If you want to change / update a subassembly parameter e.g. "Slope", you can currently use Civil 3D COM API as .NET API in the current release (Civil 3D 2013) doesn't support this.
Here is a C# .NET code snippet demonstrating usage of AeccSubassembly COM API and how to change "Slope" parameter value.
AeccRoadwayApplication roadwayApp = new AeccRoadwayApplication();
roadwayApp.Init((AcadApplication)Autodesk.AutoCAD.ApplicationServices.Application.AcadApplication);
AeccRoadwayDocument roadwayDoc = roadwayApp.ActiveDocument as AeccRoadwayDocument;
AeccRoadwayDatabase roadwayDb = roadwayDoc.Database as AeccRoadwayDatabase;
AeccSubassembly subassembly = roadwayDb.Subassemblies.Item(0);
IAeccParamsDouble paramsDouble = subassembly.ParamsDouble;
IAeccParamDouble slopeParam = paramsDouble.Item("Slope");
ed.WriteMessage("\n Subassembly Slope parameter Before Change : " + slopeParam.Value.ToString());
// Change the slopeParam Value
slopeParam.Value = -0.01;
ed.WriteMessage("\n Subassembly Slope parameter After Change : " + slopeParam.Value.ToString());
Hope this is useful to you!
