While importing (drag/drop) your webpart to any SharePoint page, if you receive an error stating “Cannot import webpart” or “unable to add selected web part(s)”! here is the check list you should look for:
Make sure the webpart class is inherited either from Microsoft.SharePoint.WebPartPages.WebPart or System.Web.UI.WebControls.WebParts.WebPart
The webpart class needs to be declared public.
Safe controls entry [...]