Kind of Open Source Licence

Question

Imagine this scenario: I have a web site where I offer some freeware applications for Windows that are not Open Source (but perhaps are going to be in the future). I'm going to upload a new application that has a plugin architecture, therefore I need to provide the source code for the plugin interfaces that are written in C#. Under which licence should I publish those interfaces? When the interfaces to the plugins are the only piece of source code you are giving away is a licence really necessary?