{"id":10341,"date":"2023-12-10T16:13:45","date_gmt":"2023-12-10T16:13:45","guid":{"rendered":"https:\/\/putridparrot.com\/blog\/?p=10341"},"modified":"2023-12-10T16:13:45","modified_gmt":"2023-12-10T16:13:45","slug":"collection-expressions-in-c-12","status":"publish","type":"post","link":"https:\/\/putridparrot.com\/blog\/collection-expressions-in-c-12\/","title":{"rendered":"Collection Expressions in C# 12"},"content":{"rendered":"

C# 12 includes something called Collection Expressions<\/em>. These offer more generic way to create our collections from array-like syntax.<\/p>\n

Let’s look first at the old style creation of an array of integers<\/p>\n

\r\nvar array = new [] { 1, 2, 3 };\r\n<\/pre>\n
This is simple enough array<\/em> is of type int[]?<\/em>. This way of creation arrays is not going away, but what if we want to change the array<\/em> to a different collection then we end up using collection initializers like this<\/p>\n
\r\nvar list = new List<int> { 1, 2, 3 };\r\n<\/pre>\n
There’s nothing much wrong with this, but essentially we’re sort of doing the same thing, just with different syntax.<\/p>\n
Collection expressions now allow us to use syntax such as (below) to create our collection regardless of type<\/p>\n
\r\nint[] array = [1, 2, 3 ];\r\nList<int> list = [1, 2, 3 ];\r\n<\/pre>\n
On the surface this may not seem a big deal, but imagine you’ve a class that accepts an int[]<\/em> and maybe you change the type to a List<\/em>, passing the values via the collection expression [] syntax means that part of your code remains unchanged, it just remains as [1, 2, 3].<\/p>\n
Along with this we get to use the spread operator .. for example<\/p>\n
\r\nList<int> list = [1, 2, 3 ];\r\nint[] array = [.. list];\r\n<\/pre>\n
In this example we’ve created a list then basically copied the items to the array, but a spread operator can be used to concatenate values (or collections), such as<\/p>\n
\r\nint[] array = [-3, -2, -1, 0, .. list];\r\n<\/pre>\n
Creating your own collections to use collection expressions<\/strong><\/p>\n
For many of the original types, such as List<T> the collection expression code is built in. But newer collections and, if we want, our own collection can take advantage of this syntax by following a minimal set of rules.<\/p>\n
All we need to do is create our collection type and add the CollectionBuilderAttribute to it like this<\/p>\n
\r\n[CollectionBuilder(typeof(MyCollection), nameof(MyCollection.Create))]\r\npublic class MyCollection<T>\r\n{\r\n   \/\/ our code\r\n}\r\n<\/pre>\n
Now this is not going to work, the typeof expects a non-generic type, so we create a simple non-generic version of this class to handle the creation of the generic version. Also notice the CollectionBuilder expects the name of the method to call and expects a method that takes a single parameter of type ReadOnlySpan<\/em> and returns the collection type, now initialized, like this<\/p>\n
\r\npublic class MyCollection\r\n{\r\n  public static MyCollection<T> Create<T>(ReadOnlySpan<T> items)\r\n  {\r\n     \/\/ returns a MyCollection<T>\r\n  }\r\n}\r\n<\/pre>\n
Let’s look at potential bare minimum implementation of this collection type which can be used with the collection expression syntax. Notice we will also need to implement IEnumerable and\/or IEnumerable<T> <\/p>\n
\r\n[CollectionBuilder(typeof(MyCollection), nameof(MyCollection.Create))]\r\npublic class MyCollection<T> : IEnumerable<T>\r\n{\r\n  public static readonly MyCollection<T> Empty = new(Array.Empty<T>());\r\n\r\n  private readonly List<T> _innerCollection;\r\n\r\n  internal MyCollection(T[]? items)\r\n  {\r\n    _innerCollection = items == null ? new List<T>() : [..items];\r\n  }\r\n\r\n  public T this[int index] => _innerCollection[index];\r\n  public IEnumerator<T> GetEnumerator() => _innerCollection.GetEnumerator();\r\n  IEnumerator IEnumerable.GetEnumerator() => _innerCollection.GetEnumerator();\r\n}\r\n\r\npublic class MyCollection\r\n{\r\n  public static MyCollection<T> Create<T>(ReadOnlySpan<T> items)\r\n  {\r\n    return items.IsEmpty ? \r\n      MyCollection<T>.Empty : \r\n      new MyCollection<T>(items.ToArray());\r\n  }\r\n}\r\n<\/pre>\n
Ofcourse this is a silly example as we’re not adding anything that the inner List<T><\/em> cannot supply, but you get the idea. Now we can use the collection expression syntax on our new collection type<\/p>\n
\r\nMyCollection<int> collection = [1, 2, 6, 7];\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
C# 12 includes something called Collection Expressions. These offer more generic way to create our collections from array-like syntax. Let’s look first at the old style creation of an array of integers var array = new [] { 1, 2, 3 }; This is simple enough array is of type int[]?. This way of creation […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[3],"tags":[],"class_list":["post-10341","post","type-post","status-publish","format-standard","hentry","category-c"],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/10341","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/comments?post=10341"}],"version-history":[{"count":2,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/10341\/revisions"}],"predecessor-version":[{"id":10343,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/10341\/revisions\/10343"}],"wp:attachment":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/media?parent=10341"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/categories?post=10341"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/tags?post=10341"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}