Using directives simplify how you work with namespaces. C# 10 includes a new global using directive and implicit usings to reduce the number of usings you need to specify at the top of each file.
If the keyword global appears prior to a using directive, that using applies to the entire project.
You can use any feature of using within a global using directive. For example, adding static imports a type and makes the type’s members and nested types available throughout your project. If you use an alias in your using directive, that alias will also affect your entire project:
global using System;
global using static System.Console;
global using Env = System.Environment;
You can put global usings in any .cs file, including Program.cs or a specifically named file like globalusings.cs. The scope of global usings is the current compilation, which generally corresponds to the current project.
The Implicit usings feature automatically adds common global using directives for the type of project you are building. To enable implicit usings set the ImplicitUsings property in your .csproj file:
<!-- Other properties like OutputType and TargetFramework -->
For more information see this MSDN article.
Traditional using directives at the top of your files, global using directives, and implicit usings work well together. Implicit usings let you include the .NET namespaces appropriate to the kind of project you’re building with a single line in your project file. Global using directives let you include additional namespaces to make them available throughout your project. The using directives at the top of your code files let you include namespaces used by just a few files in your project.