Unscary Code

I found a better approach, one that isn't as scary ...

builder.Define(
    mEntryColumn,
    aEditor,
    (column, editor) => column.AddNumericField(editor.Format)
                            .BoundTo(editor, e => e.Value)
                            .FocusBoundTo(editor, e => e.Focused));

The key difference is one of approach - instead of trying to subscribe to an arbitary event (which is difficult since you can't pass events around as data, only delegates), I went with subscribing to an arbitary property and keeping the gnarly bits under the covers.