public IReadOnlyList<Field> GetFields()

{
  //Url examples for (federated) feature services
  //(by "ref" online):
  var url =
      @"https://sampleserver6.arcgisonline.com/arcgis/rest/services/SF311/FeatureServer";
  //(federated by ref on portal)
  //https://portal.example.com/server/rest/services/FeatureServiceName/FeatureServer");
  //(federated by value - Hosted - on portal)
  //https://portal.example.com/server/rest/services/Hosted/FeatureServiceName/FeatureServer");

  await QueuedTask.Run(() =>
  {
    var uri = new Uri(url, UriKind.Absolute);
    using (var fs_db = new ArcGIS.Core.Data.Geodatabase(new ServiceConnectionProperties(uri)))
    {
      //Assuming that id 0 is a FeatureClass for the given service
      FeatureClassDefinition featureClassDefinition =
          fs_db.GetDefinition<FeatureClassDefinition>("0");
      string shapeField = featureClassDefinition.GetShapeField();
      IReadOnlyList<Field> fields = featureClassDefinition.GetFields();

      //Assuming that id 1 is a Table for the given service
      TableDefinition tableDefinition =
            fs_db.GetDefinition<TableDefinition>("1");
      string objectIDField = tableDefinition.GetObjectIDField();
    }
  });
}

{
  //Must be called within QueuedTask.Run()
  RowCursor SortWorldCities(FeatureClass worldCitiesTable)
  {
    using (FeatureClassDefinition featureClassDefinition = worldCitiesTable.GetDefinition())
    {
      Field countryField = featureClassDefinition.GetFields()
        .First(x => x.Name.Equals("COUNTRY_NAME"));

      Field cityNameField = featureClassDefinition.GetFields()
        .First(x => x.Name.Equals("CITY_NAME"));

      // Create SortDescription for Country field
      SortDescription countrySortDescription = new SortDescription(countryField);
      countrySortDescription.CaseSensitivity = CaseSensitivity.Insensitive;
      countrySortDescription.SortOrder = SortOrder.Ascending;

      // Create SortDescription for City field
      SortDescription citySortDescription = new SortDescription(cityNameField);
      citySortDescription.CaseSensitivity = CaseSensitivity.Insensitive;
      citySortDescription.SortOrder = SortOrder.Ascending;

      // Create our TableSortDescription
      TableSortDescription tableSortDescription = new TableSortDescription(
        new List<SortDescription>() { countrySortDescription, citySortDescription });

      return worldCitiesTable.Sort(tableSortDescription);
    }
  }
}

{
  // Must be called within QueuedTask.Run
  void RemoveFieldTableSnippet(Geodatabase geodatabase)
  {
    // Removing all fields from 'Parcels' table except following 
    // Tax_Code
    // Parcel_Address

    // The table to remove fields
    string tableName = "Parcels";

    TableDefinition tableDefinition = geodatabase.GetDefinition<TableDefinition>(tableName);
    IReadOnlyList<Field> fields = tableDefinition.GetFields();

    // Existing fields from 'Parcels' table
    Field taxCodeField = fields.First(f => f.Name.Equals("Tax_Code"));
    Field parcelAddressField = fields.First(f => f.Name.Equals("Parcel_Address"));

    FieldDescription taxFieldDescription = new FieldDescription(taxCodeField);
    FieldDescription parcelAddressFieldDescription = new FieldDescription(parcelAddressField);

    // Fields to retain in modified table
    List<FieldDescription> fieldsToBeRetained = new List<FieldDescription>()
      {
        taxFieldDescription, parcelAddressFieldDescription
      };

    // New description of the 'Parcels' table with the 'Tax_Code' and 'Parcel_Address' fields
    TableDescription modifiedTableDescription = new TableDescription(tableName, fieldsToBeRetained);

    SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

    // Remove all fields except the 'Tax_Code' and 'Parcel_Address' fields
    schemaBuilder.Modify(modifiedTableDescription);
    schemaBuilder.Build();
  }
}