using (var fileStream = File.OpenRead("path/to/file.osm.pbf"))
{
  // create source stream.
  var source = new PBFOsmStreamSource(fileStream);
  
  // filter all powerlines and keep all nodes.
  var filtered = from osmGeo in source
   where osmGeo.Type == OsmSharp.OsmGeoType.Node || 
    (osmGeo.Type == OsmSharp.OsmGeoType.Way && osmGeo.Tags != null && osmGeo.Tags.Contains("power", "line"))
   select osmGeo;

  // convert to a feature stream.
  // WARNING: nodes that are partof powerlines will be kept in-memory.
  //          it's important to filter only the objects you need **before** 
  //          you convert to a feature stream otherwise all objects will 
  //          be kept in-memory.
 var features = filtered.ToFeatureSource();

  // filter out only linestrings.
  var lineStrings = from feature in features
    where feature.Geometry is LineString
    select feature;
}