How to know current bitmap-scaling factor for a Windows HWND?

Question

I have a small C# program, HelloCs.cs, like this:

using System;
using System.Drawing;
using System.Windows.Forms;

class HelloWorld: Form
{
     public static void Main()
     {
          Application.Run(new HelloWorld());
     }
     public HelloWorld()
     {
          Text = "Hello World";
          BackColor = Color.White;

     }
     protected override void OnPaint(PaintEventArgs pea)
     {
          Graphics grfx = pea.Graphics;

          this.Width = 200;
          this.Height = 100;

          grfx.DrawString(
              $"({this.Width} , {this.Height}) Hello, Windows Forms!", 
              Font, Brushes.Black, 0, 0);

          base.OnPaint(pea);
     }    
}

== CASE 1.0 ==

When run on a default Windows 7 system. It shows like this, nothing special:

The window width & height is 200 & 100 respectively.

== CASE 1.5 ==

When I set system DPI scaling to 150% (and restart OS), C# program still reports window width & height as before, BUT, the window's actual on-screen size(pixel count) becomes 300 and 150. That means, Windows system has apply so-called bitmap-scaling on my window.

Then my question is: How do I know(in my own program, or with an HWND handle), whether the system has applied bitmap-scaling to the window, and what is the scaling factor.

On Window 10, things becomes more complicated, because Windows 10 may apply different scaling factor to different HWND, according to the HWND's current residing monitor.

I hope to get an answer applicable to Both Windows 7 and 10.

[GetWindowDpiAwarenessContext](https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getwindowdpiawarenesscontext) seems to be what you are looking for. This is Windows 10 only, though. — IInspectable, Jun 01 '21 at 04:57
Try my answer here: https://stackoverflow.com/a/32888078/403671 — Simon Mourier, Jun 01 '21 at 06:09
@SimonMourier Looks like your answer does not work. On Win10.1909, calling WinAPI ``GetDpiForWindow(this.Handle);`` returns 96, when DPI scaling has been set to 150%. I expect to get a DPI value of 144. — Jimm Chen, Jun 01 '21 at 08:59
I think my code works fine, I just tested it again, but you can be fooled by presence/absence and content of a manifest. It's quite complex matter... Read this carefully https://learn.microsoft.com/en-us/windows/win32/hidpi/high-dpi-desktop-application-development-on-windows or provide a reproducing project, not only code — Simon Mourier, Jun 01 '21 at 09:19
@SimonMourier Thank you. I'm finding it out. With a manifest referring to and/or , GetDpiForWindow() now returns 144. So your solution is hopefully right. I'm still trying. — Jimm Chen, Jun 01 '21 at 09:52

score 2 · Accepted Answer · answered Jul 18 '21 at 13:11

After one month's investigation, I find it a VERY complicated problem. The situation on Windows 7, Windows 8.1 and Win10.1607+ are all different.

Finally, I managed to composed a demo program(based on an open source project) that can determine the bitmap-stretching factor by the system.

Talking is cheap, so here is my working code: https://github.com/chjfth/window-finder-control

Feature:

On Windows 7 and Win10.1607+, I can tell it exactly no matter the caller program itself is DPI-unaware, SysDpi-aware, or Per-monitor-aware.
On Windows 8.1 ~ Win10.1511, the caller program must be Per-monitor-aware to tell it exactly. This is due to lack of API support for Win81's rudimentary mix-mode DPI scaling.

See my demo UI below

Figure 1:

Explanation:

WinRect shows the RECT value by calling GetWindowRect, so called virtual RECT size.
Top-left red digits show the physical dimension of the target window "calculated or deduced" by the calling program.

They are both 400x241, so the target-window is not-bitmap stretched.

Figure 2: On Windows 7 with global DPI-scaling set to 150%, physical RECT is 150% larger than the virtual RECT(compared by width or height, width 600 vs 400). So, the target windows is 150% stretched.

Figure 3: On Win10.1909, we can also see 150% bitmap-stretched window.

Figure 4: On Win10.1909, we can even see bitmap-shrunk HWND. It happens when the caller is SysDpi-aware(150%), and, the target-windows is also SysDpi-aware but resides on a 100%-scaling monitor.

How to know current bitmap-scaling factor for a Windows HWND?

== CASE 1.0 ==

== CASE 1.5 ==

1 Answers1

See my demo UI below

Linked