How to get the section geomtry/mesh of cutting plane through model

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WPF

Hi,
I have managed to add a cutting plane to model (imported as Model3D then added as content of ModelVisual3D) & it works well but the model is surface & it is open. I want to close the surface with patch/mesh to give illusion it is solid. I understand you get patch mesh detail from MeshGeometry3D but I am struggling to get the conversion from Model to mesh.What is the best way to either import the Mesh from the .stl file or convert.

I undersatnd that you would then create the point3d array like this:-

MeshGeometryHelper.GetContourSegments(mesh, New Point3D(0, 0, 0), New Vector3D(0, -1, 0)).ToArray 

But how do I get to a mesh I don't now I have not this far to try to debug.

Maybe:-

Dim mesh_Out As Mesh3D = New Mesh3D
               For Each s As Point3D In segments
                   meshModel.Positions.Add(s)
               Next

I have looked every where for days & I find promissing things but turn out to be deadends.
Maybe some sample code to help kickstart the momentum.

Thanks in advance.

What I have tried:

Dim group As Model3DGroup = New Model3DGroup
                ModelImporter.Load(My.Application.Info.DirectoryPath.ToString & "\MODEL\" & Model_Name & ".STL")
                Dim TestMesh As MeshBuilder = New MeshBuilder(False, False)
                For Each m In group.Children
                    Dim mGeo = CType(m, GeometryModel3D)
                    Dim mesh As MeshGeometry3D = CType(CType(mGeo.Geometry, Geometry3D), MeshGeometry3D)
                    If (Not (mesh) Is Nothing) Then
                        TestMesh.Append(mesh)
                    End If

                Next