Engine中如何截取线上指定两点间的线段？

//调用
IPolyline newLine = GetSubCurve(polyline, p1, p2);

ESRI.ArcGIS.Display.IScreenDisplay screenDisplay = axMapControl1.ActiveView.ScreenDisplay;
screenDisplay.StartDrawing(screenDisplay.hDC, System.Convert.ToInt16(ESRI.ArcGIS.Display.esriScreenCache.esriNoScreenCache));
IRgbColor rgbColor = new RgbColorClass();
rgbColor.Red = 255;
ESRI.ArcGIS.Display.IColor color = rgbColor; // Implicit Cast
ESRI.ArcGIS.Display.ISimpleLineSymbol simpleLineSymbol = new ESRI.ArcGIS.Display.SimpleLineSymbolClass();
simpleLineSymbol.Color = color;
ESRI.ArcGIS.Display.ISymbol symbol = (ESRI.ArcGIS.Display.ISymbol)simpleLineSymbol; // Explicit Cast
screenDisplay.SetSymbol(symbol);
screenDisplay.DrawPolyline(newLine);
screenDisplay.FinishDrawing();

private IPolyline GetSubCurve(IPolyline inpolyLine, IPoint pnt1, IPoint pnt2)
{
double d1 = GetDistAlong(inpolyLine, pnt1);
double d2 = GetDistAlong(inpolyLine, pnt2);

var c = inpolyLine as ICurve;
ICurve outCurve;
c.GetSubcurve(d1, d2, false, out outCurve);
if (c == null || c.IsEmpty)
throw new Exception(fail);
var outPolyline = outCurve as IPolyline;
if (outPolyline == null)
{
outPolyline = new PolylineClass() as IPolyline;
var sc = outPolyline as ISegmentCollection;
sc.AddSegment((ISegment)outCurve);
((IGeometry)sc).SpatialReference = outCurve.SpatialReference;
}
return outPolyline;
}

private double GetDistAlong(IPolyline polyLine, IPoint pnt)
{
var outPnt = new PointClass() as IPoint;
double distAlong = double.NaN;
double distFrom = double.NaN;
bool bRight = false;
polyLine.QueryPointAndDistance(esriSegmentExtension.esriNoExtension, pnt, false, outPnt,
ref distAlong, ref distFrom, ref bRight);
return distAlong;
}