I need to get correct size (width / height) of window's client area, when window is being resized. I tried to use GetClientRect, but it ALWAYS gives WRONG values (see details below).
User32.RECT clientRECT;
User32.GetClientRect(_handle, out clientRECT);
So I decided to store x- and y-delta, between window rect (with borders) and client rect, when window is created, and then sum up this delta values with window rect, received from lParam of WM_SIZING message.
var windowRECT = (User32.RECT)Marshal.PtrToStructure(lParam, typeof(User32.RECT));
var clientAreaWidth = ((windowRECT.right - windowRECT.left) + _xDelta);
var clientAreaHeight = ((windowRECT.bottom - windowRECT.top) + _yDelta);
This trick ALWAYS gives RIGHT size.
But why GetClientRect has such strange behavior? So I wrote this code to get more details about what is going on.
Debug.WriteLine("dX: " + (clientRECT.right - clientAreaWidth).ToString() + ", dY: " + (clientRECT.bottom - clientAreaHeight).ToString());
Then I started project and began dragging right-bottom corner of the window. Here's the output:
dX: 16, dY: 17 // fast dragging in NW-direction
dX: 21, dY: 16
dX: 34, dY: 25
dX: 92, dY: 62
dX: 110, dY: 75
dX: 94, dY: 65
dX: 60, dY: 48
dX: 22, dY: 19
...
dX: 0, dY: 1 // slow dragging in NW-direction
dX: 0, dY: 2
dX: 0, dY: 1
dX: 0, dY: 1
dX: 0, dY: 1
dX: 1, dY: 1
dX: 0, dY: 1
dX: 1, dY: 2
dX: 0, dY: 1
...
dX: -16, dY: -12 // fast dragging in SE-direction
dX: -68, dY: -65
dX: -77, dY: -75
dX: -53, dY: -43
dX: -22, dY: -17
dX: -5, dY: -4
...
dX: -1, dY: 0 // slow dragging in SE-direction
dX: -1, dY: 0
dX: -1, dY: 0
dX: -1, dY: -1
dX: -1, dY: 0
dX: -1, dY: -1
dX: -1, dY: 0
dX: 0, dY: -1
dX: -1, dY: 0
dX: 0, dY: -1
What cause this behavior?
