The GetWindowPicture subroutine uses API functions to get the window's device context (DC) and uses BitBlt to copy the window's image.

Note that this method copies what is currently visible on the window. If the window is partially covered by another window, the result includes the covering window's image.

Private Function GetWindowPicture(ByVal hWnd As Integer) As _
    Bitmap
    Dim g As Graphics
    Dim hdcDest As IntPtr
    Dim hdcSrc As Integer
    Dim bm As Bitmap
    Dim r As New RECT()
    Dim w, h As Integer

    GetWindowRect(hWnd, r)

    w = r.Right - r.Left
    h = r.Bottom - r.Top
    bm = New Bitmap(w, h)
    g = g.FromImage(bm)

    hdcSrc = GetWindowDC(hWnd)
    hdcDest = g.GetHdc

    BitBlt(hdcDest.ToInt32, 0, 0, w, h, hdcSrc, 0, 0, _
        SRCCOPY)

    g.ReleaseHdc(hdcDest)
    ReleaseDC(hWnd, hdcSrc)

    Return bm
End Function