question

Hi, if you want see line you must create triangles. Try following code:

   <Window.DataContext>
     <local:ViewModel/>
   </Window.DataContext>
    
    
     <Viewport3D>
       <Viewport3D.Camera>
         <PerspectiveCamera Position="-10,20,10" LookDirection="40,-40,-40" UpDirection="0,0,1"/>
       </Viewport3D.Camera>
       <ModelVisual3D Content="{Binding MGroup}"/>
     </Viewport3D>
    
    
   public class ViewModel
 Hi, if you want see line you must create triangles. try following code:
    
   {
     public Model3DGroup MGroup { get => GetData(); }
    
     private ObservableCollection<Line> col = new ObservableCollection<Line>();
    
     private Model3DGroup GetData()
     {
       // collection of lines
       Point pt1 = new Point(0, 0);
       Point pt2 = new Point(10, 0);
       for (int i = 0; i < 10; i++) col.Add(new Line() { X1 = pt1.X, X2 = pt2.X, Y1 = pt1.Y + i, Y2 = pt2.Y + i });
       // create ModelGroup
       Model3DGroup gr = new Model3DGroup();
       gr.Children.Add(new DirectionalLight(Colors.White, new Vector3D(-1, -1, -3)));
       foreach (var item in col)
       {
         GeometryModel3D mod = AddLine(new Point3D(item.X1, item.Y1, 0), new Point3D(item.X2, item.Y2, 0));
         gr.Children.Add(mod);
       }
       return gr;
     }
    
     private GeometryModel3D AddLine(Point3D startPoint, Point3D endPoint)
     {
       var material = new DiffuseMaterial(new SolidColorBrush(Colors.Red));
       var mesh = new MeshGeometry3D();
       mesh.Positions.Add(startPoint);
       mesh.Positions.Add(endPoint);
       mesh.Positions.Add(new Point3D(startPoint.X, startPoint.Y + .1, startPoint.Z));
       mesh.Positions.Add(new Point3D(endPoint.X, endPoint.Y + .1, endPoint.Z));
       mesh.TriangleIndices.Add(0);
       mesh.TriangleIndices.Add(1);
       mesh.TriangleIndices.Add(3);
       mesh.TriangleIndices.Add(3);
       mesh.TriangleIndices.Add(2);
       mesh.TriangleIndices.Add(0);
       return new GeometryModel3D(mesh, material);
     }
   }
    
   public class MyViewPort
   {
     private Viewport3D _vPort;
     public Viewport3D VPort
     {
       get => this._vPort;
       set
       {
         if (this._vPort == null)
         {
           this._vPort = value;
           this._vPort.MouseDown += _vPort_MouseDown;
           this._vPort.MouseMove += _vPort_MouseMove;
           this._vPort.MouseUp += _vPort_MouseUp;
           this._vPort.Loaded += _vPort_Loaded;
         }
       }
     }
    
     private AxisAngleRotation3D rot = new AxisAngleRotation3D(new Vector3D(0, 2, 0), 0);
     private void _vPort_Loaded(object sender, RoutedEventArgs e)
     {
       var mod = this._vPort.Children[0] as ModelVisual3D;
       mod.Transform = new RotateTransform3D(rot);
     }
    
     private Point pt = new Point(double.NaN, double.NaN);
     private void _vPort_MouseDown(object sender, MouseButtonEventArgs e) => pt = e.GetPosition(this._vPort);
     private void _vPort_MouseMove(object sender, MouseEventArgs e)
     {
       if (!double.IsNaN(pt.X))
       {
         Point pt1 = e.GetPosition(this._vPort);
         double ang = pt1.X - pt.X;
         rot.Angle = ang;
       }
     }
     private void _vPort_MouseUp(object sender, MouseButtonEventArgs e) => pt = new Point(double.NaN, double.NaN);
   }