this.FormBorderStyle = FormBorderStyle.None;  //设置窗体为无边框样式
this.WindowState = FormWindowState.Maximized; //最大化窗体 

 [DllImport("user32.dll", EntryPoint = "ShowWindow")]
  public static extern Int32 ShowWindow(Int32 hwnd, Int32 nCmdShow);

  [DllImport("user32.dll", EntryPoint = "FindWindow")]
  private static extern Int32 FindWindow(string lpClassName, string lpWindowName);

using System;
using System.Windows.Forms;

using System.Drawing;
using System.Runtime.InteropServices;


namespace FullScr
{
 public partial class Form1 : Form
 {

  Boolean m_IsFullScreen = false;//标记是否全屏


  public Form1()
  {
   InitializeComponent();
  }


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

  /// <summary>
  /// 全屏按钮的Click事件
  /// </summary>
  /// <param name="sender"></param>
  /// <param name="e"></param>
  private void button1_Click(object sender, EventArgs e)
  {
   m_IsFullScreen = !m_IsFullScreen;//点一次全屏，再点还原。 
   this.SuspendLayout();
   if (m_IsFullScreen)//全屏 ,按特定的顺序执行
   {
    SetFormFullScreen(m_IsFullScreen);
    this.FormBorderStyle = FormBorderStyle.None;
    this.WindowState = FormWindowState.Maximized;
    this.Activate();//
   }
   else//还原，按特定的顺序执行——窗体状态，窗体边框，设置任务栏和工作区域
   {
    this.WindowState = FormWindowState.Normal;
    this.FormBorderStyle = FormBorderStyle.Sizable;
    SetFormFullScreen(m_IsFullScreen);
    this.Activate();
   }
   this.ResumeLayout(false);
  }

  /// <summary> 
  /// 设置全屏或这取消全屏 
  /// </summary> 
  /// <param name="fullscreen">true:全屏 false:恢复</param> 
  /// <param name="rectOld">设置的时候，此参数返回原始尺寸，恢复时用此参数设置恢复</param> 
  /// <returns>设置结果</returns> 
  public Boolean SetFormFullScreen(Boolean fullscreen)//, ref Rectangle rectOld
  {
   Rectangle rectOld = Rectangle.Empty;
   Int32 hwnd = 0;
   hwnd = FindWindow("Shell_TrayWnd", null);//获取任务栏的句柄

   if (hwnd == 0) return false;

   if (fullscreen)//全屏
   {
    ShowWindow(hwnd, SW_HIDE);//隐藏任务栏

    SystemParametersInfo(SPI_GETWORKAREA, 0, ref rectOld, SPIF_UPDATEINIFILE);//get屏幕范围
    Rectangle rectFull = Screen.PrimaryScreen.Bounds;//全屏范围
    SystemParametersInfo(SPI_SETWORKAREA, 0, ref rectFull, SPIF_UPDATEINIFILE);//窗体全屏幕显示
   }
   else//还原 
   {
    ShowWindow(hwnd, SW_SHOW);//显示任务栏
    SystemParametersInfo(SPI_SETWORKAREA, 0, ref rectOld, SPIF_UPDATEINIFILE);//窗体还原
   }
   return true;
  }

  #region user32.dll

  public const Int32 SPIF_UPDATEINIFILE = 0x1;
  public const Int32 SPI_SETWORKAREA = 47;
  public const Int32 SPI_GETWORKAREA = 48;
  public const Int32 SW_SHOW = 5;
  public const Int32 SW_HIDE = 0;

  [DllImport("user32.dll", EntryPoint = "ShowWindow")]
  public static extern Int32 ShowWindow(Int32 hwnd, Int32 nCmdShow);

  [DllImport("user32.dll", EntryPoint = "FindWindow")]
  private static extern Int32 FindWindow(string lpClassName, string lpWindowName);

  [DllImport("user32.dll", EntryPoint = "SystemParametersInfo")]
  private static extern Int32 SystemParametersInfo(Int32 uAction, Int32 uParam, ref Rectangle lpvParam, Int32 fuWinIni);

