rounderButton.cs

/// https://stackoverflow.com/questions/28486521/rounded-edges-in-button-c-sharp-winforms
class RoundedButton : Button
{
   GraphicsPath GetRoundPath(RectangleF Rect, int radius)
   {
      float r2 = radius / 2f;
      GraphicsPath GraphPath = new GraphicsPath();

      GraphPath.AddArc(Rect.X, Rect.Y, radius, radius, 180, 90);
      GraphPath.AddLine(Rect.X + r2, Rect.Y, Rect.Width - r2, Rect.Y);
      GraphPath.AddArc(Rect.X + Rect.Width - radius, Rect.Y, radius, radius, 270, 90);
      GraphPath.AddLine(Rect.Width, Rect.Y + r2, Rect.Width, Rect.Height - r2);
      GraphPath.AddArc(Rect.X + Rect.Width - radius, 
                       Rect.Y + Rect.Height - radius, radius, radius, 0, 90);
      GraphPath.AddLine(Rect.Width - r2, Rect.Height, Rect.X + r2, Rect.Height);
      GraphPath.AddArc(Rect.X, Rect.Y + Rect.Height - radius, radius, radius, 90, 90);
      GraphPath.AddLine(Rect.X, Rect.Height - r2, Rect.X, Rect.Y + r2);

      GraphPath.CloseFigure();
      return GraphPath;
       }

   protected override void OnPaint(PaintEventArgs e)
   {
      base.OnPaint(e);
      RectangleF Rect = new RectangleF(0, 0, this.Width, this.Height);
      GraphicsPath GraphPath = GetRoundPath(Rect, 50);

      this.Region = new Region(GraphPath);
      using (Pen pen = new Pen(Color.CadetBlue, 1.75f))
      {
          pen.Alignment = PenAlignment.Inset;
          e.Graphics.DrawPath(pen, GraphPath);
      }
   }
}