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

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)):

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.

It's actually very hard to implement GetHashCode() correctly because, in addition to the rules Marc already mentioned, the hash code should not change during the lifetime of an object. Therefore the fields which are used to calculate the hash code must be immutable.

I finally found a solution to this problem when I was working with NHibernate. My approach is to calculate the hash code from the ID of the object. The ID can only be set though the constructor so if you want to change the ID, which is very unlikely, you have to create a new object which has a new ID and therefore a new hash code. This approach works best with GUIDs because you can provide a parameterless constructor which randomly generates an ID.

By overriding Equals you're basically stating that you are the one who knows better how to compare two instances of a given type, so you're likely to be the best candidate to provide the best hash code.

This is an example of how ReSharper writes a GetHashCode() function for you:

public override int GetHashCode()
{
    unchecked
    {
        var result = 0;
        result = (result * 397) ^ m_someVar1;
        result = (result * 397) ^ m_someVar2;
        result = (result * 397) ^ m_someVar3;
        result = (result * 397) ^ m_someVar4;
        return result;
    }
}

As you can see it just tries to guess a good hash code based on all the fields in the class, but since you know your object's domain or value ranges you could still provide a better one.