Reset field code not working when using a form inside a content place holder

hi all i am using this code to reset all my textboxes and Dropdowmlists after the button is clicked:
void resetField(object myObj)
    {
        //RadioButton rl;
        string temp = myObj.GetType().ToString();
        switch (myObj.GetType().ToString())
        {
            case "System.Web.UI.WebControls.TextBox":
                ((TextBox)myObj).Text = "";
                break;
          
            case "System.Web.UI.WebControls.DropDow ...