Question

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;          if (fooItem == null)          {            return false;         }          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 Foos table. Which is the preferred method for overriding the GetHashCode?

Why is it important to override GetHashCode?

Answer 1

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)):

In modern frameworks, the HashCode type has methods to help you create a hashcode from multiple values; on older frameworks, you’d need to go without, so something like:

unchecked // only needed if you're compiling with arithmetic checks enabled { // (the default compiler behaviour is *disabled*, so most folks won't need this)     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.

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