Using setter for TextBlock.Text property breaks some other TextBlocks

Describe the bug When I define a Style for TextBlock with Setter for Text property, it removes (not even replaces) text on some other TextBlock controls.

To Reproduce Minimal XAML example I’ve came up with:

<Window xmlns="https://github.com/avaloniaui"
		xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
		xmlns:vm="using:TextBlockTextSetterBug.ViewModels"
		xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
		xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
		mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
		x:Class="TextBlockTextSetterBug.Views.MainWindow"
		Icon="/Assets/avalonia-logo.ico"
		Title="TextBlockTextSetterBug">
	<Window.Styles>
		<Style Selector="TextBlock.myautotext">
			<Setter Property="Text" Value="Text from Style" />
		</Style>
	</Window.Styles>

	<Design.Height>200</Design.Height>
	<Design.Width>300</Design.Width>
	<Design.DataContext>
		<vm:MainWindowViewModel/>
	</Design.DataContext>

	<StackPanel>
		<!-- This text IS visible (style works as expected) -->
		<TextBlock Classes="myautotext" />
		
		<!-- This text IS visible (bindings outside DataTemplate are working) -->
		<TextBlock Text="{Binding Greeting}" />
	
		<!-- This text is NOT visible (direct setting of text doesn't work) -->
		<TextBlock Text="Can't see me" />
	
		<!-- Text of items is NOT visible (using explicit DataTemplate changes nothing) -->
		<ListBox Items="{Binding SomeStrings}"/>
	</StackPanel>
</Window>

Expected behavior TextBlock controls not matching the selector must not be affected.

Screenshots Expected:

Actual:

Desktop

• OS: Windows 10
• Avalonia 0.10.13 (latest from NuGet)

Additional context Add any other context about the problem here.

[grokys]

The reason is kind of lost in the mists of time, but I think it was done for performance reasons. Reading a CLR (i.e. direct) property is about 40x faster currently than reading a styled property. That combined with:

• It will almost always be set
• The need to style it is uncommon
• TextBlocks are very common and there will likely be a lot of them
• In CSS you can’t style element contents

I think lead us to decide that it was worth sacrificing some flexibility for performance.

[robloo]

@grokys Hm, in my opinion this feature shouldn’t be sacrificed. It is something I would expect to just work and does in all other XAML frameworks. That said, I don’t know the performance issues encountered at the time, or if they are still valid. I think this should also be documented some place along with why it was designed this way.

If the styling system is really this slow, and this is still a problem enough to keep Text a direct property, I guess there are more fundamental concerns that should be addressed. WPF can handle this type of thing just fine and it’s performance was well enough – obviously it never ran on all the form factors Avalonia does though.

Overall, this is pretty unintuitive and has/will cause a lot of tricky debugging. I definitely think compile warnings are needed but I think this should just be a StyledProperty for TextBlock (leave TextBox as-is). Optimizations have since been made to the styling system over the years as well.

Finally, are we sure there isn’t more going on?:

		<!-- This text is NOT visible (direct setting of text doesn't work) -->
		<TextBlock Text="Can't see me" />

There is no way this should ever not work. I guess it is related to the other issue of priority that might be fixed after your PR.