Create a list of Rhino geometry within a Grasshopper component using GeometryBase

class GeometryBuilder
{
    //constructor
    public GeometryBuilder()
    {
        //...
    }

    //property - declare our geometry list here
    private List<Rhino.Geometry.GeometryBase> geom = new List<Rhino.Geometry.GeometryBase>();
    public List<Rhino.Geometry.GeometryBase> Geom
    { get { return geom; } set { geom = value; }}

    //methods
    public BuildGeometry()
    {
        //add a line
        //lines don't inherit from GeometryBase, so we can't cast it to be in
        // our Geom list, but we can convert it to a NurbsCurve, which does.
        Rhino.Geometry.Line ln = new Line(new Point3d(0,0,0), new Point3d(1,1,0));
        Geom.Add((GeometryBase)(ln.ToNurbsCurve())); 

        //Meshes DO inherit from GeometryBase, so we can add them to Geom directly
        Rhino.Geometry.Mesh msh = someOtherMesh;
        Geom.Add((GeometryBase)msh);

        //Generally, things within Rhino.Geometry that are classes can be cast to GeometryBase.
        //Things which are structs can not, though there is usually a sensible way to convert them.

        //Point3d is a struct, so we can't cast it to GeometryBase.
        //The Rhino Point is a class, and it contains a constructor converting from Point3d
        Rhino.Geometry.Point3d pt3d = new Point3d(5,2,0);
        Rhino.Geometry.Point pt = new Point(pt3d);
        Geom.Add((GeometryBase)pt);
    }
}



class GrasshopperComponent : GH_Component
{
    //constructors etc...
    //...

    //set inputs
    //...

    //set outputs
    protected override void RegisterOutputParams(GH_Component.GH_OutputParamManager pManager)
    {
        //our output is created using the 'AddGeometryParameter' method. Remember to set 'access' to 'list'.
        pManager.AddGeometryParameter("My Geometry","my geom","This is a list of generic geometry", GH_ParamAccess.list);
    }

    protected override void SolveInstance(IGH_DataAccess DA)
    {
        //generate our list of geometry from the class we made
        var GeometryBuilder gb = new GeometryBuilder();
        gb.BuildGeometry();

        //the geometry is now saved within the Geom property of our gb instance of GeometryBuilder.
        //All we need to do now is send the geometry to the Grasshopper output
        DA.SetDataList(0, gb.Geom);
    }
}