I am trying to add a field to first layer in the map here is my code

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";

It throws a NullReferenceException, where I did do wrong?

2 Answers 2

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

You went wrong here. An IFeatureLayer cannot be cast to an IFeatureClass. This line should be this:

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

Field names cannot have spaces in their name, so this line is invalid:

fld.Name_2 = "country name";

The field alias is the human readable name of the field which CAN have spaces.

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