Why IEnumFeature in ArcObjects returns duplicate features?

Question

During development of ArcGis AddIn I've found strange behaviour of MapSelection when there are a lot of selected features. When I tryed to get selected features I've received duplicate features. So the code below throws an exception "Key already exist" . I have no idea why this happens and how to fix it. Can anybody hekp me?

  IEnumFeature enumFeat = (IEnumFeature) ArcMap.Document.FocusMap.FeatureSelection;
  IEnumFeatureSetup enumSetup = (IEnumFeatureSetup)enumFeat;
  enumSetup.AllFields = true;
  enumFeat.Reset();
  IDictionary<int, int> dicIds = new Dictionary<int, int>();
  while ((feat = enumFeat.Next()) != null)
  {
    dicIds.Add(feat.OID, num);
  }

Answer 1

The Line:

  IEnumFeature enumFeat = (IEnumFeature) ArcMap.Document.FocusMap.FeatureSelection;

gets all the selected features in the map, across all layers. This is where you getting duplicate OID's, since they are coming from features in different Layers.

You should use the IFeatureSelection Interface instead. Get the selectionset using the IFeatureSelection.SelectionSet Property, and then you can iterate over selected features in a particular layer.