private void Form1_Load(object sender, EventArgs e)
{
CreatRoundRect();
}

using System.Runtime.InteropServices;

[DllImport("user32", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
private static extern int SetWindowRgn(int hWnd, int hRgn, bool bRedraw);

[DllImport("gdi32", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
private static extern int CreateRoundRectRgn(int X1, int Y1, int X2, int Y2, int X3, int Y3);
private void CreatRoundRect()
{
int hRgn = CreateRoundRectRgn(0, 0, this.Width + 1, this.Height + 1, 0x17, 0x17);
SetWindowRgn(this.Handle.ToInt32(), hRgn, true);
}

this.Width + 1, this.Height + 1