Сделать скрин выбранного окна

using System;
using System.Linq;
using System.Drawing;
using System.Threading;
using System.Reflection;
using System.Diagnostics;
using System.Windows.Forms;
using System.Drawing.Imaging;
using System.Text.RegularExpressions;
using System.Runtime.InteropServices;
 
namespace ScreenShot {
  internal sealed class frmMain : Form {
    public frmMain() {
      InitializeComponent();
    }
    
    private ComboBox   cbProcess;
    private Button     btnShot;
    private PictureBox pbPreview;
    
    private const String USER32  = "user32.dll";
    private const UInt32 SW_HIDE = 0;
    private const UInt32 SW_SHOW = 5;
    
    private static _GetWindowRect GetWindowRect {get; set;}
    private static _SetForegroundWindow SetForegroundWindow {get; set;}
    private static _ShowWindow ShowWindow {get; set;}
    
    private static T GetProc<T>(String dll, String fun) where T : class {
      var meths = Assembly
          .GetAssembly(typeof(Regex))
          .GetType("Microsoft.Win32.UnsafeNativeMethods")
          .GetMethods(BindingFlags.Static | BindingFlags.Public)
          .Where(m => new Regex(@"\AGet(ProcA|ModuleH)").IsMatch(m.Name))
          .ToList();
      IntPtr ptr = (IntPtr)meths[1].Invoke(null, new Object[] {
        new HandleRef(new IntPtr(), (IntPtr)meths[0].Invoke(null, new Object[] {dll})), fun
      });
      
      if (ptr == IntPtr.Zero) {
        throw new InvalidOperationException("Could not find specified signature.");
      }
      
      return Marshal.GetDelegateForFunctionPointer(ptr, typeof(T)) as T;
    }
    
    [StructLayout(LayoutKind.Sequential)]
    internal struct RECT {
      internal Int32 left;
      internal Int32 top;
      internal Int32 right;
      internal Int32 bottom;
    }
    
    [UnmanagedFunctionPointer(CallingConvention.Winapi)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private delegate Boolean _GetWindowRect(
        IntPtr hWnd,
        ref RECT rect
    );
    
    [UnmanagedFunctionPointer(CallingConvention.Winapi)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private delegate Boolean _SetForegroundWindow(
        IntPtr hWnd
    );
    
    [UnmanagedFunctionPointer(CallingConvention.Winapi)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private delegate Boolean _ShowWindow(
        IntPtr hWnd,
        UInt32 nCmdShow
    );
    
    private static void LocateSignatures() {
      GetWindowRect = GetProc<_GetWindowRect>(USER32, "GetWindowRect");
      SetForegroundWindow = GetProc<_SetForegroundWindow>(USER32, "SetForegroundWindow");
      ShowWindow = GetProc<_ShowWindow>(USER32, "ShowWindow");
    }
    
    private void InitializeComponent() {
      this.cbProcess = new ComboBox() {
        Location = new Point(13, 13)
      };
      this.btnShot   = new Button() {
        Location = new Point(13, 37),
        Text = "Shot",
        Width = 120
      };
      this.btnShot.Click += (s, e) => {
        Int32 height, width;
        Process proc = Process.GetProcessesByName(cbProcess.SelectedItem.ToString())[0];
        RECT rect = new RECT();
        
        if (GetWindowRect(proc.MainWindowHandle, ref rect)) {
          ShowWindow(this.Handle, SW_HIDE);
          Thread.Sleep(500);
          SetForegroundWindow(proc.MainWindowHandle);
          Thread.Sleep(500);
          height = rect.bottom - rect.top;
          width  = rect.right - rect.left;
          
          using (Bitmap bmp = new Bitmap(width, height, PixelFormat.Format32bppArgb)) {
            using (Graphics g = Graphics.FromImage(bmp)) {
              g.CopyFromScreen(rect.left, rect.top, 0, 0, new Size(width, height), CopyPixelOperation.SourceCopy);
              pbPreview.Image = new Bitmap(bmp);
            }
          }
          ShowWindow(this.Handle, SW_SHOW);
          SetForegroundWindow(this.Handle);
        }
      };
      this.pbPreview = new PictureBox() {
        BorderStyle = BorderStyle.FixedSingle,
        Location = new Point(143, 13),
        Size = new Size(370, 250),
        SizeMode = PictureBoxSizeMode.StretchImage
      };
      this.ClientSize = new Size(530, 290);
      this.Controls.AddRange(new Control[] {this.cbProcess, this.btnShot, this.pbPreview});
      this.FormBorderStyle = FormBorderStyle.FixedSingle;
      this.MaximizeBox = false;
      this.StartPosition = FormStartPosition.CenterScreen;
      this.Text = "ScreenShot";
      this.Load += (s, e) => {
        LocateSignatures();
        Process
            .GetProcesses()
            .Where(p => p.MainWindowHandle != IntPtr.Zero)
            .Select(p => p.ProcessName)
            .ToList()
            .ForEach(p => cbProcess.Items.Add(p));
        cbProcess.SelectedIndex = 0;
      };
    }
  }
  
  internal sealed class Program {
    [STAThread]
    static void Main() {
      Application.EnableVisualStyles();
      Application.Run(new frmMain());
    }
  }
}