public CIMSymbolReference SoftEdgeSymbol {get; set;}

var breaklineRendererDef = new TinBreaklineRendererDefinition();
// use default symbol for regular edge but specific symbols for hard,soft,outside
breaklineRendererDef.HardEdgeSymbol = hardEdgeSymbol.MakeSymbolReference();
breaklineRendererDef.SoftEdgeSymbol = softEdgeSymbol.MakeSymbolReference();
breaklineRendererDef.OutsideEdgeSymbol = outsideEdgeSymbol.MakeSymbolReference();

if (surfaceLayer.CanCreateRenderer(breaklineRendererDef))
{
  CIMTinRenderer renderer = surfaceLayer.CreateRenderer(breaklineRendererDef);
  if (surfaceLayer.CanSetRenderer(renderer, SurfaceRendererTarget.Edges))
    surfaceLayer.SetRenderer(renderer, SurfaceRendererTarget.Edges);
}