Problem with input parameter

I am just a newbies in grasshopper component with c#  and also using Grasshopper Tekla Live link (GTlink). I found from forum so can read output parameter from a GTlink component. But I have problem with my output parameter. Can not set correct data to another GTlink component input parameter. Please help. The error is here:

Below is my code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Grasshopper.Kernel;
using Grasshopper.Kernel.Types;
using Rhino.Geometry;


namespace ReadTeklaLivelink
{
    public class PositionGoo : GH_Goo<Tekla.Structures.Model.Position>
    {
        public override bool IsValid
        {
            get
            {
                return true;
            }
        }

        public override string TypeDescription
        {
            get
            {
                return "Tekla part position";
            }
        }

        public override string TypeName
        {
            get
            {
                return typeof(Tekla.Structures.Model.Position).ToString();
            }
        }

        public PositionGoo()
        {
        }

        public PositionGoo(Tekla.Structures.Model.Position pos) : base(pos)
        {
        }

        public override IGH_Goo Duplicate()
        {
            throw new System.NotImplementedException();
        }

        public override string ToString()
        {
            return this.Value.ToString();
        }
    }


    public class PositionParam : GH_Param<PositionGoo>
    {
        public override System.Guid ComponentGuid
        {
            get
            {
                return new System.Guid("4be8ead2-02a7-45e6-900f-859ea43f6b49");
            }
        }

        protected override System.Drawing.Bitmap Icon
        {
            get
            {
                return null; //Todo, provide an icon.
            }
        }

        public PositionParam()
          : base(new GH_InstanceDescription("Cozy Tekla Postion", "Tpos", "Maintains a collection of Boat Shell data.", "WIP", "Boat"))
        {
        }
    }

    public class ReadTeklaLivelinkComponent : GH_Component
    {

        public ReadTeklaLivelinkComponent()
          : base("ReadTeklaLivelink", "Read Data",
              "Description",
              "Category", "Subcategory")
        {
        }

        protected override void RegisterInputParams(GH_Component.GH_InputParamManager pManager)
        {
            pManager.AddGenericParameter("GT_Beam", "GTB", "GH Live link beam", GH_ParamAccess.item);
        }

        protected override void RegisterOutputParams(GH_Component.GH_OutputParamManager pManager)
        {
            pManager.AddParameter(new PositionParam());
        }

        protected override void SolveInstance(IGH_DataAccess DA)
        {
            object beam = null;
            if (!DA.GetData(0, ref beam)) { return; }
            if (beam == null) { return; }
            GH_Goo<Tekla.Structures.Model.ModelObject> gr_beam = beam as GH_Goo<Tekla.Structures.Model.ModelObject>;
            Tekla.Structures.Model.ModelObject be_model = gr_beam.Value;

            Tekla.Structures.Model.Beam be = be_model as Tekla.Structures.Model.Beam;
            PositionGoo pos = new PositionGoo(be.Position);
            DA.SetData(0, pos);
        }

        public override GH_Exposure Exposure
        {
            get
            {
                return GH_Exposure.primary;
            }
        }


        protected override System.Drawing.Bitmap Icon
        {
            get
            {
                return null;
            }
        }

        public override Guid ComponentGuid
        {
            get { return new Guid("{bed8c051-7fe8-4b3c-81ec-428550f6f9c0}"); }
        }
    }

}

Thanks so much!