lst.Contains(newItem);//false
上面的代码调用Contains后得到false,我们预想1和2的对象都已经存在了,应该得到true才对呀。
要实现这个效果,需要实现IEquatable<T>接口。
public class DaichoKey : IEquatable<DaichoKey>
{
public int ID { get; set; }
public int SubID { get; set; }
public bool Equals(DaichoKey other)
{
return this.ID == other.ID && this.SubID == other.SubID;
}
}
经过上面的改良,结果如我们预期了,但是还不够完善,微软建议我们重写object的Equels方法我GetHashCode方法,以保持语义的一致性,于是有了下面的代码:
public class DaichoKey : IEquatable<DaichoKey>
{
public int ID { get; set; }
public int SubID { get; set; }
public bool Equals(DaichoKey other)
{
return this.ID == other.ID && this.SubID == other.SubID;
}
public override bool Equals(object obj)
{
if (obj == null) return base.Equals(obj);
if (obj is DaichoKey)
return Equals(obj as DaichoKey);
else
throw new InvalidCastException("the 'obj' Argument is not a DaichoKey object");
}
public override int GetHashCode()
{
return base.GetHashCode();//return object's hashcode
}
}
上面的代码依然还有缺陷,没重写==和!=运算符,但这不是本文讨论的重点。绕了一大圈,终于来到了GetHashCode函数身上,貌似他对我们的Contains函数没有啥影响呀,不重写又何妨?我们再来试试List<T>的一个扩展函数Distinct:
List<DaichoKey> lst = new List<DaichoKey>() {
new DaichoKey(){ID = 1,SubID =2},
new DaichoKey(){ID = 1,SubID = 3}
};
var newItem = new DaichoKey() { ID = 1, SubID = 2 };
lst.Add(newItem);
if (lst != null)
{
lst = lst.Distinct<DaichoKey>().ToList();
}
//result:
//1 2
//1 3
//1 2
悲剧发生了,数据1,2的重复数据没有被去掉呀,我们不是实现了IEquatable<T>接口接口吗。在园子上找到了一篇文章(c# 扩展方法奇思妙用基础篇八:Distinct 扩展),在回复中提到要将GetHashCode返回固定值,以强制调用IEquatable<T>的Equels方法。如下:
public class DaichoKey : IEquatable<DaichoKey>
{
public int ID { get; set; }
public int SubID { get; set; }
public bool Equals(DaichoKey other)
{
return this.ID == other.ID && this.SubID == other.SubID;
}
public override bool Equals(object obj)
{
if (obj == null) return base.Equals(obj);
if (obj is DaichoKey)
return Equals(obj as DaichoKey);
else
throw new InvalidCastException("the 'obj' Argument is not a DaichoKey object");
}
public override int GetHashCode()
{
return 0;//base.GetHashCode();
}
}
结果立马就对了,难道是这个Distinct函数在比较时,先比较的HashCode值?
带着这个疑问,反编译了下Distinct的代码,确实如我所猜测的那样。下面是源代码,有兴趣的同学,可以往下看看:
public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source)
{
if (source == null) throw Error.ArgumentNull("source");
return DistinctIterator<TSource>(source, null);
}
private static IEnumerable<TSource> DistinctIterator<TSource>(IEnumerable<TSource> source, IEqualityComparer<TSource> comparer)
{
<DistinctIterator>d__81<TSource> d__ = new <DistinctIterator>d__81<TSource>(-2);
d__.<>3__source = source;
d__.<>3__comparer = comparer;
return d__;
}
private sealed class <DistinctIterator>d__81<TSource> : IEnumerable<TSource>, IEnumerable, IEnumerator<TSource>, IEnumerator, IDisposable
{
// Fields
private int <>1__state;
private TSource <>2__current;
public IEqualityComparer<TSource> <>3__comparer;
public IEnumerable<TSource> <>3__source;
public IEnumerator<TSource> <>7__wrap84;
private int <>l__initialThreadId;
public TSource <element>5__83;
public Set<TSource> <set>5__82;
public IEqualityComparer<TSource> comparer;
public IEnumerable<TSource> source;
// Methods
[DebuggerHidden]
public <DistinctIterator>d__81(int <>1__state);
private void <>m__Finally85();
private bool MoveNext();
[DebuggerHidden]
IEnumerator<TSource> IEnumerable<TSource>.GetEnumerator();
[DebuggerHidden, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
IEnumerator IEnumerable.GetEnumerator();
[DebuggerHidden]
void IEnumerator.Reset();
void IDisposable.Dispose();
// Properties
TSource IEnumerator<TSource>.Current { [DebuggerHidden] get; }
object IEnumerator.Current { [DebuggerHidden] get; }
}
private sealed class <DistinctIterator>d__81<TSource> : IEnumerable<TSource>, IEnumerable, IEnumerator<TSource>, IEnumerator, IDisposable
{
// Fields
private int <>1__state;
private TSource <>2__current;
public IEqualityComparer<TSource> <>3__comparer;
public IEnumerable<TSource> <>3__source;
public IEnumerator<TSource> <>7__wrap84;
private int <>l__initialThreadId;
public TSource <element>5__83;
public Set<TSource> <set>5__82;
public IEqualityComparer<TSource> comparer;
public IEnumerable<TSource> source;
// Methods
[DebuggerHidden]
public <DistinctIterator>d__81(int <>1__state);
private void <>m__Finally85();
private bool MoveNext();
[DebuggerHidden]
IEnumerator<TSource> IEnumerable<TSource>.GetEnumerator();
[DebuggerHidden, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
