Because the backgroud color of the form is white, when I use
SmoothingMode.AntiAlias, the color of shadow seems gradient to white color.
But if I don't use SmoothingMode.AntiAlias, there are white gap between four
arc.

Here is the source code:
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Drawing.Drawing2D;

namespace WindowsApplication1
{
public class Form1 : System.Windows.Forms.Form
{
 private System.Windows.Forms.Panel panelContent;
 private System.ComponentModel.Container components = null;

 public Form1()
 {
  InitializeComponent();
 }

 protected override void Dispose( bool disposing )
 {
  if( disposing )
  {
   if (components != null)
   {
    components.Dispose();
   }
  }
  base.Dispose( disposing );
 }

 private void InitializeComponent()
 {
  this.panelContent = new System.Windows.Forms.Panel();
  this.SuspendLayout();
  //
  // panelContent
  //
  this.panelContent.Location = new System.Drawing.Point(88, 56);
  this.panelContent.Name = "panelContent";
  this.panelContent.Size = new System.Drawing.Size(272, 120);
  this.panelContent.TabIndex = 0;
  this.panelContent.Paint += new
System.Windows.Forms.PaintEventHandler(this.panelContent_Paint);
  //
  // Form1
  //
  this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
  this.BackColor = System.Drawing.Color.White;
  this.ClientSize = new System.Drawing.Size(448, 277);
  this.Controls.Add(this.panelContent);
  this.Name = "Form1";
  this.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen;
  this.Text = "Form1";
  this.Paint += new
System.Windows.Forms.PaintEventHandler(this.Form1_Paint);
  this.ResumeLayout(false);

 }

 [STAThread]
 static void Main()
 {
  Application.Run(new Form1());
 }

 private void panelContent_Paint(object sender,
System.Windows.Forms.PaintEventArgs e)
 {
  e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
  System.Drawing.Drawing2D.GraphicsPath path = new GraphicsPath();

  int width = this.panelContent.Width - 1;
  int height = this.panelContent.Height - 1;
  path.StartFigure();
  path.AddLine(16, 0, width - 16, 0);
  path.AddArc(width - 32, 0, 32, 32, 270, 90);
  path.AddLine(width, 16, width, height - 16);
  path.AddArc(width - 32, height - 32, 32, 32, 0, 90);
  path.AddLine(width - 16, height, 16, height);
  path.AddArc(0, height - 32, 32, 32, 90, 90);
  path.AddLine(0, height - 16, 0, 16);
  path.AddArc(0, 0, 32, 32, 180, 90);
  path.CloseFigure();

  Brush b = new SolidBrush(Color.FromArgb(239, 239, 239));
  e.Graphics.FillPath(b, path);
  b.Dispose();
  Pen parc = new Pen(Brushes.LightGray, 1);
  parc.LineJoin = LineJoin.Round;
  e.Graphics.DrawPath(parc, path);

  for(int i=0;i<4;i++)
  {
   Pen p1 = new Pen(Color.FromArgb(132 + 30*i, 132 + 30*i, 132 + 30*i));
   e.Graphics.DrawArc(p1, this.panelContent.Width - 32 + i,
this.panelContent.Height - 32 + i, 32, 32, 0, 90);
   p1.Dispose();
  }
 }

 private void Form1_Paint(object sender,
System.Windows.Forms.PaintEventArgs e)
 {

  e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
  for(int i=0;i<4;i++)
  {
   Pen p1 = new Pen(Color.FromArgb(132 + 30*i, 132 + 30*i, 132 + 30*i));
   e.Graphics.DrawLine(p1, this.panelContent.Right + i,
this.panelContent.Top + 16 + i,
    this.panelContent.Right + i, this.panelContent.Bottom - 13 + i);
   e.Graphics.DrawLine(p1, this.panelContent.Left + 16 + i,
this.panelContent.Bottom + i,
    this.panelContent.Right - 13 + i, this.panelContent.Bottom + i);
   e.Graphics.DrawArc(p1, this.panelContent.Right - 32 + i,
this.panelContent.Bottom - 32 + i, 32, 32, 0, 90);
   p1.Dispose();
  }
 }
}
}