Retrieving the underlying dataitems of selected gridview rows

Question

I have a button on the page and a gridview with checkboxes on the first column. The gridview is bound to a List. On buttonClick, I need to retrieve the underlying bound data item (MyEntity) for each selected row. How can I do this? I can't simply recreate MyEntity based on the gridview columns, because I am not displaying all the fields of MyEntity. I need a way to get a hold of MyEntity based on the key (MyEntityId).