ESRI.ArcGIS.Framework.IApplication app = ArcMap.Application;
ESRI.ArcGIS.Framework.IDocument doc = app.Document;
ESRI.ArcGIS.ArcMapUI.IMxDocument mxd = doc as ESRI.ArcGIS.ArcMapUI.IMxDocument;
ESRI.ArcGIS.Carto.IMap map = mxd.FocusMap;
ESRI.ArcGIS.Carto.ILayer lay = map.Layer[0];
ESRI.ArcGIS.Carto.IFeatureLayer flay = lay as ESRI.ArcGIS.Carto.IFeatureLayer;
ESRI.ArcGIS.Geodatabase.IFeatureClass fcls = flay as ESRI.ArcGIS.Geodatabase.IFeatureClass;
ESRI.ArcGIS.Geodatabase.IFieldEdit fld = new ESRI.ArcGIS.Geodatabase.FieldClass();
string nam = "map";
fld.AliasName_2 = nam;
fld.Type_2 = ESRI.ArcGIS.Geodatabase.esriFieldType.esriFieldTypeString;
fld.Length_2 = 50;
fld.Name_2 = "country name";
fcls.AddField(fld);

ESRI.ArcGIS.Geodatabase.IFeatureClass fcls = flay as ESRI.ArcGIS.Geodatabase.IFeatureClass;

ESRI.ArcGIS.Geodatabase.IFeatureClass fcls = flay.FeatureClass;

fld.Name_2 = "country name";