public IEnumerable<Item> Search( 
   string searchString
)

{
  string openProjectPath = @"D\DATA\IGPHistoryItemTestProject\IGPHistoryItemTestProject.aprx";
  Project.OpenAsync(openProjectPath);
  MapProjectItem mapProjItem = Project.Current.GetItems<MapProjectItem>().FirstOrDefault(item => item.Name.Equals("Map", StringComparison.CurrentCultureIgnoreCase));

  // Note: Needs QueuedTask to run
  var map = mapProjItem.GetMap();
  var ftrLayer = map.Layers[0] as FeatureLayer;
  string tool1 = "management.GetCount";
  var args1 = Geoprocessing.MakeValueArray(ftrLayer);
  var env = Geoprocessing.MakeEnvironmentArray(overwriteoutput: true);
  GPExecuteToolFlags executeFlags = GPExecuteToolFlags.AddToHistory;
  var t = Geoprocessing.ExecuteToolAsync(tool1, args1, env, null, null, executeFlags);

  IEnumerable<IGPHistoryItem> hisItems = Project.Current.GetProjectItemContainer(Geoprocessing.HistoryContainerKey) as IEnumerable<IGPHistoryItem>;

  string hitemID = "";
  string hitemToolPath = "";
  IGPResult hitemGPResult = null;
  DateTime hitemTimeStamp;

  foreach (var hitem in hisItems)
  {
    // common IGPHistoryItem and Item properties
    hitemID = (hitem as Item).ID;
    hitemToolPath = hitem.ToolPath;
    hitemGPResult = hitem.GPResult;
    hitemTimeStamp = hitem.TimeStamp;

    // use the properties as needed
  }
}