C# 如何使用重写 Equals() 的逻辑实现对 GetHashCode() 的重写
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how to implement override of GetHashCode() with logic of overriden Equals()
提问by rene
I have some classes as below, i have implemented the Equals(Object)method for almost all of them. But i don't know how to write GetHashCode(). As far I used these data types as value type in a DictionaryCollection, i think i should override GetHashCode().
我有一些如下类，我已经Equals(Object)为几乎所有的类实现了该方法。但是我不知道怎么写 GetHashCode()。就我使用这些数据类型作为Dictionary集合中的值类型而言，我认为我应该覆盖GetHashCode().
1.I don't know how to implement GetHashCode()with logic of Equals(Object).
1.我不知道如何GetHashCode()用Equals(Object).
2.There are some derived classes, if i override GetHashCode()and Equals(Object)for base class ( Param), is it still necessary to override it for childs?
2.有一些派生类，如果我重写GetHashCode()和Equals(Object)为基类（Param），它仍然需要覆盖它用于儿童的？
class Param
{
...
public Int16 id { get; set; }
public String name { get; set; }
...
public override bool Equals(object obj)
{
if ( obj is Param){
Param p = (Param)(obj);
if (id > 0 && p.id > 0)
return (id == p.id);
else if (name != String.Empty && p.name != String.Empty)
return (name.equals(p.name));
else
return object.ReferenceEquals(this, obj);
}
return false;
}
}
class Item
{
public int it_code { get; set; }
public Dictionary<String, Param> paramAr { get; set; }
...
public override bool Equals(Object obj)
{
Item im = new Item();
if (obj is Item)
im = (Item)obj;
else
return false;
if (this.it_code != String.Empty && im.it_code != String.Empty)
if (this.it_code.Equals(im.it_code))
return true;
bool reParams = true;
foreach ( KeyValuePair<String,Param> kvp in paramAr ){
if (kvp.Value != im.paramAr[kvp.Key]) {
reParams = false;
break;
}
}
return reParams;
}
}
class Order
{
public String or_code { get; set; }
public List <Item> items { get; set; }
...
public override bool Equals( Object obj ){
Order o = new Order();
if (obj is Order)
o = (Order)obj;
else
return false;
if (this.or_code != String.Empty && o.or_code != String.Empty)
if (this.or_code.Equals(o.or_code))
return true;
bool flag = true;
foreach( Item i in items){
if (!o.items.Contains(i)) {
flag = false;
break;
}
}
return flag;
}
}
EDIT:i get this warning:
编辑：我收到此警告：
Warning : 'Item' overrides Object.Equals(object o) but does not override Object.GetHashCode()
警告：'Item' 覆盖 Object.Equals(object o) 但不覆盖 Object.GetHashCode()
采纳答案by kaj
Firstly, as I think you understand, wherever you implement Equalsyou MUST also implement GetHashCode. The implementation of GetHashCodemust reflect the behaviour of the Equalsimplementation but it doesn't usually useit.
首先，正如我认为您所理解的，无论您在何处实施，Equals都必须实施GetHashCode. 的实现GetHashCode必须反映实现的行为，Equals但它通常不使用它。
See http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx- especially the "Notes to Implementers"
请参阅http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx- 特别是“实施者注意事项”
So if you take your example of the Itemimplementation of Equals, you're considering both the values of idand nameto affect equality. So both of these must contribute to the GetHashCodeimplementation.
因此，如果您以 的Item实现为例Equals，您将同时考虑id和name影响平等的价值。因此，这两者都必须有助于GetHashCode实施。
An example of how you could implement GetHashCodefor Itemwould be along the lines of the following (note you may need to make it resilient to a nullable namefield):
您可以如何实现GetHashCodefor的示例Item如下（请注意，您可能需要使其适应可空name字段）：
public override GetHashCode()
{
return id.GetHashCode() ^ name.GetHashCode();
}
See Eric Lippert's blog post on guidelines for GetHashCode- http://ericlippert.com/2011/02/28/guidelines-and-rules-for-gethashcode/
请参阅 Eric Lippert 关于指南的博客文章GetHashCode- http://ericlippert.com/2011/02/28/guidelines-and-rules-for-gethashcode/
As for whether you need to re-implement GetHashCodein subclasses - Yes if you also override Equals- as per the first (and main) point - the implementation of the two must be consistent - if two items are considered equal by Equalsthen they must return the same value from GetHashCode.
至于您是否需要GetHashCode在子类中重新实现- 是的，如果您还覆盖Equals- 根据第一个（和主要）点 - 两者的实现必须一致 - 如果两个项目被认为是相等的，Equals那么它们必须返回相同值来自GetHashCode.
Side note: As a performance improvement on your code (avoid multiple casts):
旁注：作为代码的性能改进（避免多次转换）：
if ( obj is Param){
Param p = (Param)(obj);
Param p = obj as Param;
if (p != null) ...
回答by Alex
I prefer Josh Bloch's aproach.
我更喜欢 Josh Bloch 的方法。
Here's the example for the Paramclass.
这是Param该类的示例。
override GetHashCode(object obj)
{
unchecked
{
int hash = 17;
hash = hash * 23 + id.GetHashCode();
hash = hash * 23 + name.GetHashCode();
return hash;
}
}
Also, check this link out : .net - best algorithm for GetHashCodeProperties used for the hashcode computation should be immutable as well.
此外，请查看此链接：.net -用于哈希码计算的GetHashCode属性的最佳算法也应该是不可变的。
