PrintScreen only active window

Question

I'm writing a simple Windows Forms utility to screenshot either the entire screen or an active window and save the file. The winform has a button called 'Capture' that, when clicked, takes the screenshot.

I have the code entirely working for the entire screen but can't figure it out for the active window.

By my current code, if I select 'Active Window', it ends up Print Screening the application itself which isn't what I want.

Here's my code (under the Capture_Click method):

try
        {
            this.Hide();

            bmpScreenshot = new Bitmap(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height, PixelFormat.Format32bppArgb);
            //bmpScreenshot = new Bitmap(this.Bounds.Width, this.Bounds.Height, PixelFormat.Format32bppArgb);

            gfxScreenshot = Graphics.FromImage(bmpScreenshot);

            if (AltPrint.Checked)
            {
                gfxScreenshot.CopyFromScreen(this.Bounds.X, this.Bounds.Y, 0, 0, this.Bounds.Size, CopyPixelOperation.SourceCopy);
            }
            else
            {
                gfxScreenshot.CopyFromScreen(Screen.PrimaryScreen.Bounds.X, Screen.PrimaryScreen.Bounds.Y, 0, 0, Screen.PrimaryScreen.Bounds.Size, CopyPixelOperation.SourceCopy);
            }

            bmpScreenshot.Save(SaveLocation, ImageFormat.Png);

            number++;
        }

Answer 1

In your button_Click method, to capture a window other than the one which is doing the capturing, the only possible way I can see is to make use of the user32 Windows API.

You will first need to platform invoke some methods like this:

[DllImport("user32.dll", SetLastError = true)]
private static extern int GetWindowLong(IntPtr hWnd, GetWindowLongNIndex nIndex);

I recommend www.pinvoke.net/ for more info about this.

Here is my own Window class (work in progress, but useable) which you are welcome to use as an example: http://pastebin.com/fj2tEaPY

Some useful properties in there for you are:

• get AllWindows
• get TaskbarWindows
• set ForegroundWindow

You'll need to get the handle of your current window (use get ForegroundWindow, then the Handle property), then find the window which you want to capture the image of, perhaps using code similar to that in TaskbarWindows. Once you have the window, you will need to send the keypresses Alt + PrtScn (again maybe using user32 API, but I won't go into it.) Then use the static ForegroundWindow property to set the orignal window as the forground using the handle which you saved at the start.

Good luck!

Answer 2

As far as I can see your code uses the bounds of its own form to capture the "active" window. That is probably not what you intend.

If you want to use the mouse to "print" a window (and not hotkey which probably is easier to implement) you need to be able to from your own form let the user point to another window to capture. Here is an idea describing how to do that:

1. When the user presses the "Capture" button capture the mouse and initiate a drag operation.
2. Let the user drag the mouse (perhaps now having a "bullseye" or a "camera" cursor) to the window to capture.
3. When the user releases the mouse determine the window it was on top and use the location and size of this window to capture the correct pixels.

You will probably need to use P/Invoke to get information about all top-level windows on the desktop.

Note that "mouse capture" is very different from "screen capture". When an application "captures the mouse" it takes ownership of the mouse cursor and doesn't the ability to track the mouse even when it is moved outside the bounds of the window of the application.

Answer 3

Your comments seem to understand that once your app gets the focus (when you click on it) the other app has lost it and is no longer 'active'.

A possible alternative: your application gets a WM_SETFOCUS message when it gets focus. The wParam of that message is a handle to whichever window has just given up the focus -- and that window belongs to the app you want to capture.

Of course, that window will only be a subwindow or control, but you can figure out which app owns that button.

Answer 4

I ended up taking a different route.

I allow the user to resize my form window to cover exactly the area they need to screenshot, and then when they click the button, the form hides itself, takes a screenshot of the area it was covering and appears again. Contrary to my initial plan, I believe this will be more beneficial, as my targeted users (friends at an overclocking forum I'm part of) seldom take screenshots of just one window. They mostly need a couple of windows covered (temp monitoring, clockspeed monitoring, stability test etc.).

Thanks for all the answers guys. They definitely got me thinking, and made me realize what my options were.

Should anyone else need it, this is the code I use for the button that captures only part of the screen:

        try
        {
            this.Hide();
            Thread.Sleep(250);

            bmpScreenshot = new Bitmap(this.Bounds.Width, this.Bounds.Height, PixelFormat.Format32bppArgb);

            gfxScreenshot = Graphics.FromImage(bmpScreenshot);

            gfxScreenshot.CopyFromScreen(this.Bounds.X, this.Bounds.Y, 0, 0, this.Bounds.Size, CopyPixelOperation.SourceCopy);

            bmpScreenshot.Save(SaveLocation, ImageFormat.Png);

            tbxStatus.AppendText(Environment.NewLine);
            tbxStatus.AppendText(Environment.NewLine);
            tbxStatus.AppendText("Screenshot saved at " + SaveLocation);

            numSuffix++;
        }
        catch (Exception ex)
        {
            tbxStatus.AppendText(Environment.NewLine);
            tbxStatus.AppendText(Environment.NewLine);
            tbxStatus.AppendText("Unable to take screenshot. Exception: " + ex.ToString());
        }
        finally
        {
            this.Show();
        }

Answer 5

You need to borrow the design of commercially available screen capturing apps, such as Psp. I think you need to use a 'hot' key to trigger the capturing of the active window. Instead of doing it on Capture_Click event. This way, you can hide your application's window so that it does not become the active window. Then when the user pushes the 'hot' key, you can capture it.

Answer 6

What about SendKeys?

Windows.Forms.SendKeys.Send("%{PRTSC}");

It will copy currently selected window in to Clipboard object.

Than calling

BitmapSource image = Clipboard.GetImage();

you can get your image back