Get a screenshot of a specific application

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);
    using (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

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;   
}

The reference to RECT above can be resolved with the following class:

[StructLayout(LayoutKind.Sequential)]
public struct RECT
{
    private int _Left;
    private int _Top;
    private int _Right;
    private int _Bottom;

    public RECT(RECT Rectangle) : this(Rectangle.Left, Rectangle.Top, Rectangle.Right, Rectangle.Bottom)
    {
    }
    public RECT(int Left, int Top, int Right, int Bottom)
    {
        _Left = Left;
        _Top = Top;
        _Right = Right;
        _Bottom = Bottom;
    }

    public int X {
        get { return _Left; }
        set { _Left = value; }
    }
    public int Y {
        get { return _Top; }
        set { _Top = value; }
    }
    public int Left {
        get { return _Left; }
        set { _Left = value; }
    }
    public int Top {
        get { return _Top; }
        set { _Top = value; }
    }
    public int Right {
        get { return _Right; }
        set { _Right = value; }
    }
    public int Bottom {
        get { return _Bottom; }
        set { _Bottom = value; }
    }
    public int Height {
        get { return _Bottom - _Top; }
        set { _Bottom = value + _Top; }
    }
    public int Width {
        get { return _Right - _Left; }
        set { _Right = value + _Left; }
    }
    public Point Location {
        get { return new Point(Left, Top); }
        set {
            _Left = value.X;
            _Top = value.Y;
        }
    }
    public Size Size {
        get { return new Size(Width, Height); }
        set {
            _Right = value.Width + _Left;
            _Bottom = value.Height + _Top;
        }
    }

    public static implicit operator Rectangle(RECT Rectangle)
    {
        return new Rectangle(Rectangle.Left, Rectangle.Top, Rectangle.Width, Rectangle.Height);
    }
    public static implicit operator RECT(Rectangle Rectangle)
    {
        return new RECT(Rectangle.Left, Rectangle.Top, Rectangle.Right, Rectangle.Bottom);
    }
    public static bool operator ==(RECT Rectangle1, RECT Rectangle2)
    {
        return Rectangle1.Equals(Rectangle2);
    }
    public static bool operator !=(RECT Rectangle1, RECT Rectangle2)
    {
        return !Rectangle1.Equals(Rectangle2);
    }

    public override string ToString()
    {
        return "{Left: " + _Left + "; " + "Top: " + _Top + "; Right: " + _Right + "; Bottom: " + _Bottom + "}";
    }

    public override int GetHashCode()
    {
        return ToString().GetHashCode();
    }

    public bool Equals(RECT Rectangle)
    {
        return Rectangle.Left == _Left && Rectangle.Top == _Top && Rectangle.Right == _Right && Rectangle.Bottom == _Bottom;
    }

    public override bool Equals(object Object)
    {
        if (Object is RECT) {
            return Equals((RECT)Object);
        } else if (Object is Rectangle) {
            return Equals(new RECT((Rectangle)Object));
        }

        return false;
    }
}