Compiled Bindings

Bindings defined in the XAML are using reflection in order to find and access the requested property in your ViewModel. In Avalonia you can also use compiled bindings, which has some benefits:

• If you use compiled bindings and the property you bind to is not found, you will get a compile-time error. Hence you get a much better debugging experience.
• Reflection is known to be slow (see this article on codeproject.com). Using compiled bindings can therefore improve the performance of your application.

Enable and disable compiled bindings

info

Depending on the template that was used to create the Avalonia project, compiled bindings may or may not be enabled by default. You can check this in the project file.

Enable and disable globally

If you want your application to use compiled bindings globally by default, you can add

<AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>

to your project file. You will still need to provide x:DataType for the objects you want to bind but you don't need to to set x:CompileBindings="[True|False]" for each UserControl or Window.

Enable and disable per UserControl or Window

To enable compiled bindings, you will need to define the DataType of the object you want to bind to first. In DataTemplates there is a property DataType, for all other elements you can set it via x:DataType. Most likely you will set x:DataType in your root node, for example in a Window or an UserControl. You can also specify the DataType in the Binding directly.

You can now enable or disable compiled bindings by setting x:CompileBindings="[True|False]". All child nodes will inherit this property, so you can enable it in your root node and disable it for a specific child, if needed.

<!-- Set DataType and enable compiled bindings -->
<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:vm="using:MyApp.ViewModels"
             x:DataType="vm:MyViewModel"
             x:CompileBindings="True">
    <StackPanel>
        <TextBlock Text="Last name:" />
        <TextBox Text="{Binding LastName}" />
        <TextBlock Text="Given name:" />
        <TextBox Text="{Binding GivenName}" />
        <TextBlock Text="E-Mail:" />
        <!-- Set DataType inside the Binding-markup -->
        <TextBox Text="{Binding MailAddress, DataType={x:Type vm:MyViewModel}}" />

        <Button Content="Send an E-Mail"
                Command="{Binding SendEmailCommand}" />
    </StackPanel>
</UserControl>

CompiledBinding-Markup

If you don't want to enable compiled bindings for all child nodes, you can also use the CompiledBinding-markup. You still need to define the DataType, but you can omit x:CompileBindings="True".

<!-- Set DataType -->
<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:vm="using:MyApp.ViewModels"
             x:DataType="vm:MyViewModel">
    <StackPanel>
        <TextBlock Text="Last name:" />
        <!-- use CompiledBinding markup for your binding -->
        <TextBox Text="{CompiledBinding LastName}" />
        <TextBlock Text="Given name:" />
        <TextBox Text="{CompiledBinding GivenName}" />
        <TextBlock Text="E-Mail:" />
        <TextBox Text="{CompiledBinding MailAddress}" />

        <!-- This command will use ReflectionBinding, as it's default -->
        <Button Content="Send an E-Mail"
                Command="{Binding SendEmailCommand}" />
    </StackPanel>
</UserControl>

ReflectionBinding-Markup

If you have compiled bindings enabled in the root node (via x:CompileBindings="True") and you either don't want to use compiled binding at a certain position or you hit one of the known limitations, you can use the ReflectionBinding-markup.

<!-- Set DataType -->
<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:vm="using:MyApp.ViewModels"
             x:DataType="vm:MyViewModel"
             x:CompileBindings="True">
    <StackPanel>
        <TextBlock Text="Last name:" />
        <TextBox Text="{Binding LastName}" />
        <TextBlock Text="Given name:" />
        <TextBox Text="{Binding GivenName}" />
        <TextBlock Text="E-Mail:" />
        <TextBox Text="{Binding MailAddress}" />

        <!-- We use ReflectionBinding instead -->
        <Button Content="Send an E-Mail"
                Command="{ReflectionBinding SendEmailCommand}" />
    </StackPanel>
</UserControl>

Type casting

In some cases the target type of the binding expression cannot be automatically evaluated. In such cases you must provide an explicit type cast in the binding expression.

<ItemsRepeater ItemsSource="{Binding MyItems}">
<ItemsRepeater.ItemTemplate>
    <DataTemplate>
    <StackPanel Orientation="Horizontal">
        <TextBlock Text="{Binding DisplayName}"/>
        <Grid>
        <Button Command="{Binding $parent[ItemsRepeater].((vm:MyUserControlViewModel)DataContext).DoItCommand}"
                CommandParameter="{Binding ItemId}"/>
        </Grid>
    </StackPanel>
    </DataTemplate>
</ItemsRepeater.ItemTemplate>
</ItemsRepeater>

In this case, the button command shall not be bound to the item's DataContext but to a command that is defined in the DataContextof the ItemsRepeater. The single item will be identified using a CommandParameter bound to the item's DataContext. Therefore, you must specify the type of the "parent" DataContext via cast expression ((vm:MyUserControlViewModel)DataContext).