0.新建窗体 及添加按钮

1.
  执行如下按钮事件
  private void btnFormMax_Click(object sender, EventArgs e)
  {
     if (this.WindowState == FormWindowState.Maximized)
     {
         this.WindowState = FormWindowState.Normal;
     }
     else
     {
         this.WindowState = FormWindowState.Maximized;
     }
  }
  窗体最大化时 非全屏 不会遮盖任务栏
  此时this.FormBorderStyle 默认为 Sizable

2.
  执行如下按钮事件
  private void btnFormMax_Click(object sender, EventArgs e)
  {
      if (this.WindowState == FormWindowState.Maximized)
      {        
         this.WindowState = FormWindowState.Normal;
      }
      else
      {
         this.FormBorderStyle = FormBorderStyle.None;
         this.WindowState = FormWindowState.Maximized;
      }
   }

窗体最大化时 会全屏 及遮盖任务栏
  此时this.FormBorderStyle 为 None 不会显示窗体标题栏等相关

3.
  执行如下按钮事件
  private void btnFormMax_Click(object sender, EventArgs e)
  {
      if (this.WindowState == FormWindowState.Maximized)
      {        
         this.WindowState = FormWindowState.Normal;
      }
      else
      {
         this.FormBorderStyle = FormBorderStyle.None;
         this.MaximumSize = new Size(Screen.PrimaryScreen.WorkingArea.Width, Screen.PrimaryScreen.WorkingArea.Height);
         this.WindowState = FormWindowState.Maximized;
      }
   }

窗体最大化时 非全屏 不会遮盖任务栏
  此时this.FormBorderStyle 为 None 不会显示窗体标题栏等相关