C# keyboard Enter key replaces Tab key implementation code

private void SetEnt()
        {
            foreach (Control txt in (("ContentPlaceHolderEntity")).Controls)
            {
                if (txt is )
                {
                    if (((TextBox)txt).CssClass != "erp_textarea")
                        ((TextBox)txt).("onkeydown", "if (==13) =9");
                }
                if (txt is )
                {
                    ((DropDownList)txt).("onkeydown", "if (==13) =9");
                }
                if (txt is )
                {
                    ((CheckBox)txt).("onkeydown", "if (==13) =9");
                }
            }
        }