using System;
using System.Windows.Forms;
using System.Drawing;
namespace CSImageFullScreenSlideShow
{
 public class FullScreen
 {
  private FormWindowState winState;
  private FormBorderStyle brdStyle;
  private bool topMost;
  private Rectangle bounds;
  public FullScreen()
  {
   IsFullScreen = false;
  }
  public bool IsFullScreen
  {
   get;
   set;
  }
  public void EnterFullScreen(Form targetForm)
  {
   if (!IsFullScreen)
   {
    Save(targetForm); // Save the original form state.
    targetForm.WindowState = FormWindowState.Maximized;
    targetForm.FormBorderStyle = FormBorderStyle.None;
    targetForm.TopMost = true;
    targetForm.Bounds = Screen.GetBounds(targetForm);
    IsFullScreen = true;
   }
  }
  /// <summary>
  /// Save the current Window state.
  /// </summary>
  private void Save(Form targetForm)
  {
   winState = targetForm.WindowState;
   brdStyle = targetForm.FormBorderStyle;
   topMost = targetForm.TopMost;
   bounds = targetForm.Bounds;
  }
  /// <summary>
  /// Leave the full screen mode and restore the original window state.
  /// </summary>
  public void LeaveFullScreen(Form targetForm)
  {
   if (IsFullScreen)
   {
    // Restore the original Window state.
    targetForm.WindowState = winState;
    targetForm.FormBorderStyle = brdStyle;
    targetForm.TopMost = topMost;
    targetForm.Bounds = bounds;
    IsFullScreen = false;
   }
  }
 }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
namespace CSImageFullScreenSlideShow
{
 public partial class Test : Form
 {
  public Test()
  {
   InitializeComponent();
   }
  private FullScreen fullScreen = new FullScreen();
  private void button1_Click(object sender, EventArgs e)
  {
   if (fullScreen.IsFullScreen)
   {
    fullScreen.LeaveFullScreen(this);
   }
   else
   {
    fullScreen.EnterFullScreen(this);
   }
  }
 }
}