How do I resolve the "Binding could be compiled to improve runtime performance if x:DataType is specified " warning

Question

I have the following page:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="PhotoManager.MainPage">

    <ScrollView>
        <VerticalStackLayout
            Padding="5,0"
            Spacing="10">
            <Image
                Source="banner2.jpg"
                HeightRequest="75"
                Aspect="AspectFit"
                SemanticProperties.Description="Organize My League" />
            
                <Line Stroke="Black" X2="{Binding Width, Source={RelativeSource Self}}" HorizontalOptions="Fill" StrokeThickness="2" />

            <ActivityIndicator x:Name="activityIndicator"
                       IsVisible="false"
                       IsRunning="True"
                       Color="{StaticResource NavigationBarColor}" ZIndex="1000"
                       VerticalOptions="Center" HorizontalOptions="Center"
                       WidthRequest="100" HeightRequest="100"/>
            <Label
                Text="Log On"
                Style="{StaticResource Headline}"
                SemanticProperties.HeadingLevel="Level1" />

            <Label
                Text="Username"
                Style="{StaticResource SubHeadline}"
                SemanticProperties.HeadingLevel="Level2"
                SemanticProperties.Description="Enter your username." />

            <Entry x:Name="TheUsername"
                Placeholder="Enter Username"/>

            <Label
                Text="Password"
                Style="{StaticResource SubHeadline}"
                SemanticProperties.HeadingLevel="Level2"
                SemanticProperties.Description="Enter your password." />

            <Entry x:Name="ThePassword"
                Placeholder="Enter Password"
                IsPassword="true" />

            <Button
                x:Name="CounterBtn"
                Text="Submit" 
                SemanticProperties.Hint="Log on"
                Clicked="OnCounterClicked"
                HorizontalOptions="Fill" />

        </VerticalStackLayout>
    </ScrollView>
</ContentPage>

The line:

<Line Stroke="Black" X2="{Binding Width, Source={RelativeSource Self}}" HorizontalOptions="Fill" StrokeThickness="2" />

Generates the subject warning. How do I resolve the warning?

Answer 1

If you break up your Binding, we have

• Width, // where does Width come from? This is what .NET is thinking
• Source={RelativeSource Self}, // "What is Self?"
• <Line ..., // Aha Self == Line

This means we need to specify x:DataType=Line. Another way to look at it, we're telling .NET that Width is Line's Width.

Perhaps, in future, an optimized compiler can deduce this information since, from your code snippet it is obvious that Width comes from Line. In more complex Binding, however, you will have to help it out, that's what x:DataType is for.

<Line Stroke="Black" X2="{Binding Width, x:DataType=Line, Source={RelativeSource Self}}" HorizontalOptions="Fill" StrokeThickness="2" />