Capture screenshot of active window?

Question

I am making a screen capturing application and everything is going fine. All I need to do is capture the active window and take a screenshot of this active window. Does an

Answer 1

KvanTTT's code worked great. I extended it a bit to allow a little more flexibility on save format, as well as the ability to save by hWnd, .NET Control/Form. You can get a bitmap or save to file, with a few options.

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace MosaiqPerformanceMonitor {
     public enum CaptureMode {
          Screen, Window
     }

     public static class ScreenCapturer {
          [DllImport("user32.dll")]
          private static extern IntPtr GetForegroundWindow();

          [DllImport("user32.dll")]
          private static extern IntPtr GetWindowRect(IntPtr hWnd, ref Rect rect);

          [StructLayout(LayoutKind.Sequential)]
          private struct Rect {
                public int Left;
                public int Top;
                public int Right;
                public int Bottom;
          }

          [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
          public static extern IntPtr GetDesktopWindow();

          /// 
 Capture Active Window, Desktop, Window or Control by hWnd or .NET Contro/Form and save it to a specified file.  
          /// Filename.
          /// * If extension is omitted, it's calculated from the type of file
          /// * If path is omitted, defaults to %TEMP%
          /// * Use %NOW% to put a timestamp in the filename
          /// Optional. The default value is CaptureMode.Window.
          /// Optional file save mode.  Default is PNG
          public static void CaptureAndSave(string filename, CaptureMode mode = CaptureMode.Window, ImageFormat format = null) {
                ImageSave(filename, format, Capture(mode));
          }

          /// 
 Capture a specific window (or control) and save it to a specified file.  
          /// Filename.
          /// * If extension is omitted, it's calculated from the type of file
          /// * If path is omitted, defaults to %TEMP%
          /// * Use %NOW% to put a timestamp in the filename
          /// hWnd (handle) of the window to capture
          /// Optional file save mode.  Default is PNG
          public static void CaptureAndSave(string filename, IntPtr handle, ImageFormat format = null) {
                ImageSave(filename, format, Capture(handle));
          }

          /// 
 Capture a specific window (or control) and save it to a specified file.  
          /// Filename.
          /// * If extension is omitted, it's calculated from the type of file
          /// * If path is omitted, defaults to %TEMP%
          /// * Use %NOW% to put a timestamp in the filename
          /// Object to capture
          /// Optional file save mode.  Default is PNG
          public static void CaptureAndSave(string filename, Control c, ImageFormat format = null) {
                ImageSave(filename, format, Capture(c));
          }
          /// 
 Capture the active window (default) or the desktop and return it as a bitmap 
          /// Optional. The default value is CaptureMode.Window.
          public static Bitmap Capture(CaptureMode mode = CaptureMode.Window) {
                return Capture(mode == CaptureMode.Screen ? GetDesktopWindow() : GetForegroundWindow());
          }

          /// 
 Capture a .NET Control, Form, UserControl, etc. 
          /// Object to capture
          ///  Bitmap of control's area 
          public static Bitmap Capture(Control c) {
                return Capture(c.Handle);
          }


          /// 
 Capture a specific window and return it as a bitmap 
          /// hWnd (handle) of the window to capture
          public static Bitmap Capture(IntPtr handle) {
                Rectangle bounds;
                var rect = new Rect();
                GetWindowRect(handle, ref rect);
                bounds = new Rectangle(rect.Left, rect.Top, rect.Right - rect.Left, rect.Bottom - rect.Top);
                CursorPosition = new Point(Cursor.Position.X - rect.Left, Cursor.Position.Y - rect.Top);

                var result = new Bitmap(bounds.Width, bounds.Height);
                using (var g = Graphics.FromImage(result))
                     g.CopyFromScreen(new Point(bounds.Left, bounds.Top), Point.Empty, bounds.Size);

                return result;
          }

          /// 
 Position of the cursor relative to the start of the capture 
          public static Point CursorPosition;


          /// 
 Save an image to a specific file 
          /// Filename.
          /// * If extension is omitted, it's calculated from the type of file
          /// * If path is omitted, defaults to %TEMP%
          /// * Use %NOW% to put a timestamp in the filename
          /// Optional file save mode.  Default is PNG
          /// Image to save.  Usually a BitMap, but can be any
          /// Image.
          static void ImageSave(string filename, ImageFormat format, Image image) {
                format = format ?? ImageFormat.Png;
                if (!filename.Contains("."))
                     filename = filename.Trim() + "." + format.ToString().ToLower();

                if (!filename.Contains(@"\"))
                     filename = Path.Combine(Environment.GetEnvironmentVariable("TEMP") ?? @"C:\Temp", filename);

                filename = filename.Replace("%NOW%", DateTime.Now.ToString("yyyy-MM-dd@hh.mm.ss"));
                image.Save(filename, format);
          }
     }
}