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 numSegments = part.Count;

  for (int j = 0; j < numSegments; 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();
return polyline3D;