일반 배열을 포함하는 객체의 GetHashCode 재정의

일반 배열을 포함하는 객체의 GetHashCode 재정의

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다음 두 가지 속성을 포함하는 클래스가 있습니다.

public int Id      { get; private set; }
public T[] Values  { get; private set; }

나는 그것을 IEquatable<T>만들고 다음 object.Equals과 같이 재정의 했습니다.

public override bool Equals(object obj)
{
    return Equals(obj as SimpleTableRow<T>);
}

public bool Equals(SimpleTableRow<T> other)
{
    // Check for null
    if(ReferenceEquals(other, null))
        return false;

    // Check for same reference
    if(ReferenceEquals(this, other))
        return true;

    // Check for same Id and same Values
    return Id == other.Id && Values.SequenceEqual(other.Values);
}

오버라이드를 할 때는 물론 object.Equals오버라이드해야합니다 GetHashCode. 하지만 어떤 코드를 구현해야합니까? 일반 배열에서 해시 코드를 생성하려면 어떻게합니까? 그리고 그것을 Id정수 와 어떻게 결합 합니까?

public override int GetHashCode()
{
    return // What?
}

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이 스레드에서 발생하는 문제로 인해 잘못된 경우 어떻게되는지 보여주는 또 다른 답변을 게시하고 있습니다. 주로 배열을 사용할 수 없다는 것입니다 GetHashCode(). 올바른 동작은 실행할 때 경고가 인쇄되지 않는 것입니다. 주석을 전환하여 수정하십시오.

using System;
using System.Collections.Generic;
using System.Linq;
static class Program
{
    static void Main()
    {
        // first and second are logically equivalent
        SimpleTableRow<int> first = new SimpleTableRow<int>(1, 2, 3, 4, 5, 6),
            second = new SimpleTableRow<int>(1, 2, 3, 4, 5, 6);

        if (first.Equals(second) && first.GetHashCode() != second.GetHashCode())
        { // proven Equals, but GetHashCode() disagrees
            Console.WriteLine("We have a problem");
        }
        HashSet<SimpleTableRow<int>> set = new HashSet<SimpleTableRow<int>>();
        set.Add(first);
        set.Add(second);
        // which confuses anything that uses hash algorithms
        if (set.Count != 1) Console.WriteLine("Yup, very bad indeed");
    }
}
class SimpleTableRow<T> : IEquatable<SimpleTableRow<T>>
{

    public SimpleTableRow(int id, params T[] values) {
        this.Id = id;
        this.Values = values;
    }
    public int Id { get; private set; }
    public T[] Values { get; private set; }

    public override int GetHashCode() // wrong
    {
        return Id.GetHashCode() ^ Values.GetHashCode();
    }
    /*
    public override int GetHashCode() // right
    {
        int hash = Id;
        if (Values != null)
        {
            hash = (hash * 17) + Values.Length;
            foreach (T t in Values)
            {
                hash *= 17;
                if (t != null) hash = hash + t.GetHashCode();
            }
        }
        return hash;
    }
    */
    public override bool Equals(object obj)
    {
        return Equals(obj as SimpleTableRow<T>);
    }
    public bool Equals(SimpleTableRow<T> other)
    {
        // Check for null
        if (ReferenceEquals(other, null))
            return false;

        // Check for same reference
        if (ReferenceEquals(this, other))
            return true;

        // Check for same Id and same Values
        return Id == other.Id && Values.SequenceEqual(other.Values);
    }
}

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FWIW, it's very dangerous to use the contents of the Values in your hash code. You should only do this if you can guarantee that it will never change. However, since it is exposed, I don't think guaranteeing it is possible. The hashcode of an object should never change. Otherwise, it loses its value as a key in a Hashtable or Dictionary. Consider the hard-to-find bug of using an object as a key in a Hashtable, its hashcode changes because of an outside influence and you can no longer find it in the Hashtable!

