WPF 2D Creating Perpendicular Lines

<Viewbox Stretch="Uniform">
 
<Grid Width="730" Height="600" 
        HorizontalAlignment="Left" 
        VerticalAlignment="Top">
	<Grid.ColumnDefinitions>
		<ColumnDefinition Width="250" />
		<ColumnDefinition Width="*" />
	</Grid.ColumnDefinitions>
	<Grid  Margin="5,10,5,5" >
		<Grid.ColumnDefinitions>
			<ColumnDefinition Width="39" />
			<ColumnDefinition Width="*" />
		</Grid.ColumnDefinitions>
		<Grid.RowDefinitions>
			<RowDefinition Height="Auto" />
			<RowDefinition Height="Auto" />
			<RowDefinition Height="Auto" />
			<RowDefinition Height="Auto" />
			<RowDefinition Height="Auto" />
			<RowDefinition Height="Auto" />
			<RowDefinition Height="Auto" />
			<RowDefinition Height="Auto" />
			<RowDefinition Height="Auto" />
		</Grid.RowDefinitions>
 		<TextBlock HorizontalAlignment="Right" 
                    Grid.Column="0" Grid.Row="0"  
                    Margin="5,5,10,5">X1</TextBlock>
		<Slider Name="tbX1" Grid.Column="1"  
                    Grid.Row="0"  Value="50" Minimum="-1000" Maximum="1000" 
                     ValueChanged="tbX1_ValueChanged" ></Slider>
		<TextBlock HorizontalAlignment="Right"  
                    Grid.Column="0" Grid.Row="1"  Margin="5,5,10,5">Y1</TextBlock>
		<Slider Name="tbY1" Grid.Column="1" 
                    Grid.Row="1"  Value="200"  Minimum="-1000" Maximum="1000"  
                     ValueChanged="tbX1_ValueChanged" ></Slider>
		<TextBlock HorizontalAlignment="Right" 
                    Grid.Column="0" Grid.Row="2" Margin="5,5,10,5">X2</TextBlock>
		<Slider Name="tbX2" Grid.Column="1" 
                    Grid.Row="2" Value="150" Minimum="-1000" Maximum="1000"  
                     ValueChanged="tbX1_ValueChanged" ></Slider>
		<TextBlock HorizontalAlignment="Right" 
                    Grid.Column="0" Grid.Row="3" Margin="5,5,10,5">Y2</TextBlock>
		<Slider Name="tbY2" Grid.Column="1"  
                    Grid.Row="3" Value="100"  Minimum="-1000" Maximum="1000"  
                     ValueChanged="tbX1_ValueChanged"></Slider>
		<TextBlock HorizontalAlignment="Right" 
                    Grid.Column="0" Grid.Row="4"  Margin="5,5,10,5">Length</TextBlock>
		<Slider Name="tbLength" Grid.Column="1" 
                     Grid.Row="4" Value="100"  Minimum="-1000" Maximum="1000"  
                     ValueChanged="tbX1_ValueChanged"></Slider>
		<TextBlock HorizontalAlignment="Right" Grid.Column="0" Grid.Row="6" 
                     Margin="5,5,10,5">Line2</TextBlock>
		<CheckBox Grid.Column="1" Grid.Row="6" Content="Visible" x:Name="chk2" 
                     IsChecked="True" Click="chk2_Click"></CheckBox>
		<Button Click="BtnApply_Click" 
                    Margin="15,20,15,5"  
                    Grid.Row="7" Height="25"  
                    Grid.ColumnSpan="2" 
                    Grid.Column="0">Apply</Button>
		<Button Click="BtnClose_Click" 
                    Margin="15,0,15,5" 
                    Grid.Row="8" Height="25"  
                    Grid.ColumnSpan="2" 
                    Grid.Column="0">Close</Button>
	</Grid>
	<Border  Grid.Column="1"   BorderThickness="1" BorderBrush="Blue">
		<Canvas Name="canvas1" Grid.Column="1" Margin="10" ClipToBounds="True">
		<TextBlock Name="tbPoint1" Canvas.Top="10">Point1</TextBlock>
		<TextBlock Name="tbPoint2" Canvas.Top="25">Point2</TextBlock>
		<TextBlock Name="tbPoint3" Canvas.Top="40">Point3</TextBlock>
		<TextBlock Name="tbPoint4" Canvas.Top="55">Point4</TextBlock>
		</Canvas>
	</Border>
</Grid>
</Viewbox>

private Line line1;
private Line line2;
 
public MainWindow()
{
    InitializeComponent();
 
