Avalonia based stuff.

error.log

System.ArgumentNullException: Value cannot be null.
Parameter name: xamlType
   at Portable.Xaml.XamlWriterInternalBase.WriteStartObject(XamlType xamlType)
   at Portable.Xaml.XamlObjectWriter.WriteStartObject(XamlType xamlType)
   at Portable.Xaml.XamlWriter.WriteNode(XamlReader reader)
   at Portable.Xaml.XamlServices.Transform(XamlReader xamlReader, XamlWriter xamlWriter, Boolean closeWriter)
   at Avalonia.Markup.Xaml.AvaloniaXamlLoaderPortableXaml.LoadFromReader(XamlReader reader, AvaloniaXamlContext context)
   at Avalonia.Markup.Xaml.AvaloniaXamlLoaderPortableXaml.Load(Stream stream, Object rootInstance, Uri uri)
   at Avalonia.DesignerSupport.DesignWindowLoader.LoadDesignerWindow(String xaml, String assemblyPath)
   at Avalonia.DesignerSupport.Remote.RemoteDesignerEntryPoint.<>c__DisplayClass17_0.<OnTransportMessage>b__0()

MainWindow.xaml

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="clr-namespace:CanvasTesting.ViewModels;assembly=CanvasTesting"
        Icon="resm:CanvasTesting.Assets.avalonia-logo.ico"
        Title="CanvasTesting"
        Width="500"
        Height="300">

    <Design.DataContext>
        <vm:MainWindowViewModel/>
    </Design.DataContext>
    <DockPanel>
        <StackPanel DockPanel.Dock="Top" Orientation="Vertical">
            <DockPanel>
                <TextBox DockPanel.Dock="Right" Text="{Binding Sides}"/>
                <Slider Orientation="Horizontal" Minimum="3" Maximum="30" Value="{Binding Sides}"/>
            </DockPanel>
            <DockPanel>
                <TextBox DockPanel.Dock="Right" Text="{Binding CenterX}"/>
                <Slider Orientation="Horizontal" Minimum="0" Maximum="500" Value="{Binding CenterX}"/>
            </DockPanel>
            <DockPanel>
                <TextBox DockPanel.Dock="Right" Text="{Biding CenterY}"/>
                <Slider Orientation="Horizontal" Minimum="0" Maximum="300" Value="{Biding CenterY}"/>
            </DockPanel>
        </StackPanel>
        <ItemsControl Items="{Binding Lines}"
                      Width="{Binding Path=CanvasWidth, Mode=OneWayToSource}"
                      Height="{Binding Path=CanvasHeight, Mode=OneWayToSource}">
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <Canvas/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
            <ItemsControl.DataTemplates>
                <DataTemplate DataType="{x:Type vm:LinePoints}">
                    <Line StartPoint="{Binding Start}" EndPoint="{Binding End}" Stroke="Silver" StrokeThickness="3"/>
                </DataTemplate>
            </ItemsControl.DataTemplates>
        </ItemsControl>
    </DockPanel>
</Window>

MainWindowsViewModel.cs

using System;
using System.Collections.Generic;
using System.Text;
using Avalonia;
using ReactiveUI;
using System.Linq;
using static System.Math;
using Avalonia.Controls.Shapes;
using Avalonia.Media.Immutable;

namespace CanvasTesting.ViewModels
{
    public class MainWindowViewModel : ReactiveObject
    {
        private int sides = 5;
        private int radius = 10;
        private int centerY;
        private int centerX;
        public int Sides
        {
            get => sides;
            set
            {
                if (sides != value)
                {
                    sides = value;
                    this.RaisePropertyChanged();
                    this.RaisePropertyChanged(nameof(Lines));
                }
            }
        }
        public int CenterX
        {
            get => centerX;
            set
            {
                if (centerX != value)
                {
                    centerX = value;
                    this.RaisePropertyChanged();
                    this.RaisePropertyChanged(nameof(Lines));
                }
            }
        }
        public int CenterY
        {
            get => centerY;
            set
            {
                if (centerY != value)
                {
                    centerY = value;
                    this.RaisePropertyChanged();
                    this.RaisePropertyChanged(nameof(Lines));
                }
            }
        }
        public int Radius
        {
            get => radius;
            set
            {
                if (radius != value)
                {
                    radius = value;
                    this.RaisePropertyChanged();
                    this.RaisePropertyChanged(nameof(Lines));
                }
            }
        }
        private readonly ImmutableSolidColorBrush brush = new ImmutableSolidColorBrush(0xC0C0C0);
        public List<LinePoints> Lines
        {
            get => Enumerable.Range(0, sides).Select(i =>
            new LinePoints
            {
                Start = new Point(GenX(i), GenY(i)),
                End = new Point(GenX((i + sides / 2) % sides), GenY((i + sides / 2) % sides)),
            }).ToList();
        }

        private double GenX(int ctr) => radius * Cos(2 * PI * ctr / sides) + centerX;
        private double GenY(int ctr) => radius * Sin(2 * PI * ctr / sides) + centerY;
    }

    public class LinePoints : ReactiveObject
    {
        public Point Start { get; set; }
        public Point End { get; set; }
    }
}