public class QueryFilter 

{
  await QueuedTask.Run(() =>
  {
    using (Geodatabase geodatabase = new Geodatabase(new DatabaseConnectionFile(new Uri("path\\to\\sde\\file\\sdefile.sde"))))
    using (Table table = geodatabase.OpenDataset<Table>("WaterPipeInfo"))
    using (TableDefinition tableDefinition = table.GetDefinition())
    {
      bool supportsFullText = geodatabase.GetDatastoreProperties().SupportsFullTextIndex;
      if (!supportsFullText)
      {
        return;
      }

      IReadOnlyList<Index> indexes = tableDefinition.GetIndexes();
      IReadOnlyList<Field> fieldsToSearch = indexes.First().GetFields();

      FullTextSearchTermExpression searchRepairStatus = new FullTextSearchTermExpression()
      {
        SearchFields = fieldsToSearch[0].Name,
        SearchTerm = "Repaired",
        SearchType = FullTextSearchType.Simple
      };

      FullTextSearchTermExpression searchPipeMaterial = new FullTextSearchTermExpression()
      {
        SearchFields = fieldsToSearch[1].Name,
        SearchTerm = "Copper",
        SearchType = FullTextSearchType.Simple
      };

      FullTextOrExpression repairedOrCopperEx = new FullTextOrExpression(searchRepairStatus, searchPipeMaterial);

      QueryFilter queryfilter = new QueryFilter
      {
        FullTextExpression = repairedOrCopperEx,
        Offset = 1,
        RowCount = 20,
        PostfixClause = "Order By Size"
      };

      using (RowCursor rowCursor = table.Search(queryfilter, false))
      {
        // Use row cursor ...
      }
    }
  });
}

{
    QueryFilter qf = new QueryFilter()
    {
        WhereClause = "Class = 'city'"
    };
    int count = 0;
    using (RowCursor rows = selectedLayer.Search(qf)) //execute
    {
        //Looping through to count
        while (rows.MoveNext()) count++;
    }
    System.Diagnostics.Debug.WriteLine(String.Format(
       "Total features that matched the search criteria: {0}", count));
}