Get a screenshot of a specific application

Question

I know I can get the screenshot of the entire screen using Graphics.CopyFromScreen(). However, what if I just want the screenshot of a specific application?

Answer 1

You could look into P/Invoking the win32 way of doing this, an article to this effect... sort of.

Basically, go through the trouble of setting up a DC to a bitmap and send WM_PRINT to the application window in question. Its pretty nasty, all told, but may work for you.

Functions you may need: SendMessage, GetDC, CreateCompatibleBitmp, and SelectObject.

I can't say I've ever done this before, but this is how I'd attack the problem. (Well, I'd probably do it in pure C but still; roughly the way I'd attack it).

Answer 2

Here's some code to get you started:

public void CaptureApplication(string procName)
{
    var proc = Process.GetProcessesByName(procName)[0];
    var rect = new User32.Rect();
    User32.GetWindowRect(proc.MainWindowHandle, ref rect);

    int width = rect.right - rect.left;
    int height = rect.bottom - rect.top;

    var bmp = new Bitmap(width, height, PixelFormat.Format32bppArgb);
    Graphics graphics = Graphics.FromImage(bmp);
    graphics.CopyFromScreen(rect.left, rect.top, 0, 0, new Size(width, height), CopyPixelOperation.SourceCopy);

    bmp.Save("c:\\tmp\\test.png", ImageFormat.Png);
}

private class User32
{
    [StructLayout(LayoutKind.Sequential)]
    public struct Rect
    {
     public int left;
     public int top;
     public int right;
     public int bottom;
    }

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

It works, but needs improvement:

• You may want to use a different mechanism to get the process handle (or at least do some defensive coding)
• If your target window isn't in the foreground, you'll end up with a screenshot that's the right size/position, but will just be filled with whatever is in the foreground (you probably want to pull the given window into the foreground first)
• You probably want to do something other than just save the bmp to a temp directory

Answer 3

Based on Alconja's answer, I made a few improvements:

[StructLayout(LayoutKind.Sequential)]
public struct Rect
{
    public int left;
    public int top;
    public int right;
    public int bottom;
}

[DllImport("user32.dll")]
private static extern int SetForegroundWindow(IntPtr hWnd);

private const int SW_RESTORE = 9;

[DllImport("user32.dll")]
private static extern IntPtr ShowWindow(IntPtr hWnd, int nCmdShow);

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

public Bitmap CaptureApplication(string procName)
{
    Process proc;

    // Cater for cases when the process can't be located.
    try
    {
        proc = Process.GetProcessesByName(procName)[0];
    }
    catch (IndexOutOfRangeException e)
    {
        return null;
    }

    // You need to focus on the application
    SetForegroundWindow(proc.MainWindowHandle);
    ShowWindow(proc.MainWindowHandle, SW_RESTORE);

    // You need some amount of delay, but 1 second may be overkill
    Thread.Sleep(1000);

    Rect rect = new Rect();
    IntPtr error = GetWindowRect(proc.MainWindowHandle, ref rect);

    // sometimes it gives error.
    while (error == (IntPtr)0)
    {
        error = GetWindowRect(proc.MainWindowHandle, ref rect);
    }

    int width = rect.right - rect.left;
    int height = rect.bottom - rect.top;

    Bitmap bmp = new Bitmap(width, height, PixelFormat.Format32bppArgb);
    Graphics.FromImage(bmp).CopyFromScreen(rect.left,
                                           rect.top,
                                           0,
                                           0,
                                           new Size(width, height),
                                           CopyPixelOperation.SourceCopy);

    return bmp;
}

Answer 4

The PrintWindow win32 api will capture a window bitmap even if the window is covered by other windows or if it is off screen:

[DllImport("user32.dll")]
public static extern bool GetWindowRect(IntPtr hWnd, out RECT lpRect);
[DllImport("user32.dll")]
public static extern bool PrintWindow(IntPtr hWnd, IntPtr hdcBlt, int nFlags);

public static Bitmap PrintWindow(IntPtr hwnd)    
{       
    RECT rc;        
    GetWindowRect(hwnd, out rc);

    Bitmap bmp = new Bitmap(rc.Width, rc.Height, PixelFormat.Format32bppArgb);        
    Graphics gfxBmp = Graphics.FromImage(bmp);        
    IntPtr hdcBitmap = gfxBmp.GetHdc();        

    PrintWindow(hwnd, hdcBitmap, 0);  

    gfxBmp.ReleaseHdc(hdcBitmap);               
    gfxBmp.Dispose(); 

    return bmp;   
}