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C# 11 – File Scoped Types

One of the new features in C# 11 is File Scoped Types also known as File Local Types. Although it sounds kindof familiar to File Scoped Namespaces introduced in C# 10, it is completely unrelated.

As the name implies File Scoped types restrict the usage of a type to the current file.

The file modifier

To use this feature a new modifier was introduced; file. Applying this modifier to any type definition restrict its usage to the current file.

In the screenshot above you can see that I created a File class in both the FileReader.cs file and FileWriter.cs file without any issues.

If I try to use this File class outside these 2 files, I get an error:

Good to know:

  • The file modifier can be applied to class, interface, record, struct, enum, delegate.
  • It cannot be combined with another modifier like internal or public.

When should I use this feature?

There are a few use cases I can think about where I see this feature useful:

  • Code generation: Code generation is one of the places where naming collisions can easily occur. We typically solve this by using different namespaces With the file keyword it can make code generation a lot easier.
  • Nested classes: One way I use to avoid naming collisions is through private nested classes. Here the file keyword can be a cleaner alternative.

 

What is happening behind the scenes?

To have  a look at what is happening behind the scenes, I opened the compiled code in dotPeek:

As you can see above, the C# compiler renames the types to avoid naming collisions.

Nice!

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