Allow set font application-wide

[RussKie]

Is your feature request related to a problem? Please describe.

The default font has been updated in .NET Core 3.0 (#656) and documented. However for some users there is still an element of surprise when they migrate their apps to .NET Core.
We've received several questions regarding different sizes of forms (e.g. #1122, #1827, etc.).

Whilst the new default font is here to stay, some users may wish to retain the original font (e.g. due to a design of their app). However for an application with more than a handful of forms, setting the original font may be tedious and cumbersome exercise.

Describe the solution you'd like

Add the ability to set an application-wide font, similar to SetHighDpiMode() or SetCompatibleTextRenderingDefault() methods.

• This method must be "run only once" kind, i.e. a user may not be allowed to invoke it once an app has started.
• Any form that doesn't explicitly specify its own font, must inherit the application default font.

API Proposal

namespace System.Windows.Forms
{
    public partial class Application
    {
        public void SetDefaultFont(Font font);
    }
}

API Usage

class Program
{
    [STAThread]
    static void Main()
    {
        Application.SetHighDpiMode(HighDpiMode.SystemAware);
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);

        Application.SetDefaultFont(new Font(new FontFamily("Microsoft Sans Serif"), 8f));

        Application.Run(new Form1());
    }
}

Will this feature affect UI controls?

No

[merriemcgaw]

We should totally do this for .NET 5, I think this is a great idea.

[terrajobst]

• This method must be "run only once" kind, i.e. a user may not be allowed to invoke it once an app has started.

Are you going to throw an exception when the SetDefaultFont is called after Run? If so, I'd throw InvalidOperationException.

[RussKie]

• This method must be "run only once" kind, i.e. a user may not be allowed to invoke it once an app has started.

Are you going to throw an exception when the SetDefaultFont is called after Run? If so, I'd throw InvalidOperationException.

I propose we follow the same pattern set by SetCompatibleTextRenderingDefault() method and throw if SetDefaultFont() is called after the application has started:

winforms/src/System.Windows.Forms/src/System/Windows/Forms/Application.cs

Lines 1208 to 1215 in 18f1c81

	public static void SetCompatibleTextRenderingDefault(bool defaultValue)
	{
	if (NativeWindow.AnyHandleCreated)
	{
	throw new InvalidOperationException(SR.Win32WindowAlreadyCreated);
	}
	Control.UseCompatibleTextRenderingDefault = defaultValue;
	}

The rationale behind it, if a form has been created, it may have been laid out and rendered using one font, the font may have been cached etc., and setting a new font could be detrimental from both performance and UX.

[bartonjs]

Is there already a way to get the default font? (Get/set symmetry)

[weltkante]

probably Control.DefaultFont ?

[terrajobst]

Might be worthwhile to add a corresponding GetDefaultFont() to Application so that folks can just tweak parts, like:

[STAThread]
static void Main()
{
    Application.SetHighDpiMode(HighDpiMode.SystemAware);
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);

    // Set default to 10pt
    var font = Application.GetDefaultFont();
    font = new Font(font.FontFamily, 10f);
    Application.SetDefaultFont(font);

    Application.Run(new Form1());
}

It would be odd if users had to call Control.DefaultFont to do this.

[RussKie]

The rationale behind this FR is to allow users to set the original font, that may have been used to design the app (i.e. Font-scaling pixel-perfect design scenario), to help with a migration from .NET Framework to .NET Core/.NET.
To this end, users are expected to set both FontFamily and FontSize explicitly, and not leverage the current default font.

I don't feel that leveraging the default font to change its properties fits the above scenario. And, to be honest, I'm struggling to think of a use-case that would require this.
Do you have a use-case that would necessitate this?

[terrajobst]

No objections, you're the domain expert. We just generally strive to avoid asymmetries like set-only APIs.

[RussKie]

We just generally strive to avoid asymmetries like set-only APIs.

