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<Button x:Name="btnBack" Grid.Row="2" Width="300"
                            Click="btnBack_Click">
                    <Button.Template>
                        <ControlTemplate>
                            <Border CornerRadius="10" Background="#463190">
                                <TextBlock Text="Retry" Foreground="White" 
                                           HorizontalAlignment="Center"                                           
                                           Margin="0,5,0,0"
                                           Height="40"
                                           FontSize="20"></TextBlock>
                            </Border>
                        </ControlTemplate>
                    </Button.Template>
                </Button>					

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using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

        }
        public class ButtonEx : Button
        {
            private byte _cornerRadian = 0;

            private Rectangle _rect = new Rectangle();
            private Color _rectColor = SystemColors.Control;
            private byte _rectWidth = 0;
            private Pen _pen = new Pen(SystemColors.Control);


            public ButtonEx() { }

            protected override void OnPaint(PaintEventArgs e)
            {
                base.OnPaint(e);
                _rect = ClientRectangle;
                _rect.Inflate(-_rectWidth + 2, -_rectWidth + 2);
                _pen.Color = _rectColor;
                _pen.Width = _rectWidth;
                e.Graphics.DrawRectangle(_pen, _rect);
            }

            [Browsable(true)]
            public Color RectColor
            {
                get { return _rectColor; }
                set
                {
                    if (_rectColor != value)
                    {
                        _rectColor = value;
                        //_pen.Color = _rectColor; 
                        this.Refresh();
                    }
                }
            }

            [Browsable(true)]
            public byte RectWidth
            {
                get { return _rectWidth; }
                set
                {
                    if (_rectWidth != value)
                    {
                        _rectWidth = value;
                        //_pen.Width = _rectWidth; 
                        this.Refresh();
                    }
                }
            }

            [Browsable(true)]
            public byte CornerRadian
            {
                get { return _cornerRadian; }
                set
                {
                    if (_cornerRadian != value)
                    {
                        _cornerRadian = value;
                        IntPtr hRgn = CreateRoundRectRgn(0, 0, Width, Height, _cornerRadian * 2, _cornerRadian * 2);  // eventuell (optische) Anpassungen vornehmen 
                        SetWindowRgn(Handle, hRgn, true);
                    }
                }
            }

            [DllImport("Gdi32.dll")]
            public static extern IntPtr CreateRoundRectRgn(
                int nLeftRect,
                int nTopRect,
                int nRightRect,
                int nBottomRect,
                int nWidthEllipse,
                int nHeightEllipse);

            [DllImport("User32.dll")]
            public static extern int SetWindowRgn(
                IntPtr hWnd,
                IntPtr hRgn,
                bool bRedraw);
        }

        // Aufruf
        
        private void button1_Click(object sender, EventArgs e)
        {
            buttonEx1.CornerRadian = 6;  // Kantenabrundung(-sradius)
            buttonEx1.RectColor = Color.Blue;  // blauer Rahmen innerhalb ...
            buttonEx1.RectWidth = 6;  // ..
        }
    }
}					

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  private void button1_Click(object sender, EventArgs e)
        {
            ButtonEx button1x = new ButtonEx();
            button1x.CornerRadian = 2;  // Kantenabrundung(-sradius)
            button1x.RectColor = Color.Blue;  // blauer Rahmen innerhalb ...
            button1x.RectWidth = 2;  // ..
            Controls.Add(button1x);
        }