Skip to main content

GetHashCode() in .NET Core

If you ever had to implement the Equals() method to compare two instances of a type in .NET, you had to implement the GetHashCode() method too.

The GetHashCode method returns a numeric value which is used to identify an object during equality testing. It can also serve as an index for an object in a collection. The purpose of the method is to create a key for hashtable.

It is by design useful for only one thing: putting an object in a hash table.

It is faster to use the return value of GetHashCode to determine whether two objects are equal than to call the default implementation of Equals on the object type.

In other words, GetHashCode is used to generate a unique identifier for an object that can be used to compare it with other objects. It is used internally by the .NET framework for quick comparisons.

If you had to implement the GetHashCode method, there were some rules that should be followed which could make it quite a challenge to implement it correctly:

  • Equal items have equal hashes
  • The integer returned by GetHashCode must never change while the object is contained in a data structure that depends on the hash code remaining stable
  • GetHashCode must never throw an exception, and must return
  • GetHashCode must be performant

To follow all those rules we had a lot of magic going one. Here is an example from one of our applications:

We see the usage of the unchecked keyword to gain some extra performance by stopping overflow checks, magic numbers like 5 and 397. Most of the developers just copy this code without any clue why we are doing this.

Starting from .NET Core 2.1, we can leave all this magic to the framework itself and use the built-in HashCode struct:

No more magic!

Remark: If you are still working with pre .NET Core 2.1 code, you can use the Microsoft.Bcl.HashCode NuGet package.

More information

Guidelines and rules for GetHashCode | Fabulous adventures in coding (ericlippert.com)

Object.GetHashCode Method (System) | Microsoft Learn

HashCode Struct (System) | Microsoft Learn

NuGet Gallery | Microsoft.Bcl.HashCode 1.1.1

Labels:

Popular posts from this blog

.NET 8–Keyed/Named Services

A feature that a lot of IoC container libraries support but that was missing in the default DI container provided by Microsoft is the support for Keyed or Named Services. This feature allows you to register the same type multiple times using different names, allowing you to resolve a specific instance based on the circumstances. Although there is some controversy if supporting this feature is a good idea or not, it certainly can be handy. To support this feature a new interface IKeyedServiceProvider got introduced in .NET 8 providing 2 new methods on our ServiceProvider instance: object? GetKeyedService(Type serviceType, object? serviceKey); object GetRequiredKeyedService(Type serviceType, object? serviceKey); To use it, we need to register our service using one of the new extension methods: Resolving the service can be done either through the FromKeyedServices attribute: or by injecting the IKeyedServiceProvider interface and calling the GetRequiredKeyedServic...
Read more

Azure DevOps/ GitHub emoji

I’m really bad at remembering emoji’s. So here is cheat sheet with all emoji’s that can be used in tools that support the github emoji markdown markup: All credits go to rcaviers who created this list.
Read more

Kubernetes–Limit your environmental impact

Reducing the carbon footprint and CO2 emission of our (cloud) workloads, is a responsibility of all of us. If you are running a Kubernetes cluster, have a look at Kube-Green . kube-green is a simple Kubernetes operator that automatically shuts down (some of) your pods when you don't need them. A single pod produces about 11 Kg CO2eq per year( here the calculation). Reason enough to give it a try! Installing kube-green in your cluster The easiest way to install the operator in your cluster is through kubectl. We first need to install a cert-manager: kubectl apply -f https://github.com/cert-manager/cert-manager/releases/download/v1.14.5/cert-manager.yaml Remark: Wait a minute before you continue as it can take some time before the cert-manager is up & running inside your cluster. Now we can install the kube-green operator: kubectl apply -f https://github.com/kube-green/kube-green/releases/latest/download/kube-green.yaml Now in the namespace where we want t...
Read more