  #endregion

 }
}

Modified Code
 public partial class FrmFullScreen : Form
 {
  Boolean m_IsFullScreen = false;//标记是否全屏

  public FrmFullScreen()
  {
   InitializeComponent();
  }
  /// <summary>
  /// 全屏按钮的Click事件
  /// </summary>
  /// <param name="sender"></param>
  /// <param name="e"></param>
  private void btnFullScreen_Click(object sender, EventArgs e)
  {
   m_IsFullScreen = !m_IsFullScreen;//点一次全屏，再点还原。 
   this.SuspendLayout();
   if (m_IsFullScreen)//全屏 ,按特定的顺序执行
   {
    SetFormFullScreen(m_IsFullScreen);
    this.FormBorderStyle = FormBorderStyle.None;
    this.WindowState = FormWindowState.Maximized;
    this.Activate();//
   }
   else//还原，按特定的顺序执行——窗体状态，窗体边框，设置任务栏和工作区域
   {
    this.WindowState = FormWindowState.Normal;
    this.FormBorderStyle = FormBorderStyle.Sizable;
    SetFormFullScreen(m_IsFullScreen);
    this.Activate();
   }
   this.ResumeLayout(false);
  }
  /// <summary>
  /// 全屏的快捷功能，F11相当于单机按钮；Esc健，如果全屏则退出全屏
  /// </summary>
  /// <param name="sender"></param>
  /// <param name="e"></param>
  private void btnFullScreen_KeyDown(object sender, KeyEventArgs e)
  {
   if (e.KeyCode == Keys.F11)
   {
    btnFullScreen.PerformClick();
    e.Handled = true;
   }
   else if (e.KeyCode == Keys.Escape)//esc键盘退出全屏
   {
    if (m_IsFullScreen)
    {
     e.Handled = true;
     this.WindowState = FormWindowState.Normal;//还原 
     this.FormBorderStyle = FormBorderStyle.Sizable;
     SetFormFullScreen(false);
    }
   }
  }
  /// <summary> 
  /// 设置全屏或这取消全屏 
  /// </summary> 
  /// <param name="fullscreen">true:全屏 false:恢复</param> 
  /// <param name="rectOld">设置的时候，此参数返回原始尺寸，恢复时用此参数设置恢复</param> 
  /// <returns>设置结果</returns> 
  public Boolean SetFormFullScreen(Boolean fullscreen)//, ref Rectangle rectOld
  {
   Rectangle rectOld=Rectangle.Empty;
   Int32 hwnd = 0;
   hwnd = FindWindow("Shell_TrayWnd", null);//获取任务栏的句柄

   if (hwnd == 0) return false;

   if (fullscreen)//全屏
   {
    ShowWindow(hwnd, SW_HIDE);//隐藏任务栏

    SystemParametersInfo(SPI_GETWORKAREA, 0, ref rectOld, SPIF_UPDATEINIFILE);//get 屏幕范围
    Rectangle rectFull = Screen.PrimaryScreen.Bounds;//全屏范围
    SystemParametersInfo(SPI_SETWORKAREA, 0, ref rectFull, SPIF_UPDATEINIFILE);//窗体全屏幕显示
   }
   else//还原 
   {
    ShowWindow(hwnd, SW_SHOW);//显示任务栏

    SystemParametersInfo(SPI_SETWORKAREA, 0, ref rectOld, SPIF_UPDATEINIFILE);//窗体还原
   }
   return true;
  }

  #region user32.dll

  [DllImport("user32.dll", EntryPoint = "ShowWindow")]
  public static extern Int32 ShowWindow(Int32 hwnd, Int32 nCmdShow);
  public const Int32 SW_SHOW = 5; public const Int32 SW_HIDE = 0;

  [DllImport("user32.dll", EntryPoint = "SystemParametersInfo")]
  private static extern Int32 SystemParametersInfo(Int32 uAction, Int32 uParam, ref Rectangle lpvParam, Int32 fuWinIni);
  public const Int32 SPIF_UPDATEINIFILE = 0x1;
  public const Int32 SPI_SETWORKAREA = 47;
  public const Int32 SPI_GETWORKAREA = 48;

  [DllImport("user32.dll", EntryPoint = "FindWindow")]
  private static extern Int32 FindWindow(string lpClassName, string lpWindowName);

  #endregion
 }