C# 12- Type Aliasing

During a code review I noticed a file I had not seen before in this application; a GlobalUsings.cs file. When opening the file I noticed it had a combination of global using statements and type aliases. Turns out that this is an emerging pattern emerge in .NET applications where developers are defining a GlobalUsings.cs file to encapsulate all (or most) using directives into a single file.

	global using static System.Math;
	global using Coordinates = (double Latitude, double Longitude);

view raw GlobalUsings.cs hosted with ❤ by GitHub

If you have no clue what  these 2 language features are, here is a short summary for you.

Global Usings

Global usings were introduced in C# 10 and allows you to declare a namespace once in your application and make it available everywhere. So if we want to use a specific namespace, we need to add the following line to any source file:

global using <fully-qualified-namespace>;

view raw Example.cs hosted with ❤ by GitHub

Remark: As I mentioned in the introduction, I would recommend to centralize these global usings in one file instead of spreading them out over multiple files in your codebase.

What we can also do is instead of declaring this inside a source file, is to include our global namespaces in our csproj file:

	<ItemGroup>
	<Using Include="Fully.Qualified.Namespace" />
	</ItemGroup>

view raw Example.csproj hosted with ❤ by GitHub

Type Aliasing for (almost) any type

The global usings feature can be nicely combined with type aliasing for any type. This feature allows you to specify aliases that map to other types. This includes tuple types, pointer types, array types, and even non-open generic types, all of which are then available in your code.

I especially like this feature when using value tuple types that should be used at multiple places inside a codebase. Being an F# developer, this gets a step closer to Type Aliasing and Type Abbrevations I like to use in F#.

Here is an example where I created a type alias for a value tuple:

global using Coordinates = (double Latitude, double Longitude);

view raw Coordinates.cs hosted with ❤ by GitHub

Now I can use this type alias for example as a return type for a method:

	static Coordinates GetRandomCoordinates(Random? random = default)
	{
	random ??= Random.Shared;
	double minLat = -90.0, maxLat = 90.0;
	double minLon = -180.0, maxLon = 180.0;
	// Generate random latitude and longitude
	double latitude = minLat + (maxLat - minLat) * random.NextDouble();
	double longitude = minLon + (maxLon - minLon) * random.NextDouble();
	return (
	Latitude: Round(latitude, 8),
	Longitude: Round(longitude, 8)
	);
	}

view raw GetCoordinates.cs hosted with ❤ by GitHub

Remark: Although this feature is supported for a lot of types, it doesn’t work for nullable reference types.

More information

GitHub - IEvangelist/alias-any-type: A demo app, intended to demonstrate "alias any type" from C# 12.

using directive - C# reference | Microsoft Learn

MSBuild properties for Microsoft.NET.Sdk - .NET | Microsoft Learn

Refactor your code using alias any type - .NET Blog (microsoft.com)

Type Abbreviations - F# | Microsoft Learn