Compiled bindings

Browse the sample

.NET Multi-platform App UI (.NET MAUI) data bindings have two main issues:

1. There's no compile-time validation of binding expressions. Instead, bindings are resolved at runtime. Therefore, any invalid bindings aren't detected until runtime when the application doesn't behave as expected or error messages appear.
2. They aren't cost efficient. Bindings are resolved at runtime using general-purpose object inspection (reflection), and the overhead of doing this varies from platform to platform.

Compiled bindings improve data binding performance in .NET MAUI applications by resolving binding expressions at compile-time rather than runtime. In addition, this compile-time validation of binding expressions enables a better developer troubleshooting experience because invalid bindings are reported as build errors.

Important

Compiled bindings are required instead of string-based bindings in NativeAOT apps, and in apps with full trimming enabled. For more information, see Trim a .NET MAUI app and Native AOT deployment.

Compiled bindings in XAML

To use compiled bindings in XAML, set an x:DataType attribute on a VisualElement to the type of the object that the VisualElement and its children will bind to. It's recommended to set the x:DataType attribute at the same level in the view hierarchy as the BindingContext is set. However, this attribute can be re-defined at any location in a view hierarchy.

Important

Compiled bindings require the use of XAML compilation, which is enabled by default in .NET MAUI. If you've disabled XAML compilation, you'll need to enable it. For more information, see XAML Compilation.

To use compiled bindings in XAML, the x:DataType attribute must be set to a string literal, or a type using the x:Type markup extension. At XAML compile time, any invalid binding expressions will be reported as build errors. However, the XAML compiler will only report a build error for the first invalid binding expression that it encounters. Any valid binding expressions that are defined on the VisualElement or its children will be compiled, regardless of whether the BindingContext is set in XAML or code. Compiling a binding expression generates compiled code that will get a value from a property on the source, and set it on the property on the target that's specified in the markup. In addition, depending on the binding expression, the generated code may observe changes in the value of the source property and refresh the target property, and may push changes from the target back to the source.

By default, .NET MAUI produces build warnings for XAML bindings that don't use compiled bindings.

For more information about XAML compiled bindings warnings, see XAML compiled bindings warnings.

Use compiled bindings in XAML

The following example demonstrates using compiled bindings between .NET MAUI views and viewmodel properties:

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:DataBindingDemos"
             x:Class="DataBindingDemos.CompiledColorSelectorPage"
             x:DataType="local:HslColorViewModel"
             Title="Compiled Color Selector">
    <ContentPage.BindingContext>
        <local:HslColorViewModel Color="Sienna" />
    </ContentPage.BindingContext>
    ...
    <StackLayout>
        <BoxView Color="{Binding Color}"
                 ... />
        <StackLayout Margin="10, 0">
            <Label Text="{Binding Name}" />
            <Slider Value="{Binding Hue}" />
            <Label Text="{Binding Hue, StringFormat='Hue = {0:F2}'}" />
            <Slider Value="{Binding Saturation}" />
            <Label Text="{Binding Saturation, StringFormat='Saturation = {0:F2}'}" />
            <Slider Value="{Binding Luminosity}" />
            <Label Text="{Binding Luminosity, StringFormat='Luminosity = {0:F2}'}" />
        </StackLayout>
    </StackLayout>    
</ContentPage>

The ContentPage instantiates the HslColorViewModel and initializes the Color property within property element tags for the BindingContext property. The ContentPage also defines the x:DataType attribute as the viewmodel type, indicating that any binding expressions in the ContentPage view hierarchy will be compiled. This can be verified by changing any of the binding expressions to bind to a non-existent viewmodel property, which will result in a build error. While this example sets the x:DataType attribute to a string literal, it can also be set to a type with the x:Type markup extension. For more information about the x:Type markup extension, see x:Type Markup Extension.

Important

The x:DataType attribute can be re-defined at any point in a view hierarchy.

The BoxView, Label elements, and Slider views inherit the binding context from the ContentPage. These views are all binding targets that reference source properties in the viewmodel. For the BoxView.Color property, and the Label.Text property, the data bindings are OneWay – the properties in the view are set from the properties in the viewmodel. However, the Slider.Value property uses a TwoWay binding. This allows each Slider to be set from the viewmodel, and also for the viewmodel to be set from each Slider.

When the example is first run, the BoxView, Label elements, and Slider elements are all set from the viewmodel based on the initial Color property set when the viewmodel was instantiated. As the sliders are manipulated, the BoxView and Label elements are updated accordingly:

For more information about this color selector, see ViewModels and property-change notifications.

