Why is a winform shown different on the same 4k monitor

Question

We have a legacy .net windows forms application. We've already replaced most of it with a web application, but still use the windows forms application for administration tasks.

When this application is used on a 4k monitor, it looks really bad, and that's understandable.

But the look changes depending of other monitors on the system. The test is done on a lenovo 430 which has a 4k monitor connected via display port. The resolution on the laptop monitor is 1920/1080 and on the 4k monitor its 3840/2160. The display scaling is always 200%.

All of the following screen shots were taken on the 4k monitor.

1. One test with both monitor active
2. One test with second screen only
3. One test with the notebook monitor closed (which means only one monitor available for the system)

I am aware that our application will not look good on 4k without changes and setting dpiAware/dpiAwareness.

But my question is: Why does the applications look change only depending on which monitors are active? And is there a way to control which "kind" of look is taken?

Answer 1

Maybe you are aware of this, but it was not mentioned before:

There are 3 kinds of applications:

• Not DPI aware
• System DPI aware
• Per monitor DPI aware

Details here: https://msdn.microsoft.com/de-de/library/windows/desktop/dn469266(v=vs.85).aspx

You can set the DPI-awareness on your application in app.config:

<appSettings>
    <add key="EnableWindowsFormsHighDpiAutoResizing" value="false" />
</appSettings>

and app.manifest:

<application xmlns="urn:schemas-microsoft-com:asm.v3">
  <windowsSettings>
    <dpiAware xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">false</dpiAware>
  </windowsSettings>
</application>

When you have a DPI >= 150% scaling can additionally controlled via RMB on application -> Properties -> Compatibility -> Scaling on high DPI

For Windows Forms the default is DPI-Scaling disables for high DPI. So everything shoud work out of the box. Maybe one of this settings is not on default? Especially the configuration in the RMB properties on the exe?