Developer: openNURBS
Summary: Discusses openNURBS.NET and extracting curve data.

I am trying to write a little test program to read a .3DM file using Microsoft Visual C# 2008 Express Edition. It is working fine up to the point I want to extract an object from the object table in the model as shown below:

OnNurbsCurve nc = (OnNurbsCurve)model.m_object_table[0].m_object;

I know the object I stored is a OnNurbsCurve. I can not seem to find a way to get the object back to a OnNurbsCurve so I can use it. I get the error.

Unable to cast object of type 'RMA.OpenNURBS.OnObject' to type 'RMA.OpenNURBS.OnNurbsCurve'.

OnXModel.m_object_table is an array of OnXModel_Object objects. An OnXModel_Object object contains two components:

1. An object that inherits from OnObject (OnXModel_Object.m_object). This is the geometry component.
2. An On3dmObjectAttributes object (OnXModel_Object.m_attributes) which contains the object's attributes (e.g. color, layer, etc.)

As you can see, an OnXModel_Object cannot be cast as an OnNurbsCurve object. But, you can try to cast an OnObject as an OnNurbsCurve object.

The following code example demonstrates how to walk through the object table and attempt to cast each object as some type of curve object.

OnXModel model = ...;
for (int i = 0; i < model.m_object_table.Count(); i++)
{
  // Get a model object
  IOnXModel_Object model_obj = model.m_object_table[i];
  If( model_obj == null)
    continue;
 
  // Get the geometry of the object
  IOnObject obj = model_obj.m_object;
  If (obj == null)
    continue;
 
  // Is the geometry a curve?
  IOnCurve crv = OnCurve.ConstCast(obj);
  If (crv != null)
  {
    // OnCurve is a virtual class. So, if we want the curve's data,
    // then we need to cast the curve object as one of the implemented
    // curve types
 
    // Is the curve a line?
    IOnLineCurve line_crv = OnLineCurve.ConstCast(crv);
    If (line_crv != null)
    {
      // TODO: process line curve
      continue;
    }
 
    // Is the curve a polyline?
    IOnPolylineCurve pline_crv = OnPolylineCurve.ConstCast(crv);
    If (pline_crv != null)
    {
      // TODO: process polyline curve
      continue;
    }
 
    // Is the curve an arc (or circle)?
    IOnArcCurve arc_crv = OnArcCurve.ConstCast(crv);
    If (arc_crv != null)
    {
      // TODO: process arc curve
      continue;
    }
 
    // Is the curve a polycurve?
    IOnPolyCurve poly_crv = OnPolyCurve.ConstCast(crv);
    If (plinepoly_crv_crv != null)
    {
      // TODO: process poly curve
      continue;
    }
 
    // Is the curve a NURBS curve?
    IOnNurbsCurve nurbs_crv = OnNurbsCurve.ConstCast(crv);
    If (nurbs_crv != null)
    {
      // TODO: process NURBS curve
      continue;
    }
  }
}

ONX_Model model = ...;
for (int i = 0; i < model.m_object_table.Count(); i++)
{
  // Get a model object
  const ONX_Model_Object& model_obj = model.m_object_table[i];
 
  // Get the geometry of the object
  const ON_Object* obj = model_obj.m_object;
  if( 0 == obj )
    continue;
 
  // Is the geometry a curve?
  const ON_Curve* crv = ON_Curve::Cast( obj );
  if( crv )
  {
    // ON_Curve is a virtual class. So, if we want the curve's data,
    // then we need to cast the curve object as one of the implemented
    // curve types
 
    // Is the curve a line?
    const ON_LineCurve* line_crv = ON_LineCurve::Cast( crv );
    if( line_crv )
    {
      // TODO: process line curve
      continue;
    }
 
    // Is the curve a polyline?
    const ON_PolylineCurve* pline_crv = ON_PolylineCurve::Cast( crv );
    if( pline_crv )
    {
      // TODO: process polyline curve
      continue;
    }
 
    // Is the curve an arc (or circle)?
    const ON_ArcCurve* arc_crv = ON_ArcCurve::Cast( crv );
    if( arc_crv )
    {
      // TODO: process arc curve
      continue;
    }
 
    // Is the curve a polycurve?
    const ON_PolyCurve* poly_crv = ON_PolyCurve::Cast( crv );
    if( poly_crv )
    {
      // TODO: process poly curve
      continue;
    }
 
    // Is the curve a NURBS curve?
    const ON_NurbsCurve* nurbs_crv = ON_NurbsCurve::Cast( crv );
    if( nurbs_crv )
    {
      // TODO: process NURBS curve
      continue;
    }
  }
}