public void IIndex_Example(IFeatureClass featureClass, string nameOfField){//get the field for which we will add the indexIFields fields = new FieldsClass();IFieldsEdit fieldsEdit = fields as IFieldsEdit;fieldsEdit.FieldCount_2 = 1;IField field = featureClass.Fields.get_Field(featureClass.Fields.FindField(nameOfField)); fieldsEdit.set_Field(0, field); //create new Index IIndex index = new IndexClass(); //cast for IindexEdit IIndexEdit indexEdit = (IIndexEdit)index; indexEdit.Fields_2 = fields; indexEdit.IsAscending_2 = false; indexEdit.IsUnique_2 = false; indexEdit.Name_2 = "Idx_1"; //add index to the feature class featureClass.AddIndex(index); }