David Basarab David Basarab - 2 months ago 13
C# Question

Why is it important to override GetHashCode when Equals method is overridden?

Given the following class

public class Foo
{
public int FooId { get; set; }
public string FooName { get; set; }

public override bool Equals(object obj)
{
Foo fooItem = obj as Foo;

return fooItem.FooId == this.FooId;
}

public override int GetHashCode()
{
// Which is preferred?

return base.GetHashCode();

//return this.FooId.GetHashCode();
}
}


I have overridden the
Equals
method because
Foo
represent a row for the
Foo
s table. Which is the preferred method for overriding the
GetHashCode
?

Why is it important to override
GetHashCode
?

Answer

Yes, it is important if your item will be used as a key in a dictionary, or HashSet<T>, etc - since this is used (in the absence of a custom IEqualityComparer<T>) to group items into buckets. If the hash-code for two items does not match, they may never be considered equal (Equals will simply never be called).

The GetHashCode() method should reflect the Equals logic; the rules are:

  • if two things are equal (Equals(...) == true) then they must return the same value for GetHashCode()
  • if the GetHashCode() is equal, it is not necessary for them to be the same; this is a collision, and Equals will be called to see if it is a real equality or not.

In this case, it looks like "return FooId;" is a suitable GetHashCode() implementation. If you are testing multiple properties, it is common to combine them using code like below, to reduce diagonal collisions (i.e. so that new Foo(3,5) has a different hash-code to new Foo(5,3)):

int hash = 13;
hash = (hash * 7) + field1.GetHashCode();
hash = (hash * 7) + field2.GetHashCode();
...
return hash;

Oh - for convenience, you might also consider providing == and != operators when overriding Equals and GetHashCode.


A demonstration of what happens when you get this wrong is here.

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