In order to avoid collisions with other types (such as when generating code) you can prefix a type with the file keyword to indicate it can only be referenced from the file it is defined in.

Code

Now

file class CantBeSeenOutside
{
}

Before

internal class CantBeSeenOutside234456aze
{
}

Notes

• A file type can't appear in the signature of a member of a non-file type, as it would then be visible outside the file

More information

• Language reference