So do we, but in this case we want to have set-once kind of API, akin to SetHighDpiMode() or EnableVisualStyles().
Thank you

[terrajobst]

Reviewed and approved offline:

namespace System.Windows.Forms
{
    public partial class Application
    {
        public void SetDefaultFont(Font font);
    }
}

[RussKie]

We can now implement this proposal. #4909 will facilitate a communication of the the font information to the designer process.

[kirsan31]

@RussKie #4009 is grate! But, I want to point out some one thing that related to this issue and #4009 too. It's system font scaling:

Currently explicitly set font of the control (most time in designer), will stop it's scaling with this technique. What will be if we set app wide font?
My thoughts on this:
Current behavior is not good at all. The System.Windows.Forms.AutoScaleMode.Font is default to the form and disabling this scale if you set the font is not obvious.
I want an ability to set the font and still be the part of system font scaling (at least if we use default font but different size / style). #3263 related. Good question what to do if we use not default (system) font at all? But in 90% of time we need a proportional increase of all controls, regardless of the font. For other 10% we have AutoScaleMode.Dpi or AutoScaleMode.None.

P.s. I want to remind some old annoying DataGridView Designer issue, that become more annoying due to working font scaling in .net. And I think it definitely needs to be addressed 😋

[mdtauk]

With Windows 10 21H2 changing the System font to Segoe UI Variable with its Small, Text, and Display OpticalSize variants - will WinForms be ready to handle that, and will there be suitable fallbacks for Windows 8, and older Windows 10 versions?

https://blogs.windows.com/windows-insider/2021/05/06/announcing-windows-10-insider-preview-build-21376/

[kirsan31]

@RussKie

Changing the text size I didn't observe the any changes to the font metrics in net472
...
but net6.0 appears to be updating the default font metrics

Yes, becouse .net framework doesn't respect system font, and .net core does (#656). But font scaling in .net will work only if you not set font explicitly (then it will be taken from Control.DefaultFont). And I see that size of the form is the same on all of your screenshots, it means that font scaling not working... That's all is my previous post about.

[RussKie]

And I see that size of the form is the same on all of your screenshots, it means that font scaling not working...

They are in fact different in net6.0 scenarios :)

[RussKie]

With Windows 10 21H2 changing the System font to Segoe UI Variable with its Small, Text, and Display OpticalSize variants - will WinForms be ready to handle that, and will there be suitable fallbacks for Windows 8, and older Windows 10 versions?

Thanks for the link. We get the default font from .NET runtime, and take it for granted:

winforms/src/System.Windows.Forms/src/System/Windows/Forms/Control.cs

Lines 1807 to 1819 in c93d8c8

	public static Font DefaultFont
	{
	get
	{
	if (s_defaultFont is null)
	{
	s_defaultFont = SystemFonts.MessageBoxFont;
	Debug.Assert(s_defaultFont is not null, "defaultFont wasn't set!");
	}
	return s_defaultFont;
	}
	}

If you're running that build already you could probably run a test yourself and see:
https://github.com/dotnet/runtime/blob/64303750a9198a49f596bcc3aa13de804e421579/src/libraries/System.Drawing.Common/src/System/Drawing/SystemFonts.Windows.cs#L85-L99

I think it may just work out of the box... @safern any thoughts?

[Zheng-Li01]

Verified the issue with latest 6.0.0-preview.5.21275.7, the main functions have been fully merged/implemented, found a new issue too and filed it as #4978

[imba-tjd]

Question: Can WPF set default font? rather than set FontFamily in every xaml. I had googled it but got no answer.

Edit:

I managed to set it by typeof(SystemFonts).GetField("_messageFontFamily", BindingFlags.Static | BindingFlags.NonPublic).SetValue(null, new System.Windows.Media.FontFamily("xxx"));

It's the first time I monkey patch in C#, interesting. Previously I only did this kind of things in Python.

[weltkante]

@imba-tjd you can try setting the default value of the FontFamily and related properties in the metadata before creating any window or control. In general this is the wrong repo to ask in though, you'll find more people knowing about WPF in the WPF repo.