private string _StyleClass;
public string StyleClass
{
  get { return _StyleClass; }
  set { _StyleClass = value; }
}
private void StyleForm()
{
  HtmlForm Form1 = this.FindControl("form1") as HtmlForm;
  Form1.Attributes.Add("class", _StyleClass);
}
protected override void OnPreRender(EventArgs e)
{
  //控件呈现调用方法
  StyleForm();
  base.OnPreRender(e);
}