One way to access existing features is to use the FeatureClass.GetFeatures() method. This method returns a FeatureList object (a linked list of Feature objects using the IList interface). Certain overrides of this method allow filters and SQL Where clauses to limit the number of features returned.
Another way to get a list of features is to prompt the user to select them after they have generated graphics within the AutoCAD Map 3D Client. Calling the Autodesk.Map.IM.Forms.Document.Map.GetFeatures() method pauses execution of your program while the user selects features using the Map interface. It returns a linked list of features, which may be of different kinds of feature classes.