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Since the hashCode is kinda a key for storing the object (lllike in a hashtable), i would use just Id.GetHashCode()

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How about something like:

    public override int GetHashCode()
    {
        int hash = Id;
        if (Values != null)
        {
            hash = (hash * 17) + Values.Length;
            foreach (T t in Values)
            {
                hash *= 17;
                if (t != null) hash = hash + t.GetHashCode();
            }
        }
        return hash;
    }

This should be compatible with SequenceEqual, rather than doing a reference comparison on the array.

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I just had to add another answer because one of the more obvious (and easiest to implement) solutions were not mentioned - not including the collection in your GetHashCode calculation!

The main thing that seemed to have forgotten here is that the uniqueness from the result of GetHashCode isn't required (or in many cases even possible). Unequal objects don't have to return unequal hash codes, the only requirement is that equal objects return equal hash codes. So by that definition, the following implementation of GetHashCode is correct for all objects (assuming there's a correct Equals implementation):

public override int GetHashCode() 
{ 
    return 42; 
} 

Of course this would yield the worst possible performance in hashtable lookup, O(n) instead of O(1), but it is still functionally correct.

With that in mind, my general recommendation when implementing GetHashCode for an object that happens to have any kind of collection as one or more of its members is to simply ignore them and calculate GetHashCode solely based on the other scalar members. This would work pretty well except if you put into a hash table a huge number of objects where all their scalar members have identical values, resulting in identical hash codes.

Ignoring collection members when calculating the hash code can also yield a performance improvement, despite the decreased distribution of the hash code values. Remember that using a hash code is supposed to improve performance in a hash table by not requiring to call Equals N times, and instead will only require calling GetHashCode once and a quick hash table lookup. If each object has an inner array with 10,000 items which all participate in the calculation of the hash code, any benefits gained by the good distribution would probably be lost. It would be better to have a marginally less distributed hash code if generating it is considerably less costly.

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public override int GetHashCode() {
   return Id.GetHashCode() ^ Values.GetHashCode();  
}

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There are several good points in the comments and other answers. The OP should consider whether the Values would be used as part of the "key" if the object were used as a key in a dictionary. If so, then they should be part of the hash code, otherwise, not.

On the other hand, I'm not sure why the GetHashCode method should mirror SequenceEqual. It's meant to compute an index into a hash table, not to be the complete determinant of equality. If there are many hash table collisions using the algorithm above, and if they differ in the sequence of the Values, then an algorithm should be chosen that takes sequence into account. If sequence doesn't really matter, save the time and don't take it into account.

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I would do it this way:

long result = Id.GetHashCode();
foreach(T val in Values)
    result ^= val.GetHashCode();
return result;

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I know this thread is pretty old, but I wrote this method to allow me to calculate hashcodes of multiple objects. It's been very helpful for this very case. It's not perfect, but it does meet my needs and most likely yours too.

I can't really take any credit for it. I got the concept from some of the .net gethashcode implementations. I'm using 419 (afterall, it's my favorite large prime), but you can choose just about any reasonable prime (not too small . . . not too large).

So, here's how I get my hashcodes:

using System.Collections.Generic;
using System.Linq;

public static class HashCodeCalculator
{
    public static int CalculateHashCode(params object[] args)
    {
        return args.CalculateHashCode();
    }

    public static int CalculateHashCode(this IEnumerable<object> args)
    {
        if (args == null)
            return new object().GetHashCode();

        unchecked
        {
            return args.Aggregate(0, (current, next) => (current*419) ^ (next ?? new object()).GetHashCode());
        }
    }
}

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Provided that Id and Values will never change, and Values is not null...

public override int GetHashCode()
{
  return Id ^ Values.GetHashCode();
}

Note that your class is not immutable, since anyone can modify the contents of Values because it is an array. Given that, I wouldn't try to generate a hashcode using its contents.

ReferenceURL : https://stackoverflow.com/questions/638761/gethashcode-override-of-object-containing-generic-array