What is namespace that includes: esriGeoDatabase.Fields ?

Hi,

Are there anyone know what is namespace that includes: esriGeoDatabase.Fields ?

I try to use below namspace, but not successful:

using ESRI.ArcGIS.Geodatabase;
using ESRI.ArcGIS.DataSourcesFile;
using ESRI.ArcGIS.esriSystem;
using ESRI.ArcGIS.Geometry;

Thanks and regards,