C# 11 – file access modifier & file-scoped types

The file modifier restricts a type’s scope and visibility to the file in which it is declared. This is especially useful in situations where you want to avoid name collisions among types – like in the case of generated types using source generators.

A quick example:

file class MyLocalType { }

In terms of restrictions we have the following:

• types nested inside a file-scoped type will only be visible within the file in which they are declared
• other types in the assembly may use the same fully qualified name as the file-scoped type without creating a name collision
• file-local types can’t be used as the return type or parameter of any member that is more visible than the file scope
• similarly, a file-scoped type can’t be a field member of a type that is more visible than the file scope

On the other hand:

• A more visible type can implicitly implement a file-scoped interface
• A more visible type can also explicitly implement a file-scoped interface with the condition that explicit implementations can only be used within the file scope

Implementing a file-scoped interface implicitly

A public class can implement a file-scoped interface as long as they are defined in the same file. In the example below you have the file-scoped interface ICalculator which is implemented by a public class Calculator.

file interface ICalculator
{
    int Sum(int x, int y);
}

public class Calculator : ICalculator
{
    public int Sum(int x, int y) => x + y;
}