Menu
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?
410
If you're implementing a reference type, you should consider overriding the Equals method if your type looks like a base type, such as Point, String, BigNumber, and so on. Override the GetHashCode method to allow a type to work correctly in a hash table.
You must override hashCode() in every class that overrides equals(). Failure to do so will result in a violation of the general contract for Object. hashCode(), which will prevent your class from functioning properly in conjunction with all hash-based collections, including HashMap, HashSet, and Hashtable.
We can override the equals method in our class to check whether two objects have same data or not.
Only Override HashCode, Use the default Equals: Only the references to the same object will return true. In other words, those objects you expected to be equal will not be equal by calling the equals method. Only Override Equals, Use the default HashCode: There might be duplicates in the HashMap or HashSet.
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:
Equals(...) == true) then they must return the same value for GetHashCode() 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.
155
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
