C++ MFC button disappears at window resize - c++

Question

I have a Dialog in MFC C++ that has a CButton attached.
I want to modify OnSize() so that the button will anchor to bottom-left.
if (btn.m_hWnd) {
CRect winRect;
GetWindowRect(&winRect);
int width = winRect.right - winRect.left;
int height = winRect.bottom - winRect.top;
int x = width - wndWidth;
int y = height - wndHeight;
CRect rect;
btn.GetWindowRect(&rect);
rect.left += x;
rect.top += y;
btn.MoveWindow(&rect);
ScreenToClient(&rect);
btn.ShowWindow(SW_SHOW);
}
x and y are the difference of how much the window has changed and will be added to the button's start coordinates.
I am not sure about the last 2 commands (I might as well delete them) but then I run the program the button disappears.
I need to know a way to move the button by x and y.

Answer 1

The original code was using the wrong coordinate system for both the parent dialog, and the button.
A correct way to dock to the bottom left would be like this:
if (btn.m_hWnd) {
CRect winRect;
GetClientRect(&winRect);
CRect rect;
btn.GetWindowRect(&rect);
ScreenToClient(&rect);
int btnWidth = rect.Width();
int btnHeight = rect.Width();
rect.left = winRect.right-btnWidth;
rect.top = winRect.bottom-btnHeight;
rect.right = winRect.right;
rect.bottom = winRect.bottom;
btn.MoveWindow(&rect);
}
OR
if (btn.m_hWnd) {
CRect winRect;
GetClientRect(&winRect);
CRect rect;
btn.GetWindowRect(&rect);
ScreenToClient(&rect);
int btnWidth = rect.Width();
int btnHeight = rect.Width();
btn.SetWindowPos(NULL,winRect.right-btnWidth,winRect.bottom-btnHeight,0,0,SWP_NOSIZE|SWP_NOZORDER);
}

Answer 2

Basically, the answer should be to do ScreenToClient before MoveWindow!
Some more detail: You need to familiarize yourself with what function returns or uses client coordinates (and relative to what these client coordinates are) and which screen coordinates; this is one of the parts of MFC which can be really confusing. As to your question:
CWnd::GetWindowRect returns screen coordinates; CWnd::MoveWindow expects coordinates relative to the parent CWnd (or to the screen if it's a top level window). So before calling MoveWindow, you have to convert the rect returned by GetWindowRect to client coordinates of your parent window; suppose pParent is the CWnd * to the parent window of btn, then your moving code should look like this:
CRect rect;
btn.GetWindowRect(&rect);
pParent->ScreenToClient(&rect); // this line and it's position is important
rect.left += x;
rect.top += y;
btn.MoveWindow(&rect);
If you're in a method of the parent window (as I think you are, since you mention OnSize of the dialog), then just leave out the pParent->. The way you do ScreenToClient at the moment, it has no effect on MoveWindow, since it's executed after it!