Question

This shouldnt be a difficult question, but it is difficult to google the question and get the idea across.

The problem is simple: I have a windows form where the user presses a button, then, it will wait on the user to click another window. It stores that selected window information for manipulation later (specifically the dimensions).

How can I get the active window of the next user click after a button is pressed?

Thanks

Answer 1

You need to get the foreground window.

[DllImport("user32.dll")]
static extern IntPtr GetForegroundWindow();

[DllImport("user32.dll", SetLastError = true)]
static extern bool GetWindowRect(IntPtr hWnd, out Rectangle lpRect); 


Rect rect = new Rect ();
GetWindowRect(GetForegroundWindow(), out rect);

//calculate width and height from rect

using (Bitmap bitmap = new Bitmap(width, height))
{
    using (Graphics g = Graphics.FromImage(bitmap))
    {
        Size size = new System.Drawing.Size(width, height);
        g.CopyFromScreen(new Point(rect.Left, rect.Top), Point.Empty, size);
    }
    bitmap.Save("C://test.jpg", ImageFormat.Jpeg);
}

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

I found most of the code in these two answers on SO. Modifed it to suit your question

Capture window

Find window width and height