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MultiBinding

MultiBinding combines values from multiple source properties into a single target property. It is useful when a display value depends on more than one data source, such as formatting a full name from separate first and last name properties, or computing a composite value.

Basic usage with StringFormat​

The simplest use of MultiBinding is combining multiple values into a formatted string:

<TextBlock>
    <TextBlock.Text>
        <MultiBinding StringFormat="{}{0} {1}">
            <Binding Path="FirstName" />
            <Binding Path="LastName" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

Each Binding inside the MultiBinding maps to a placeholder ({0}, {1}, and so on) in the StringFormat pattern. The format string follows standard .NET string.Format rules.

Formatting numbers​

<TextBlock>
    <TextBlock.Text>
        <MultiBinding StringFormat="Total: {0:C2} ({1} items)">
            <Binding Path="TotalPrice" />
            <Binding Path="ItemCount" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>
tip

When StringFormat starts with {0, you must escape the opening brace. Prefix the pattern with {} or use backslash escaping: StringFormat='\{0\} items'.

Using an IMultiValueConverter​

For logic beyond string formatting, implement IMultiValueConverter. This converter receives an array of values from all child bindings and returns a single result.

Defining the converter​

using System;
using System.Collections.Generic;
using System.Globalization;
using Avalonia.Data.Converters;

public class AllTrueConverter : IMultiValueConverter
{
    public object? Convert(
        IList<object?> values,
        Type targetType,
        object? parameter,
        CultureInfo culture)
    {
        foreach (var value in values)
        {
            if (value is not true)
                return false;
        }
        return true;
    }
}

Using the converter in XAML​

Declare the converter as a resource, then reference it from the MultiBinding:

<Window.Resources>
    <local:AllTrueConverter x:Key="AllTrue" />
</Window.Resources>

<Button Content="Submit"
        IsEnabled="{MultiBinding Converter={StaticResource AllTrue}}">
    <!-- Intentionally empty: MultiBinding must use property element syntax
         for child bindings. See below for the full form. -->
</Button>

Because MultiBinding with child bindings requires property element syntax, the full form is:

<Button Content="Submit">
    <Button.IsEnabled>
        <MultiBinding Converter="{StaticResource AllTrue}">
            <Binding Path="IsFormValid" />
            <Binding Path="HasAcceptedTerms" />
            <Binding Path="IsNotBusy" />
        </MultiBinding>
    </Button.IsEnabled>
</Button>

The button is enabled only when all three bound properties are true.

Binding to controls​

Child bindings within a MultiBinding support the same source options as regular bindings, including ElementName, RelativeSource, and Avalonia's #elementName shorthand:

<StackPanel>
    <NumericUpDown x:Name="width" Value="100" Minimum="0" Maximum="500" />
    <NumericUpDown x:Name="height" Value="50" Minimum="0" Maximum="500" />

    <TextBlock>
        <TextBlock.Text>
            <MultiBinding StringFormat="Area: {0} x {1} = {2}">
                <Binding Path="Value" ElementName="width" />
                <Binding Path="Value" ElementName="height" />
                <Binding Path="#width.Value"
                         Converter="{x:Static local:MultiplyConverter.Instance}"
                         ConverterParameter="{Binding #height.Value}" />
            </MultiBinding>
        </TextBlock.Text>
    </TextBlock>
</StackPanel>

MultiBinding properties​

PropertyDescription
BindingsThe collection of child Binding objects.
ConverterAn IMultiValueConverter that processes the bound values.
ConverterParameterA parameter passed to the converter.
StringFormatA format string applied when no converter is specified (or when the converter returns a string).
FallbackValueThe value used when the multi-binding cannot produce a result.
TargetNullValueThe value used when the converter returns null.
ModeThe binding mode. MultiBinding supports OneWay and OneTime modes.
PriorityThe binding priority.
info

MultiBinding is one-way by default. Two-way multi-bindings are not supported because there is no general way to reverse a multi-value conversion back to individual source properties.

tip

Unlike WPF, Avalonia supports nesting a MultiBinding inside another MultiBinding. Each nested MultiBinding resolves to a single value in the parent converter's input array.

FuncMultiValueConverter​

Avalonia provides FuncMultiValueConverter<TIn, TOut> for simple scenarios where you want to define the conversion logic inline without creating a full class.

The converter function receives an IReadOnlyList<TIn>, so you can iterate over values or access them by index:

public static class Converters
{
    // Iterate over all values
    public static readonly FuncMultiValueConverter<string, string> FullName =
        new(parts => string.Join(" ", parts.Where(p => !string.IsNullOrEmpty(p))));

    // Access values by index
    public static readonly FuncMultiValueConverter<string, string> FormattedName =
        new(parts => $"{parts[1]}, {parts[0]}");
}
<TextBlock>
    <TextBlock.Text>
        <MultiBinding Converter="{x:Static local:Converters.FullName}">
            <Binding Path="FirstName" />
            <Binding Path="MiddleName" />
            <Binding Path="LastName" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

Common patterns​

Visibility from multiple conditions​

public class AnyTrueConverter : IMultiValueConverter
{
    public static readonly AnyTrueConverter Instance = new();

    public object? Convert(
        IList<object?> values,
        Type targetType,
        object? parameter,
        CultureInfo culture)
    {
        return values.Any(v => v is true);
    }
}
<Border IsVisible="{MultiBinding Converter={x:Static local:AnyTrueConverter.Instance}}">
    <!-- Shown when any condition is true -->
    <Border.IsVisible>
        <MultiBinding Converter="{x:Static local:AnyTrueConverter.Instance}">
            <Binding Path="HasErrors" />
            <Binding Path="HasWarnings" />
        </MultiBinding>
    </Border.IsVisible>
</Border>

Computing a value from multiple inputs​

public class RectangleAreaConverter : IMultiValueConverter
{
    public object? Convert(
        IList<object?> values,
        Type targetType,
        object? parameter,
        CultureInfo culture)
    {
        if (values.Count >= 2
            && values[0] is double width
            && values[1] is double height)
        {
            return width * height;
        }
        return 0.0;
    }
}

See also​

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