converting-binding-values.md

Converting Binding Values

Negating Values

Often you will need to negate a value that you're binding to. A frequent use for this is to show or hide and control or to enable/disable it. You can negate a binding value by prepending a "bang" operator: !.

For example you might want to show one control when another control is disabled.

<StackPanel>
  <TextBox Name="input" IsEnabled="{Binding AllowInput}"/>
  <TextBlock IsVisible="{Binding !AllowInput}">Sorry, no can do!</TextBlock>
</StackPanel>

Negation also works when binding to non-boolean values. First of all, the value is converted to a boolean using Convert.ToBoolean and the result from this is negated. Because the integer value 0 is considered false and all other integer values are considered true, you can use this to show a message when a collection is empty:

<Panel>
  <ListBox Items="{Binding Items}"/>
  <TextBlock IsVisible="{Binding !Items.Count}">No results found</TextBlock>
</Panel>

A "double-bang" can be used to convert a non-boolean value to a boolean value. For example to hide a control when a collection is empty:

<Panel>
  <ListBox Items="{Binding Items}" IsVisible="{Binding !!Items.Count}"/>
</Panel>

Binding Converters

For more advanced conversions, Avalonia supports IValueConverter the same as other XAML frameworks.

Note: The IValueConverter interface is not available in .NET standard 2.0 so we ship our own, in the Avalonia.Data.Converters namespace.

Usage is identical to other XAML frameworks:

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:ExampleApp;assembly=ExampleApp">

  <Window.Resources>
    <local:MyConverter x:Key="myConverter"/>
  </Window.Resources>

  <TextBlock Text="{Binding Value, Converter={StaticResource myConverter}}"/>
</Window>

Built-in Converters

Avalonia supplies a number of built-in value converters for common scenarios:

Converter	Description
StringConverters.IsNullOrEmpty	Returns true if the input string is null or empty
StringConverters.IsNotNullOrEmpty	Returns false if the input string is null or empty
ObjectConverters.IsNull	Returns true if the input is null
ObjectConverters.IsNotNull	Returns false if the input is null
BoolConverters.And	A multi-value converter that returns true if all inputs are true.

Examples

Hiding a TextBlock if the bound text is null or empty:

<TextBlock Text="{Binding MyText}"
           IsVisible="{Binding MyText, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>

Hiding a ContentControl if the bound content is null or empty:

<ContentControl Content="{Binding MyContent}"
                IsVisible="{Binding MyContent, Converter={x:Static ObjectConverters.IsNotNull}}"/>