public void ReplaceSegment( 
   int partIndex,
   int segmentIndex,
   Segment segment
)

{
  PolylineBuilderEx polylineBuilder = new PolylineBuilderEx(polyline);
  polylineBuilder.HasZ = true;

  // The HasZ property is set to true for all the points in
  // polyline3D when you call ToGeometry().
  Polyline polyline3D = polylineBuilder.ToGeometry();

  // For this example, create Z-values. 
  // You may want to pass them in as a parameter.
  int numPoints = polyline3D.PointCount;
  double[] zValues = new double[numPoints];
  for (int i = 0; i < numPoints; i++)
  {
    zValues[i] = i % 2 == 0 ? 2 : 1;
  }

  // We need to know at which point index each part starts
  int partPointIndex = 0;
  int numParts = polyline3D.PartCount;

  for (int i = 0; i < numParts; i++)
  {
    var part = polyline3D.Parts[i];
    int numSegmentsInPolyline3D = part.Count;

    for (int j = 0; j < numSegmentsInPolyline3D; j++)
    {
      Segment segment = part[j];

      MapPointBuilderEx pointBuilder = new MapPointBuilderEx(segment.StartPoint);
      pointBuilder.Z = zValues[partPointIndex++];
      MapPoint startPoint = pointBuilder.ToGeometry();

      // Make sure that the end point of this segment is the same as the start point of the next segment
      pointBuilder = new MapPointBuilderEx(segment.EndPoint);
      pointBuilder.Z = zValues[partPointIndex];
      MapPoint endPoint = pointBuilder.ToGeometry();

      SegmentType segmentType = segment.SegmentType;
      SegmentBuilderEx segmentBuilder = null;
      switch (segmentType)
      {
        case SegmentType.Line:
          segmentBuilder = new LineBuilderEx((LineSegment)segment);
          break;
        case SegmentType.Bezier:
          segmentBuilder = new CubicBezierBuilderEx((CubicBezierSegment)segment);
          break;
        case SegmentType.EllipticArc:
          segmentBuilder = new EllipticArcBuilderEx((EllipticArcSegment)segment);
          break;
      }

      // Only change the start and end point which now have Z-values set. 
      // This will preserve the curve if the segment is an EllipticArcSegment or CubicBezierSegment.
      segmentBuilder.StartPoint = startPoint;
      segmentBuilder.EndPoint = endPoint;
      segment = segmentBuilder.ToSegment();

      polylineBuilder.ReplaceSegment(i, j, segment);
    }

    // Move point index for the next part
    partPointIndex++;
  }

  polyline3D = polylineBuilder.ToGeometry();
}