Snippet to create basic Unity UI button from script, using built in resources

builtinButton

// create new button...
var o = new GameObject( id, typeof( Image ), typeof( Button ) );
o.transform.SetParent( parent );

var image = o.GetComponent<Image>();
image.sprite =
        AssetDatabase.LoadAllAssetsAtPath( "Resources/unity_builtin_extra" ).OfType<Sprite>().SingleOrDefault(
                                                                                                              sprite => sprite.name.Equals( "UISprite", StringComparison.OrdinalIgnoreCase ) );
image.type = Image.Type.Sliced;