MainPage.xaml.cs

using System.Diagnostics;

namespace MauiApp1;

public partial class MainPage : ContentPage
{

	public const double MyFontSize = 20;
	public MainPage()
	{
		InitializeComponent();
	}

	async void OnNewButtonClicked(object sender, EventArgs e)
	{
		await Navigation.PushAsync(new Pages.Template1());
	}
}

[ContentProperty("Member")]
public class StaticExtension : IMarkupExtension
{
	public string Member { get; set; }
	public object ProvideValue(IServiceProvider serviceProvider)
	{
		return MainPage.MyFontSize;
	}
}