IExternalCommand

public Autodesk.Revit.UI.Result Execute(ExternalCommandData commandData,
    ref string message, ElementSet elements)
{
    try
    {
        Document doc = commandData.Application.ActiveUIDocument.Document;
        UIDocument uidoc = commandData.Application.ActiveUIDocument;
        // Delete selected elements

        ICollection<Autodesk.Revit.DB.ElementId> ids =
            doc.Delete(uidoc.Selection.GetElementIds());

        TaskDialog taskDialog = new TaskDialog("Revit"); 
        taskDialog.MainContent = 
           ("Click Yes to return Succeeded. Selected members will be deleted.\n" +
           "Click No to return Failed.  Selected members will not be deleted.\n" +
           "Click Cancel to return Cancelled.  Selected members will not be deleted.");
        TaskDialogCommonButtons buttons = TaskDialogCommonButtons.Yes | 
            TaskDialogCommonButtons.No | TaskDialogCommonButtons.Cancel;
        taskDialog.CommonButtons = buttons;
        TaskDialogResult taskDialogResult = taskDialog.Show();

        if (taskDialogResult == TaskDialogResult.Yes)
        {
            return Autodesk.Revit.UI.Result.Succeeded;
        }
        else if (taskDialogResult == TaskDialogResult.No)
        {
            ICollection<ElementId> selectedElementIds = uidoc.Selection.GetElementIds();
            foreach (ElementId id in selectedElementIds)
            {
                elements.Insert( doc.GetElement(id) );
            }
            message = "Failed to delete selection.";
            return Autodesk.Revit.UI.Result.Failed;
        }
        else
        {
            return Autodesk.Revit.UI.Result.Cancelled;
        }
    }
    catch
    {
        message = "Unexpected Exception thrown.";
        return Autodesk.Revit.UI.Result.Failed;
    }

}