Global Using Directives in C# allows you to specify namespaces that are automatically available across all files in your project, reducing the need to repeat using statements in every file.

It is introduced in C#10 and is particularly helpful for large projects by improving code maintainability and readability

What is Global?

General, global refers to something applicable or relevant universally, across all instances, contexts, or locations. Global can have different meanings depending on the context.

The Global Modifier

C# 10 introduces the new global using directive language feature so that you don't have to repeat the same using directives in hundreds of files.

You turn a regular directive into a global one by adding a global variable. Example:

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global using System;

By doing so on a single source file, the effect is similar to setting up with System; The instructions are located at the top of each file in your project.

How to Use Global Using Directives

Create a global using Statement: 

To create a global directive, you simply add the global keyword before the using directive in any .cs file.

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global using System;
global using System.Collections.Generic;

Once declared, you do not need to add using System; or use System.Collections.Generic; in each file where you use those namespaces

Where to Place Global Usings

It's good to create a common dedicated file (such as GlobalUsings.cs) to keep them organized. But you can also place global using statements in any file.

The file can be located at the root of your project or within a folder, and the global usings will apply to the entire project regardless of the file's location. For example:

GlobalUsings.cs file:

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// GlobalUsings.cs
global using System;
global using System.Linq;
global using System.Threading.Tasks;

Global operations in .csproj files

You can also add global operations to a .csproj file by setting the ImplicitUsings property to enabled. This will allow you to add common directives for you (such as System, System.Linq, etc.).

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<PropertyGroup>
    <ImplicitUsings>enable</ImplicitUsings>
</PropertyGroup>

Scoped Global

If you want to define global usings but limit their scope to a particular project or assembly, make sure that declared in a project-specific file, such as within a shared project.

Example

In this example, global using ensures that System and System.Collections.Generic are available throughout the project without the need to write them in every file.

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// GlobalUsings.cs file
global using System;
global using System.Collections.Generic;

// Program.cs file
class Program
{
    static void Main(string[] args)
    {
        List<string> names = new List<string> { "Alice", "Bob" };
        Console.WriteLine(string.Join(", ", names)); // Uses System and Collections.Generic
    }
}

In summary, global using directives provides an efficient way to manage common namespaces and keep your code clean.