Overriding GetHashCode()

Question

In this article, Jon Skeet mentioned that he usually uses this kind of algorithm for overriding GetHashCode().

public override int GetHashCode()
{
          


        

Answer 1

I personally tend to return a different numeric value for each implementation of GetHashCode() in a class which has no immutable fields. This means if I have a dictionary containing different implementing types, there is a chance the different instances of different types will be put in different buckets.

For example

public class A
{
    // TODO Equals override

    public override int GetHashCode()
    {
        return 21313;
    } 
}

public class B
{
    // TODO Equals override

    public override int GetHashCode()
    {
        return 35507;
    } 
}

Then if I have a Dictionary containing instances of A, B and other types , the performance of the lookup will be better than if all the implementations of GetHashCode returned the same numeric value.

It should also be noted that I make use of prime numbers to get a better distribution.

As per the comments I have provided a LINQPad sample here which demonstrates the performance difference between using return 1 for different types and returning a different value for each type.