Why is it important to override GetHashCode when Equals method is overridden?
c#, hashcode, overriding
Solution
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`.
A demonstration of what happens when you get this wrong is here.
Problem
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 `Foo`s table. Which is the preferred method for overriding the `GetHashCode`? Why is it important to override `GetHashCode`?