I would like to use the
SP.ListOperation.Selection class but I cannot import it in C#. I am building a SharePoint 2013 solution.
It is part of Client Side Object Model. If you are looking to get selected items in Server Side. Then you need to handle this in client side and then pass information using say a hidden input field.
For more refer this thread https://social.msdn.microsoft.com/Forums/sharepoint/en-US/e34e59eb-dc77-47e5-8f42-5d27b6ae925b/sharepoint-2010-get-id-of-selected-list-item-using-c?forum=sharepointdevelopmentprevious
You need to add the the microsoft.sharepoint.client dll as reference in your project and then import (using Microsoft.SharePoint), then you can use all the predefined SharePoint classed in your class without any issue.
Below link details all the classes that are available under the namespace.