Use compiled bindings in XAML in a DataTemplate

Bindings in a DataTemplate are interpreted in the context of the object being templated. Therefore, when using compiled bindings in a DataTemplate, the DataTemplate needs to declare the type of its data object using the x:DataType attribute. Failure to do this could result in the DataTemplate inheriting an incorrect x:DataType from its parent scope:

<ContentPage ...
             x:DataType="local:AnimalsPageViewModel">
    <!-- Binding to AnimalsPageViewModel.Animals -->
    <CollectionView ItemsSource="{Binding Animals}">
        <CollectionView.ItemTemplate>
            <DataTemplate>
                <!-- incorrect: compiler thinks you want to bind to AnimalsPageViewModel.Name -->  
                <Label Text="{Binding Name}" />
            </DataTemplate>
        </CollectionView.ItemTemplate>
    </CollectionView>
</ContentPage>

The following example demonstrates correctly setting the x:DataType on a DataTemplate:

<ContentPage ...
             x:DataType="local:AnimalsPageViewModel">
    <!-- Binding to AnimalsPageViewModel.Animals -->
    <CollectionView ItemsSource="{Binding Animals}">
        <CollectionView.ItemTemplate>
            <DataTemplate x:DataType="local:Animal">
                <!-- correct: compiler knows you want to bind to Animal.Name -->
                <Label Text="{Binding Name}" />
            </DataTemplate>
        </CollectionView.ItemTemplate>
    </CollectionView>
</ContentPage>

While this example sets the x:DataType attribute to a string literal, it can also be set to a type with the x:Type markup extension. For more information about the x:Type markup extension, see x:Type Markup Extension.

Compile bindings that define the Source property

Prior to .NET MAUI 9, the XAML compiler would skip compilation of bindings that define the Source property instead of the BindingContext. From .NET MAUI 9, these bindings can be compiled to take advantage of better runtime performance. However, this optimization isn't enabled by default to avoid breaking existing app code. To enable this optimization, set the $(MauiEnableXamlCBindingWithSourceCompilation) build property to true in your app's project file:

<MauiEnableXamlCBindingWithSourceCompilation>true</MauiEnableXamlCBindingWithSourceCompilation>

Then, ensure that all your bindings are annotated with the correct x:DataType and that they don't inherit incorrect data types from their parent scope:

<HorizontalStackLayout BindingContext="{x:Reference slider}" x:DataType="Slider">
    <Label Text="{Binding Value}" />
    <Label Text="{Binding Text, Source={x:Reference entry}, x:DataType=Entry}" />
</HorizontalStackLayout>

Note

In cases where there's a binding with a Source, but it inherits the x:DataType from the parent, there can be a mismatch between the x:DataType and the type of the Source. In this scenario, a warning will be generated and a fallback to a reflection-based binding that resolves the binding path at runtime will occur.

Combine compiled bindings with classic bindings in XAML

Binding expressions are only compiled for the view hierarchy that the x:DataType attribute is defined on. Conversely, any views in a hierarchy on which the x:DataType attribute is not defined will use classic bindings. It's therefore possible to combine compiled bindings and classic bindings on a page. For example, in the previous section the views within the DataTemplate use compiled bindings, while the BoxView that's set to the color selected in the ListView does not.

Careful structuring of x:DataType attributes can therefore lead to a page using compiled and classic bindings. Alternatively, the x:DataType attribute can be re-defined at any point in a view hierarchy to null using the x:Null markup extension. Doing this indicates that any binding expressions within the view hierarchy will use classic bindings. The following example demonstrates this approach:

<StackLayout x:DataType="local:HslColorViewModel">
    <StackLayout.BindingContext>
        <local:HslColorViewModel Color="Sienna" />
    </StackLayout.BindingContext>
    <BoxView Color="{Binding Color}"
             VerticalOptions="FillAndExpand" />
    <StackLayout x:DataType="{x:Null}"
                 Margin="10, 0">
        <Label Text="{Binding Name}" />
        <Slider Value="{Binding Hue}" />
        <Label Text="{Binding Hue, StringFormat='Hue = {0:F2}'}" />
        <Slider Value="{Binding Saturation}" />
        <Label Text="{Binding Saturation, StringFormat='Saturation = {0:F2}'}" />
        <Slider Value="{Binding Luminosity}" />
        <Label Text="{Binding Luminosity, StringFormat='Luminosity = {0:F2}'}" />
    </StackLayout>
</StackLayout>   

The root StackLayout sets the x:DataType attribute to be the HslColorViewModel type, indicating that any binding expression in the root StackLayout view hierarchy will be compiled. However, the inner StackLayout redefines the x:DataType attribute to null with the x:Null markup expression. Therefore, the binding expressions within the inner StackLayout use classic bindings. Only the BoxView, within the root StackLayout view hierarchy, uses compiled bindings.

For more information about the x:Null markup expression, see x:Null Markup Extension.

