One of the more brilliant innovations in C# 12 was collection expressions.

Typically, initializing a collection required code specific to that expression. Take the following examples:

// Initialize a list
List<string> namesList = new List<string>() { "Brenda", "Latisha", "Linda", "Felicia" };
// Initialize an array
string[] nameArray = new string[] { "Brenda", "Latisha", "Linda", "Felicia" };
// Initialize a HashSet
HashSet<string> nameHashSet = new HashSet<string> { "Brenda", "Latisha", "Linda", "Felicia" };

You can simplify the code for the List and the HashSet as follows:

// Initialize a list
List<string> namesList = new() { "Brenda", "Latisha", "Linda", "Felicia" };
// Initialize a HashSet
HashSet<string> nameHashSet = new() { "Brenda", "Latisha", "Linda", "Felicia" };

Unfortunately, for a string array, it cannot get any simpler.

This problem is solved with collection expressions.

This means there is a universal syntax for initializing collections.

// Initialize a list
List<string> namesList = ["Brenda", "Latisha", "Linda", "Felicia"];
// Initialize an array
string[] nameArray = ["Brenda", "Latisha", "Linda", "Felicia"];
// Initialize a HashSet
HashSet<string> nameHashSet = ["Brenda", "Latisha", "Linda", "Felicia"];

You can see here that it does not matter the type of collection; initialization is the same.

The problem with this syntax is that there are times you need to initialize some properties on the collection.

For example, with a List, you can set its size in advance to prevent the runtime from having to recreate the object as it grows.

var names = new List<string>(40);

Here we want our List to have a preallocated size of 40.

It has not been possible to do this with collection expressions.

This is now possible in .NET 11.

Your code would look like this:

List<string> namesList = [with(capacity: 4), "Brenda", "Latisha", "Linda", "Felicia"];

If you are initializing the collection from another collection, your code would look like this:

string[] otherCollection = ["Brenda", "Latisha", "Linda", "Felicia"];
List<string> namesList = [with(capacity: 4), .. otherCollection];

Important: This is a preview language feature, and to get it to compile, you must include the following in your .csproj file.

<LangVersion>preview</LangVersion>

It should now look like this:

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net11.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
        <LangVersion>preview</LangVersion>
    </PropertyGroup>
</Project>

TLDR

You can pass parameters to collections during initialization using the [with] keyword.

The code is in my GitHub.

Happy hacking!