The below code snippet shows browsing all the levels and printing the names of used levels in current DGN.

 public void BrowseAllLevels()
        {
            //Get Active Dgn Model
            DgnModel model = Session.Instance.GetActiveDgnModel();

            //Get levels information from current DGN model
            FileLevelCache fileLevelCache = model.GetFileLevelCache();

            //Get collection of all levels
            LevelHandleCollection levelHandles = fileLevelCache.GetHandles();

            foreach (LevelHandle lh in levelHandles)
            {
                //Check if level is used in current DGN file
                bool isUsed = fileLevelCache.IsLevelUsedInFile(lh.LevelId);
                if (isUsed)
                {
                    //Print used levels name
                    System.Diagnostics.Debug.Print("Level Used: " + lh.Name);
                }
            }
        }

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The below image shows list of used levels in current DGN file from Visual studio Output window

Output