    Rectangle rect = new Rectangle();
    rect.Stroke = Brushes.Black;
    rect.Width = canvas1.Width;
    rect.Height = canvas1.Height;
 
    canvas1.Children.Add(rect);
    line1 = new Line();
    line2 = new Line();
    AddLines();
}
 
private void AddLines()
{
    Point pt1 = new Point();
    Point pt2 = new Point();
 
 
    pt1.X = Convert.ToDouble(tbX1 == null ? 200 : tbX1.Value);
    pt1.Y = Convert.ToDouble(tbY1 == null ? 150 : tbY1.Value);
    pt2.X = Convert.ToDouble(tbX2 == null ? 100 : tbX2.Value);
    pt2.Y = Convert.ToDouble(tbY2 == null ? 100 : tbY2.Value);
    double length =                                                                            0.5 * Convert.ToDouble(tbLength == null ? 100 : tbLength.Value);
 
    line1 = new Line();
    line1.X1 = pt1.X;
    line1.Y1 = pt1.Y;
    line1.X2 = pt2.X;
    line1.Y2 = pt2.Y;
    line1.Stroke = Brushes.Gray;
    line1.StrokeThickness = 4;
 
    canvas1.Children.Add(line1);
 
    Canvas.SetLeft(tbPoint1, pt1.X);
    Canvas.SetTop(tbPoint1, pt1.Y);
    Canvas.SetLeft(tbPoint2, pt2.X);
    Canvas.SetTop(tbPoint2, pt2.Y);
 
    tbPoint1.Text = "Pt1(" + pt1.ToString() + ")";
    tbPoint2.Text = "Pt2(" + pt2.ToString() + ")";
 
    //////////////////////////////////////////////////////////////////////////
 
    if (chk2.IsChecked == true)
    {
        Vector v1 = pt1 - pt2;
        Matrix m1 = new Matrix();
        Point pt3 = new Point();
        Point pt4 = new Point();
        m1.Rotate(-90);
        v1.Normalize();
        v1 *= length;
        line2 = new Line();
        line2.Stroke = Brushes.Gray;
        line2.StrokeThickness = 4;
        line2.StrokeDashArray = DoubleCollection.Parse("3, 1");
        pt3 = pt2 + v1 * m1;
        m1 = new Matrix();
        m1.Rotate(90);
        pt4 = pt2 + v1 * m1;
        line2.X1 = pt3.X;
        line2.Y1 = pt3.Y;
        line2.X2 = pt4.X;
        line2.Y2 = pt4.Y;
 
        canvas1.Children.Add(line2);
 
        Canvas.SetLeft(tbPoint3, pt3.X);
        Canvas.SetTop(tbPoint3, pt3.Y);
        Canvas.SetLeft(tbPoint4, pt4.X);
        Canvas.SetTop(tbPoint4, pt4.Y);
        pt3.X = Math.Round(pt3.X, 0);
        pt3.Y = Math.Round(pt3.Y, 0);
        pt4.X = Math.Round(pt4.X, 0);
        pt4.Y = Math.Round(pt4.Y, 0);
 
        tbPoint3.Text = "Pt3(" + pt3.ToString() + ")";
        tbPoint4.Text = "Pt4(" + pt4.ToString() + ")";
    }
 
}
 
private void BtnApply_Click(object sender, RoutedEventArgs e)
{
    if (line1 != null)
        canvas1.Children.Remove(line1);
    if (line2 != null)
        canvas1.Children.Remove(line2);
    AddLines();
}
 
private void BtnClose_Click(object sender, RoutedEventArgs e)
{
    this.Close();
}
 
private void tbX1_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
{
    if (canvas1 != null)
    {
        if (line1 != null)
            canvas1.Children.Remove(line1);
        if (line2 != null)
            canvas1.Children.Remove(line2);
        AddLines();
    }
 
}
 
private void chk2_Click(object sender, RoutedEventArgs e)
{
    if (line1 != null)
        canvas1.Children.Remove(line1);
    if (line2 != null)
        canvas1.Children.Remove(line2);
    AddLines();
}

reference : Practical WPF Graphics Programming