Draw Triangle from "for cycles" iusse

I need to draw from code behind, (not from xaml), a large number of triangle. For calculation I calc half Triangleindices with a for cycle and the other half with another. Here the code : xaml:

<Window x:Name="MainWindowX" x:Class="MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:HelixTest"
    xmlns:HelixToolkit="clr-namespace:HelixToolkit.Wpf;assembly=HelixToolkit.Wpf"
   
    mc:Ignorable="d"
    Title="MainWindow" Height="450" Width="800">
<Grid>
    <HelixToolkit:HelixViewport3D Name="myviewport" ZoomExtentsWhenLoaded="True"  Background="Black"  ShowCoordinateSystem="True" >
        <!-- Remember to add light to the scene -->
        <HelixToolkit:SunLight/>
        <ModelVisual3D>
            <ModelVisual3D.Content>
                <GeometryModel3D x:Name="myGeometryModel3D">
                    
                    <GeometryModel3D.Geometry>
                        <MeshGeometry3D x:Name="myMeshGeometry3D">


                        </MeshGeometry3D>
                    </GeometryModel3D.Geometry >
                    <GeometryModel3D.Material>
                        <MaterialGroup>
                            <DiffuseMaterial>
                                <DiffuseMaterial.Brush>
                                    
                                        <LinearGradientBrush StartPoint="0,0.5" EndPoint="1,0.5">
                                            <LinearGradientBrush.GradientStops>
                                                <GradientStop Color="red" Offset="2" />
                                                <GradientStop Color="Red" Offset="3" />
                                                <GradientStop Color="Blue" Offset="4" />
                                                <GradientStop Color="LimeGreen" Offset="5" />
                                            </LinearGradientBrush.GradientStops>
                                    </LinearGradientBrush>
                                </DiffuseMaterial.Brush>
                            </DiffuseMaterial>
                        </MaterialGroup>
                    </GeometryModel3D.Material>
                </GeometryModel3D>
            </ModelVisual3D.Content>
        </ModelVisual3D>

       
     
        <HelixToolkit:BillboardTextVisual3D Position="11 0 0" Text="X" Foreground="Yellow"/>
        <HelixToolkit:BillboardTextVisual3D Position="0 11 0" Text="Y" Foreground="Yellow" />
        <HelixToolkit:BillboardTextVisual3D Position="0 0 11" Text="Z"  Foreground="Yellow"/>
    </HelixToolkit:HelixViewport3D>

</Grid>

and vb.net code behind

Imports System.Windows.Media.Media3D
Imports HelixToolkit

Class MainWindow
    Dim punti = 5
    Dim curve = 5
    Dim valori As New List(Of Integer)
    Dim indices As New List(Of Integer)
    Dim triangoli = (punti - 1) * (curve - 1) * 2

    Private Sub MainWindowX_Loaded(sender As Object, e As RoutedEventArgs) Handles MyBase.Loaded, MyBase.Loaded

        myMeshGeometry3D.Positions.Clear()
        myMeshGeometry3D.TriangleIndices.Clear()

        valori.Add(0)
        valori.Add(1)
        valori.Add(2)
        valori.Add(3)
        valori.Add(3)
        valori.Add(0)
        valori.Add(1)
        valori.Add(2)
        valori.Add(3)
        valori.Add(3)
        valori.Add(0)
        valori.Add(1)
        valori.Add(2)
        valori.Add(3)
        valori.Add(3)
        valori.Add(0)
        valori.Add(1)
        valori.Add(2)
        valori.Add(3)
        valori.Add(3)
        valori.Add(0)
        valori.Add(1)
        valori.Add(2)
        valori.Add(3)
        valori.Add(3)



        'load myMeshGeometry3d.Positions
        For I = 0 To curve - 1
            'per ogni curva
            For b = 0 To punti - 1
                myMeshGeometry3D.Positions.Add(New Point3D(b, I, valori(I * punti + b)))

            Next


        Next

        'First half
        For d = punti + 1 To triangoli - 2 Step punti
            For f = 0 To punti - 2 Step 1
                myMeshGeometry3D.TriangleIndices.Add(d + f)
                myMeshGeometry3D.TriangleIndices.Add((d - 1) + f)
                myMeshGeometry3D.TriangleIndices.Add((d - punti) + f)
            Next
        Next
        'second half
        For j = 0 To triangoli Step punti
            For i = 0 To punti - 2 Step 1
                myMeshGeometry3D.TriangleIndices.Add(j + i)
                myMeshGeometry3D.TriangleIndices.Add((j + 1) + i)
                myMeshGeometry3D.TriangleIndices.Add((j + punti) + i)
            Next
        Next
    End Sub
End Class

My iusse is that only the triagles that are generate from the first cycle are showed, the second not. I tried to swap the cycle and they are both correct. I also tryed to add 2 button, one for cycle. they work to show half triangle, but only it first I clear the triangleindices.What am I missing? Any advice?