XAML compiled bindings warnings

The following table lists the compiler warnings for compiled bindings, and how to resolve them:

Code	Message
XC0022	Binding could be compiled to improve runtime performance if x:DataType is specified. To fix this warning, add x:DataType to your XAML to specify the type of the current BindingContext. It's best practice to add x:DataType to all elements where the binding context changes.
XC0023	Binding could be compiled to improve runtime performance if x:DataType is not explicitly null. To fix this warning, replace x:DataType="{x:Null}" with the right type.
XC0024	Binding might be compiled incorrectly since the x:DataType annotation comes from an outer scope. Make sure you annotate all DataTemplate XAML elements with the correct x:DataType. To fix this warning, ensure that all DataTemplate elements are annotated with the correct x:DataType.
XC0025	Binding was not compiled because it has an explicitly set Source property and compilation of bindings with Source is not enabled. Consider enabling this optimization by setting the <MauiEnableXamlCBindingWithSourceCompilation>true</MauiEnableXamlCBindingWithSourceCompilation> in your project file and make sure the correct x:DataType is specified for this binding. To fix this warning, enable the $(MauiEnableXamlCBindingWithSourceCompilation) build property in your project file, and annotate all your bindings with the appropriate x:DataType.

To ensure these warnings aren't ignored, consider changing specific warnings to build errors with the $(WarningsAsErrors) build property:

<WarningsAsErrors>$(WarningsAsErrors);XC0022;XC0023</WarningsAsErrors>

To ignore these warnings, use the $(NoWarn) build property with specific warning codes:

<NoWarn>$(NoWarn);XC0022;XC0023</NoWarn>

If you set the $(TreatWarningsAsErrors) build property to true in your app's project file, but you want to ignore certain XAML compiler warnings, use either the $(NoWarn) build property to silence these warnings or the $(WarningsNotAsErrors) build property to reduce the severity of some specific codes.

By default, .NET MAUI produces build warnings for XAML bindings that don't use compiled bindings. You can opt into compiled bindings warnings being treated as errors by setting the $(MauiStrictXamlCompilation) and $(TreatWarningsAsErrors) build properties to true in your app's project file (*.csproj):

<TreatWarningsAsErrors>true</TreatWarningsAsErrors>
<MauiStrictXamlCompilation>true</MauiStrictXamlCompilation>

Note

By default, the $(MauiStrictXamlCompilation) build property is false unless you are publishing your app using full trimming or NativeAOT.

Compiled bindings in code

Bindings written in code typically use string paths that are resolved at runtime with reflection. However, the SetBinding extension method also has an overload that defines bindings using a Func argument instead of a string path:

MyLabel.SetBinding(Label.TextProperty, static (Entry entry) => entry.Text);

Not all methods can be used to define a compiled binding. The expression must be a simple property access expression. The following examples show valid and invalid binding expressions:

// Valid: Property access
static (PersonViewModel vm) => vm.Name;
static (PersonViewModel vm) => vm.Address?.Street;

// Valid: Array and indexer access
static (PersonViewModel vm) => vm.PhoneNumbers[0];
static (PersonViewModel vm) => vm.Config["Font"];

// Valid: Casts
static (Label label) => (label.BindingContext as PersonViewModel).Name;
static (Label label) => ((PersonViewModel)label.BindingContext).Name;

// Invalid: Method calls
static (PersonViewModel vm) => vm.GetAddress();
static (PersonViewModel vm) => vm.Address?.ToString();

// Invalid: Complex expressions
static (PersonViewModel vm) => vm.Address?.Street + " " + vm.Address?.City;
static (PersonViewModel vm) => $"Name: {vm.Name}";

Warning

A CS0272 compiler error will occur if the set accessor for a property or indexer is inaccessible. If this occurs, increase the accessibility of the accessor.

In addition, the BindingBase.Create method sets the binding directly on the object with a Func, and returns the binding object instance:

myEntry.SetBinding(Entry.TextProperty, new MultiBinding
{
    Bindings = new Collection<BindingBase>
    {
        Binding.Create(static (Entry entry) => entry.FontFamily, source: RelativeBindingSource.Self),
        Binding.Create(static (Entry entry) => entry.FontSize, source: RelativeBindingSource.Self),
        Binding.Create(static (Entry entry) => entry.FontAttributes, source: RelativeBindingSource.Self),
    },
    Converter = new StringConcatenationConverter()
});

These compiled binding approaches provide the following benefits:

• Improved data binding performance by resolving binding expressions at compile-time rather than runtime.
• A better developer troubleshooting experience because invalid bindings are reported as build errors.
• Intellisense while editing.

Performance

Compiled bindings improve data binding performance, with the performance benefit varying:

• A compiled binding that uses property-change notification (i.e. a OneWay, OneWayToSource, or TwoWay binding) is resolved approximately 8 times quicker than a classic binding.
• A compiled binding that doesn't use property-change notification (i.e. a OneTime binding) is resolved approximately 20 times quicker than a classic binding.
• Setting the BindingContext on a compiled binding that uses property change notification (i.e. a OneWay, OneWayToSource, or TwoWay binding) is approximately 5 times quicker than setting the BindingContext on a classic binding.
• Setting the BindingContext on a compiled binding that doesn't use property change notification (i.e. a OneTime binding) is approximately 7 times quicker than setting the BindingContext on a classic binding.

These performance differences can be magnified on mobile devices, dependent upon the platform being used, the version of the operating system being used, and the device on which the application is running.