How can I know whether a form is showing to user? - winforms

Question

I have a system monitor program which cost system resource heavly. I hope the program run tasks only when the form is showing to users. One way is to check whether the form is foreground window or active window.
But if the form is not activated but is showing to users (for example, you can see two windows in following picture, one of the window is not activated but showing to users), I can't handle the situation well.
window example http://www.chip.de/ii/117599040_a933fb45fe.gif
Any ideas? Thanks in advanced.
Edit:
I have found the solution according to my colleague's suggest.
Here is my solution:
enum GetWindow_Cmd : uint
{
GW_HWNDFIRST = 0,
GW_HWNDLAST = 1,
GW_HWNDNEXT = 2,
GW_HWNDPREV = 3,
GW_OWNER = 4,
GW_CHILD = 5,
GW_ENABLEDPOPUP = 6
}
[DllImport("user32.dll", SetLastError = true)]
static extern bool IsZoomed(IntPtr hwnd);
[DllImport("user32.dll", SetLastError = true)]
static extern bool IsWindowVisible(IntPtr hwnd);
[DllImport("user32.dll", EntryPoint = "GetWindow", SetLastError = true)]
static extern IntPtr GetNextWindow(IntPtr hWnd, uint cmd);
[DllImport("user32.dll", EntryPoint = "GetWindowText", SetLastError = true)]
public static extern int GetWindowText(IntPtr hWnd, StringBuilder lpWindowText, int nMaxCount);
[DllImport("user32.dll", EntryPoint = "GetForegroundWindow")]
private static extern IntPtr GetForegroundWindow();
//I have made some simplifications. My form is maximized. If the form is not
// maximized, you should calculate the region of current form and top level form
// to check it.
private bool IsShowing()
{
if(this.Visible == false || this.WindowState = FormWindowState.Minimized)
return false;
IntPtr currentHandler = this.Handle;
if (currentHandler == GetForegroundWindow())
return true;
while (currentHandler != GetForegroundWindow())
{
IntPtr hwnd = GetNextWindow(currentHandler, (uint)GetWindow_Cmd.GW_HWNDPREV);
currentHandler = hwnd;
if (IsZoomed(currentHandler) && IsWindowVisible(currentHandler))
{
StringBuilder sb = new StringBuilder(255);
GetWindowText(currentHandler, sb, sb.Capacity + 1);
Debug.WriteLine("masked by:" + sb.ToString());
return false;
}
}
if (IsZoomed(currentHandler) && IsWindowVisible(currentHandler))
{
StringBuilder sb = new StringBuilder(255);
GetWindowText(currentHandler, sb, sb.Capacity + 1);
Debug.WriteLine("masked by:" + sb.ToString());
return false;
}
return true;
}

Answer 1

It is not clear when you want to run or not your tasks, but in any case, I think that the best approach is to let the system notify you when your form acvtive or visible state changes. In this case you should write the activation and deactivation tasks's code in the forms event Activated, Deactivate and/or VisibleChanged
From MSDN
Deactivate: Occurs when the form loses focus and is no longer the
active form.
Activated: Occurs when the form is activated in code or by the user.
VisibleChanged: Occurs when the Visible property value changes.
You could also find useful to refer to this article that shows the order of events in the loading and closing of a form