<local:CustomWindow x:Class="Hexagon_V2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:local="clr-namespace:Hexagon_V2"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="450" Width="800" WindowStyle="None"
        AllowsTransparency="True" CornerRadius="30" Background="Aqua">
    

<Grid>
    
</Grid>

</local:CustomWindow>

 
<local:CustomWindow x:Class="Hexagon_V2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:local="clr-namespace:Hexagon_V2"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="450" Width="800" WindowStyle="None"
        AllowsTransparency="True" CornerRadius="30" Background="Aqua">
    

<Grid>
    
</Grid>

</local:CustomWindow>

public partial class MainWindow : CustomWindow
{
    public MainWindow()
    {
        InitializeComponent();
    }
}

public partial class MainWindow : CustomWindow
{
    public MainWindow()
    {
        InitializeComponent();
    }
}

using System.Windows;

namespace Hexagon_V2
{
    public class CustomWindow : Window
    {

        public int CornerRadius
        {
            get { return (int)GetValue(CornerRadiusProperty); }
            set { SetValue(CornerRadiusProperty, value); }
        }

        public static readonly DependencyProperty CornerRadiusProperty =
            DependencyProperty.Register("CornerRadius", typeof(int), typeof(CustomWindow), new PropertyMetadata(0));

        static CustomWindow()
        {

            DefaultStyleKeyProperty.OverrideMetadata(typeof(CustomWindow), new FrameworkPropertyMetadata(typeof(CustomWindow)));
        }
    }
}

using System.Windows;

namespace Hexagon_V2
{
    public class CustomWindow : Window
    {

        public int CornerRadius
        {
            get { return (int)GetValue(CornerRadiusProperty); }
            set { SetValue(CornerRadiusProperty, value); }
        }

        public static readonly DependencyProperty CornerRadiusProperty =
            DependencyProperty.Register("CornerRadius", typeof(int), typeof(CustomWindow), new PropertyMetadata(0));

        static CustomWindow()
        {

            DefaultStyleKeyProperty.OverrideMetadata(typeof(CustomWindow), new FrameworkPropertyMetadata(typeof(CustomWindow)));
        }
    }
}