何时重写GetHashCode（）？

Question

我们什么时候应该覆盖' System '命名空间中' Object '类提供的GetHashCode （）方法？

Answer 1

When you override Equals , basically. When you want to provide a different idea of equality than simple reference equality.

String is a good example of this - two strings are equal (under a simple Equals call) if they represent the same sequence of characters. The hash code reflects this, such that if two strings are equal they will have the same hash code. (The reverse isn't necessarily true - two unequal strings can have the same hash code, but it's unlikely.)

(Strings are tricky in other ways, mind you - there are lots of different ideas of equality based on culture and casing, but String.Equals just looks at the UTF-16 code points which make up the string, and compares them in the simplest conceivable fashion.)

Answer 2

如果你的类型应该遵循值语义（比较内容）而不是引用语义（比较对象标识），你应该编写自己的实例对象覆盖.Equals（）。

Answer 3

如果重写Equals ，则必须覆盖GetHashCode 。

Answer 4

"The GetHashCode method can be overridden by a derived type. Value types must override this method to provide a hash function that is appropriate for that type and to provide a useful distribution in a hash table. For best results, the hash code must be based on the value of an instance field or property instead of a static field or property.

Objects used as a key in a Hashtable object must also override the GetHashCode method because those objects must generate their own hash code. If an object used as a key does not provide a useful implementation of GetHashCode, you can specify a hash code provider when the Hashtable object is constructed. Prior to the .NET Framework version 2.0, the hash code provider was based on the System.Collections..::.IHashCodeProvider interface. Starting with version 2.0, the hash code provider is based on the System.Collections..::.IEqualityComparer interface."

http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx