https://msdn.microsoft.com/en-us/library/windows/desktop/ms533826(v=vs.85).aspx

The purpose of hit testing is to determine whether the cursor is over a given object, such as an icon or a button. The following example creates a plus-shaped region by forming the union of two rectangular regions. Assume that the variable point holds the
location of the most recent click. The code checks to see whether point is in the plus-shaped region. If point is in the region (a hit), the region is filled with an opaque red brush. Otherwise, the region is filled with a semitransparent red brush.

Point point(60, 10);
// Assume that the variable "point" contains the location
// of the most recent click.
// To simulate a hit, assign (60, 10) to point.
// To simulate a miss, assign (0, 0) to point.
SolidBrush solidBrush(Color());
Region region1(Rect(50, 0, 50, 150));
Region region2(Rect(0, 50, 150, 50));
// Create a plus-shaped region by forming the union of region1 and region2.
// The union replaces region1.
region1.Union(&region2);
if(region1.IsVisible(point, &graphics))
{
// The point is in the region. Use an opaque brush.
solidBrush.SetColor(Color(255, 255, 0, 0));
}
else
{
// The point is not in the region. Use a semitransparent brush.
solidBrush.SetColor(Color(64, 255, 0, 0));
}
graphics.FillRegion(&solidBrush, &region1);

LRESULT CMovableWinView::OnMouseMove(UINT uMsg, WPARAM wParam, LPARAM lParam, BOOL& bHandled)
{

curPos.X = GET\_X\_LPARAM(lParam);  
curPos.Y = GET\_Y\_LPARAM(lParam);  
if (wParam & MK\_LBUTTON)  
{  
    ;  
}  
Gdiplus::Graphics g(m\_hWnd);  
Gdiplus::Point point(curPos.X, curPos.Y);

Gdiplus::Rect rect(100, 100, 300, 300);  
Gdiplus::Pen pen(penColor);  
g.DrawRectangle(&pen, rect);

Gdiplus::Region region(rect);

Gdiplus::SolidBrush solidBrush(Gdiplus::Color(255, 255, 255, 255));  
if (region.IsVisible(point, &g))  
{  
    solidBrush.SetColor(Gdiplus::Color(255, 255, 0, 0));

}  
else  
{  
    solidBrush.SetColor(Gdiplus::Color(255, 255, 255, 255));  
}

rect.X += 1;  
rect.Y += 1;  
rect.Width -= 1;  
rect.Height -= 1;  
g.FillRectangle(&solidBrush, rect);  
return TRUE;  

}