WPF: Value produced by BindingExpression is not valid for target

Question

Can't figure this out, i'm trying to make this Template as versatile as possible. I'm getting the following errors when attempting to centre a TextBlock, HorizontalAlignment doesn't work.

System.Windows.Data Error: 5 : Value produced by BindingExpression is not valid for target property.; Value='0' BindingExpression:Path=ActualWidth; DataItem='Grid' (Name='PathGrid'); target element is 'TextBlock' (Name='PathPercentage'); target property is 'FontSize' (type 'Double')

System.Windows.Data Error: 5 : Value produced by BindingExpression is not valid for target property.; Value='-6 -6 -6 -6' MultiBindingExpression:target element is 'TextBlock' (Name='PathPercentage'); target property is 'Margin' (type 'Thickness')

System.Windows.Data Error: 5 : Value produced by BindingExpression is not valid for target property.; Value='-17 -17 -17 -17' MultiBindingExpression:target element is 'TextBlock' (Name='PathPercentage'); target property is 'Margin' (type 'Thickness')

System.Windows.Data Error: 5 : Value produced by BindingExpression is not valid for target property.; Value='83 83 83 83' MultiBindingExpression:target element is 'TextBlock' (Name='PathPercentage'); target property is 'Margin' (type 'Thickness')

This is the ControlTemplate with my bindings

<Grid x:Name="PathGrid" Margin="2" Width="200">
                        <Canvas>
                            <TextBlock x:Name="PathPercentage" Text="{Binding RelativeSource={RelativeSource TemplatedParent},Path=Value}"
                                       Foreground="White"
                                       FontSize="{Binding ElementName=PathGrid, Path=ActualWidth, Converter={StaticResource SizeTextOnParent}}">
                                <TextBlock.Margin>
                                    <MultiBinding Converter="{StaticResource CenterElement}">
                                        <Binding ElementName="PathGrid" Path="ActualWidth"/>
                                        <Binding ElementName="PathPercentage" Path="FontSize"/>
                                    </MultiBinding>
                                </TextBlock.Margin>
                            </TextBlock>
                            <TextBlock Text="{Binding ElementName=PathPercentage, Path=Margin}" />
                            <Ellipse Fill="Transparent" 
                                 Stroke="#434953" 
                                 StrokeThickness="3" 
                                 Width="{Binding ElementName=PathGrid, Path=ActualWidth}" 
                                 Height="{Binding ElementName=PathGrid, Path=ActualWidth}" />

And here is my converter which should produce margin values which will centre my textblock:

using System;
using System.Globalization;
using System.Linq;
using System.Windows;
using System.Windows.Data;

namespace Test_Project.Converters
{
    public class CenterElement : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            double parentWidth = (double)values[0];
            double fontSize = (double)values[1];

            double t = Math.Round(((parentWidth / 2) - (fontSize / 2)), 0);
            string margin = t + " " + t + " " + t + " " + t;

            return margin;
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

Martyn Ball Martyn Ball · Accepted Answer · 2015-11-01T22:31:33

Woops, managed to fix it. Just make sure it returns type Thickness. I was assuming it should return Margin as a type which was why I was struggling find it!

double t = Math.Round(((parentWidth / 2) - (fontSize / 2)), 0);
            Thickness margin = new Thickness(t, t, t, t);

            return margin;

WPF: Value produced by BindingExpression is not valid for target

1 Answers