打开文件对话框HelpRequest事件在哪里定义？

Question

private void btnOpenFileDialog_Click(object sender, System.EventArgs e)
{
OpenFileDialog dlg=new OpenFileDialog();
dlg.AddExtension=cbAddExtension.Checked;
dlg.CheckFileExists=cbCheckFileExists.Checked;
dlg.CheckPathExists=cbCheckPathExists.Checked;
dlg.Multiselect=cbMultiSelect.Checked;
dlg.ReadOnlyChecked=cbReadOnlyChecked.Checked;
dlg.RestoreDirectory=cbRestoreDirectory.Checked;
dlg.ShowHelp=cbShowHelp.Checked;
dlg.ShowReadOnly=cbShowReadOnly.Checked;
dlg.ValidateNames=cbValidateNames.Checked;
dlg.DefaultExt=txtDefaultExt.Text;
dlg.FileName=txtFileName.Text;
dlg.HelpRequested += new System.Windows.Forms.HelpEventHandler(Form1.ShowOpenFileDialogHelp);
}
出错说 System.Windows.Forms.OpenFileDialog 并不包括 对HelpRequested 的定义 错误在dlg.HelpRequested 这里。