Skip to main content

Unlocking Collection Expressions for your own types

C# 12 introduced collection expressions, a new, simplified syntax for initializing collections:

This syntax works great with built-in collection types, but what about your own custom collections?

That's where CollectionBuilderAttribute comes in, allowing you to extend this modern syntax to your own types.

Custom Collections left behind

Imagine you've built a custom immutable collection type:

Before CollectionBuilderAttribute, you couldn't use the new collection expression syntax with this type. You'd be stuck with the old way:

Not exactly elegant compared to the modern syntax.

Enter CollectionBuilderAttribute

The CollectionBuilderAttribute bridges this gap by telling the compiler how to construct your collection from a collection expression. Here's an example:

Now you can write:

Beautiful! The compiler automatically calls your Create method with the items.

How it works

The attribute takes two parameters:

  1. Builder Type: A type containing the factory method (typeof(ImmutableStackBuilder))
  2. Method Name: The name of the static method that creates instances ("Create")

The builder method must:

  • Be static
  • Accept either ReadOnlySpan<T> (preferred) or T[]
  • Return an instance of your collection type
  • Have generic type parameters matching your collection

Remark: it is also necessary that the collection is an "iteration type", i.e. it should have a GetEnumerator() method that returns an IEnumerator (or IEnumerator<T>). The typical way to achieve this is by implementing the IEnumerable or IEnumerable<T> interface.

When all these things are in place, the compiler can generate the necessary code to construct your collection in the most efficient way.

More information

Collection expressions (Collection literals) - C# reference | Microsoft Learn

Adding support for collection expressions to yor own types

CollectionBuilderAttribute Class (System.Runtime.CompilerServices) | Microsoft Learn

Labels:

Popular posts from this blog

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

.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

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