The below code shows how to get the terrain from geometry model and calculate the high, low, and mean terrain elevation from all terrain points.

//Required References
using System;
using System.Collections.Generic;
using System.Diagnostics;
using Bentley.CifNET.GeometryModel.SDK;
using Bentley.CifNET.SDK;
using Bentley.GeometryNET;
using Bentley.MstnPlatformNET;
using Bentley.TerrainModelNET;

 public void QuickStatisticsOfTerrainModelAttached()
        {
            Bentley.CifNET.GeometryModel.SDK.TerrainSurface terrainSurface = null;
            try
            {
                //Get active dgn model
                Bentley.DgnPlatformNET.DgnModel dgnModel = Session.Instance.GetActiveDgnModel();
                //Create connection to dgnModel
                Bentley.CifNET.SDK.ConsensusConnection consensusConnection = new ConsensusConnection(dgnModel);
                //Get active geometric model
                Bentley.CifNET.GeometryModel.SDK.GeometricModel geometricModel = consensusConnection.GetActiveGeometricModel();
                if (geometricModel.ActiveSurface == null) return;
                List<DPoint3d> pointList = new List<DPoint3d>();
                //Get active terrain
                terrainSurface = geometricModel.ActiveSurface as TerrainSurface;
                if (terrainSurface == null) return;

                //Get the low and high elevation value from all terrain points
                var range = terrainSurface.DTM.Range3d;
                double dLowElevation = range.Low.Z;
                double dHighElevation = range.High.Z;

                double sumOfElevations = 0.0;
                long count = terrainSurface.DTM.VerticesCount;

                //Browse all points of terrain surface
                terrainSurface.DTM.BrowsePoints(new PointsBrowsingCriteria(), (DPoint3d[] tPoint, object oArg) =>
                {
                    foreach (DPoint3d point in tPoint)
                    {
                        sumOfElevations += point.Z;
                    }
                    return true;
                }
                , null);

                //Calculate the mean elevation
                double dMeanElevation = sumOfElevations / count;

                Trace.WriteLine("Highest Elevation : " + dHighElevation);
                Trace.WriteLine("Mean Elevation : " + dMeanElevation);
                Trace.WriteLine("Lowest Elevation : " + dLowElevation);
            }
            catch (Exception ex)
            {
                Trace.WriteLine(ex.Message);
            }
        }

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Output