While the one class per file policy is strictly enforced in Java, it’s not required by C#. However, it’s generally a good idea.
I typically break this rule if I have a very small helper class that is only used by the main class, but I prefer to do that as a nested inner class for clarity’s sake.
You can however, split a single class into multiple files using the partial keyword. This is useful for separating your code from wizard-generated code.