IEnumerator IEnumerable.GetEnumerator();
[DebuggerHidden]
void IEnumerator.Reset();
void IDisposable.Dispose();
// Properties
TSource IEnumerator<TSource>.Current { [DebuggerHidden] get; }
object IEnumerator.Current { [DebuggerHidden] get; }
}
private bool MoveNext()
{
bool flag;
try
{
switch (this.<>1__state)
{
case 0:
this.<>1__state = -1;
this.<set>5__82 = new Set<TSource>(this.comparer);
this.<>7__wrap84 = this.source.GetEnumerator();
this.<>1__state = 1;
goto Label_0092;
case 2:
this.<>1__state = 1;
goto Label_0092;
default:
goto Label_00A5;
}
Label_0050:
this.<element>5__83 = this.<>7__wrap84.Current;
if (this.<set>5__82.Add(this.<element>5__83))
{
this.<>2__current = this.<element>5__83;
this.<>1__state = 2;
return true;
}
Label_0092:
if (this.<>7__wrap84.MoveNext()) goto Label_0050;
this.<>m__Finally85();
Label_00A5:
flag = false;
}
fault
{
this.System.IDisposable.Dispose();
}
return flag;
}
internal class Set<TElement>
{
// Fields
private int[] buckets;
private IEqualityComparer<TElement> comparer;
private int count;
private int freeList;
private Slot<TElement>[] slots;
// Methods
[TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
public Set();
public Set(IEqualityComparer<TElement> comparer);
public bool Add(TElement value);
[TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
public bool Contains(TElement value);
private bool Find(TElement value, bool add);
internal int InternalGetHashCode(TElement value);
public bool Remove(TElement value);
private void Resize();
// Nested Types
[StructLayout(LayoutKind.Sequential)]
internal struct Slot
{
internal int hashCode;
internal TElement value;
internal int next;
}
}
public bool Add(TElement value)
{
return !this.Find(value, true);
}
public bool Contains(TElement value)
{
return this.Find(value, false);
}
private bool Find(TElement value, bool add)
{
int hashCode = this.InternalGetHashCode(value);
for (int i = this.buckets[hashCode % this.buckets.Length] - 1; i >= 0; i = this.slots[i].next)
{
if (this.slots[i].hashCode == hashCode && this.comparer.Equals(this.slots[i].value, value)) return true;//就是这一句了
}
if (add)
{
int freeList;
if (this.freeList >= 0)
{
freeList = this.freeList;
this.freeList = this.slots[freeList].next;
}
else
{
if (this.count == this.slots.Length) this.Resize();
freeList = this.count;
this.count++;
}
int index = hashCode % this.buckets.Length;
this.slots[freeList].hashCode = hashCode;
this.slots[freeList].value = value;
this.slots[freeList].next = this.buckets[index] - 1;
this.buckets[index] = freeList + 1;
}
return false;
}
在这段代码中可以看出,扩展函数Distinct在内部使用了一个Set<T>的类来帮助踢掉重复数据,而这个内部类使用的是hash表的方式存储数据,所以会调用到我们自定义类的GetHashCode函数,如果返回的hashcode值不等,它就不会再调用Equels方法进行比较了。
原因已经一目了然了,得出的结论就是:
1,重写Equles方法的时候,尽量重写GetHashCode函数,并且不要简单的调用object的GetHashCode函数,返回一个设计合理的hash值,以保证结果如我们的预期。上面的做法直接返回了0,虽然解决了问题,但明显不是每个对象的hash值都是0,做法欠妥。
2,List<T>的Contains,IndexOf方法,不会用到GetHashCode函数。
3,扩展函数Distinct,Except用到了GetHashCode函数,必须重写这个函数。其他还有哪些函数用到了GetHashCode函数,以后再做补充,使用时多加注意就是了。
4,如果对象要作为字典类(Dictionary)的主键,必须重写GetHashCode函数。
2014/07/08 补充
5,HashSet等容器的Add方法内部,也是先判断GetHashCode,如果GetHashCode值相等,进一步判断Equals方法是否相等来确定对象的相等性。
所以,Equals是相等的,那么GetHashCode也必须要保证相等。相反却不一定,GetHashCode相等,Equals方法可以不等。
6,改变影响GetHashCode返回值的字段值,会造成对象的HashCode值变化,如果对象已经存入了HashSet等容器中,将会是HashSet找不到这个对象,从而使得Remove等方法失败。
Point a = new Point(1, 2); Point b = new Point(1, 2); HashSet<Point> hashSet = new HashSet<Point>(); hashSet.Add(a); hashSet.Remove(b); //能删除a吗?答案是可以 //hashset的Count变为0,原因就是我们重新了Equals方法,a和 //b被认为相等的。
7,记录一个自定义值类型重写GetHashCode等方法的完整实现,作为参考。


1 public struct Point 2 { 3 private int x; 4 private int y; 5 public Point(int x, int y) 6 { 7 this.x = x; 8 this.y = y; 9 } 10 public int X 11 { 12 get { return x; } 13 } 14 public int Y 15 { 16 get { return y; } 17 } 18 19 public static bool operator ==(Point left,Point right) 20 { 21 if (object.ReferenceEquals(left, null)) 22 return object.ReferenceEquals(right, null); 23 return left.Equals(right); 24 } 25 26 public static bool operator !=(Point left, Point right) 27 { 28 return !(left == right); 29 } 30 31 public override bool Equals(object obj) 32 { 33 if (obj.GetType() != typeof(Point)) 34 return false; 35 Point other = (Point)obj; 36 return this.x == other.x && this.y == other.y; 37 } 38 39 public override int GetHashCode() 40 { 41 return x.GetHashCode() ^ y.GetHashCode(); 42 } 43 }