How to use a custom generic type as a key in a Dictionary?

Question

I want to use a custom generic class as a key in a dictionary. How should I override Equals and GetHashCode?

Eg,

public class SomeKey<T,V>
{
    public T Value1 { get; set; }
    public V Value2 { get; set; }

    public SomeKey(T val1, V val2)
    {
        this.Value1 = val1;
        this.Value2 = val2;
    }

    public override bool Equals(SomeKey<T,V> otherKey)
    {
        //whats the best option here?
    }

    public override int GetHashCode()
    {
        //whats the best option here?
    }
}

Thanks

score 5 · Accepted Answer · edited May 23 '17 at 12:12

5

Equality is simple: test for Value1 and Value2 being equal.

For the hash code the simplest approach is to use xor to combine the hash codes from Value1 and Value2.

public override bool Equals(SomeKey<T,V> otherKey)
{
    return Value1.Equals(otherKey.Value1) && Value2.Equals(otherKey.Value2);
}

public override int GetHashCode()
{
    return Value1.GetHashCode() ^ Value2.GetHashCode();
}

There are lots of alternative ways of calculating the hash code. If performance is a bottleneck then you should consider something more tailored than xor.

For example, Stack Overflow offers these links (and many more):

edited May 23 '17 at 12:12

Community

1
1

answered Apr 30 '11 at 13:55

David Heffernan

601,492
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sounds good. care to elaborate or give a link on why xor is a good idea? – Nicolas78 Apr 30 '11 at 13:57
Well, for the hash code the best choice depends very much on the distribution of values. xor is a pretty reliable, simple, approach that you can use if you know nothing about the distribution of values. If OP knows such information, then other choices may be appropriate. All this only matters if the hash code is a performance bottleneck. – David Heffernan Apr 30 '11 at 14:03
But if he creates two SomeKey with the same values, should them be threated the same? – Marino Šimić Apr 30 '11 at 14:07
@Marino they are treated the same. – David Heffernan Apr 30 '11 at 14:08
another comment on you latest addition, equality comparison == wont work on generic type definitions if there is no where constraint that says this is possible – Marino Šimić Apr 30 '11 at 14:11
@Marino I think I've fixed that now with `Equals()`. Is that right? – David Heffernan Apr 30 '11 at 14:12
Any particular reason you’re using `&` and not `&&`? – Timwi Apr 30 '11 at 15:16

score 1 · Answer 2 · answered Apr 30 '11 at 13:56

1

You should override it in a way that GetHashcode reliably returns the same value for the same object, and Equals to always return true for the same objects. The msdn has some advices for implementing GetHashcode (the remarks section).

answered Apr 30 '11 at 13:56

Femaref

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7
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176

How to use a custom generic type as a key in a Dictionary?

2 Answers2