Can WPF Databinding source include a parameter? - wpf

Question

Can I pass an index number into this list (SomeList)?
FontSize="{Binding FontSize, Source={x:Static ut:ViewSetupData.SomeList}, FallbackValue=12}"

Answer 1

You can put a constant indexer in the Path:
{Binding Path=[(sys:Int32)0], Source={x:Static ut:ViewSetupData.SomeList}}
But you can't bind a property of a Binding, so there's no way to stuff a parameter in there. However, you can combine multiple bindings in a MultiBinding, so you could use one of those with a multi-value converter:
C#:
public class IListIndexerConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
// You might want a little more error-checking than this...
return ((IList)values[0])[(int)values[1]];
}
public virtual object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
{
return null;
}
}
XAML:
<TextBlock>
<TextBlock.Resources>
<local:IListIndexerConverter x:Key="ListIndexer" />
</TextBlock.Resources>
<TextBlock.Text>
<MultiBinding Converter="{StaticResource ListIndexer}">
<Binding Source="{x:Static ut:ViewSetupData.SomeList}" />
<Binding
ElementName="MyComboBox"
Path="SelectedIndex"
/>
</MultiBinding>
</TextBlock.Test>
</TextBlock>
Update
While you were marking this as the solution, I was writing a more complete solution that addressed your need to grab a property from the list item:
C#:
public class ListItemPropertyGetter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
try
{
var list = values[0] as IList;
var index = (int)(values[1] ?? 0);
var propname = values[2] as String;
object item = list[index];
var prop = item.GetType().GetProperty(propname);
var propvalue = prop.GetValue(item);
return propvalue;
}
catch
{
return null;
}
}
public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
// Gotta put these somewhere
public static List<FontSizeThing> FontSizeThings { get; } =
new List<FontSizeThing>
{
new FontSizeThing(10),
new FontSizeThing(10.5),
new FontSizeThing(11),
new FontSizeThing(12),
new FontSizeThing(14),
new FontSizeThing(15),
};
}
public class FontSizeThing {
public FontSizeThing(double n) { FontSize = n; }
public double FontSize { get; set; }
}
XAML:
<ComboBox x:Name="FontSizeOptionCombo">
<sys:Int32>0</sys:Int32>
<sys:Int32>1</sys:Int32>
<sys:Int32>2</sys:Int32>
<sys:Int32>3</sys:Int32>
<sys:Int32>4</sys:Int32>
</ComboBox>
<TextBlock Text="Testing">
<TextBlock.Resources>
<hconv:ListItemPropertyGetter x:Key="ListItemPropertyGetter" />
</TextBlock.Resources>
<TextBlock.FontSize>
<MultiBinding Converter="{StaticResource ListItemPropertyGetter}" StringFormat="{}{0}">
<Binding Source="{x:Static hconv:ListItemPropertyGetter.FontSizeThings}" />
<Binding ElementName="FontSizeOptionCombo" Path="SelectedItem" />
<Binding Source="FontSize" />
</MultiBinding>
</TextBlock.FontSize>
</TextBlock>
FINAL UPDATE
Note that if I had merely populated FontSizeOptionCombo with the FontThings themselves, I could very simply have bound like this:
<ComboBox
x:Name="OtherCombo"
ItemsSource="{x:Static hconv:ListItemPropertyGetter.FontSizeThings}"
DisplayMemberPath="FontSize"
FontSize="{Binding SelectedItem.FontSize, ElementName=OtherCombo, FallbackValue=20}"
/>
If that fits in with what you're doing, it's by far the nicest way.