help how to search in a listview from an text file

If you want to do this based on the listview items, one way is to loop through the listviewitems and their subitems. For example the following selects all the items where desired text is found in the item or in the subitem.

foreach(ListViewItem item in mylistview.Items) {
   if (item.Text.Contains("text to search")) {
      item.Selected = true;
   }
   foreach (ListViewItem.ListViewSubItem subitem in item.SubItems) {
      if (item.Text.Contains("text to search")) {
         item.Selected = true;
      }
   }
}