WPF. FormatException when binding an ImageBrush Viewbox property to an object

Question

I'm trying to bind the "Viewbox" property of an ImageBrush to a class property at code-behind.

I have two images, the idea is that while i move the mouse over the top image, the bottom one shows a part of the top image in a Viewbox, relative to the mouse position. The code is just for testing while i'm learning WPF, so i know it could be much better, sorry.

XAML Code:

<Window x:Class="WpfApplication2.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    DataContext="{Binding RelativeSource={RelativeSource Self}}"
    Title="MainWindow" Height="950" Width="525">

...

<Rectangle Grid.Row="0" MouseMove="Rectangle_MouseMove">
        <Rectangle.Fill>
            <ImageBrush Stretch="UniformToFill"  ImageSource="descarga.png"/>
        </Rectangle.Fill>
    </Rectangle>
    <Rectangle Grid.Row="1">
        <Rectangle.Fill>
            <ImageBrush Stretch="UniformToFill"  ImageSource="descarga.png" ViewboxUnits="Absolute" Viewbox="{Binding Source=ViewboxRect}"/>
        </Rectangle.Fill>
    </Rectangle>

C# Code behind:

private Rect _rect;

    public MainWindow()
    {
        InitializeComponent();
    }

    public Rect ViewboxRect
    {
        get
        {                
            return _rect;
        }
        set
        {
            _rect = value;
        }
    }
private void Rectangle_MouseMove(object sender, MouseEventArgs e)
    {
        Point p = e.GetPosition(this);
        _rect.X = p.X;
        _rect.Y = p.Y;
        _rect.Width = p.X + 10;
        _rect.Height = p.Y + 10;
    }

The code above throws the following exception: System.Windows.Data Error: 6 : 'TargetDefaultValueConverter' converter failed to convert value 'ViewboxRect' (type 'String'); fallback value will be used, if available. BindingExpression:Path=; DataItem='String' (HashCode=1651705780); target element is 'ImageBrush' (HashCode=47805141); target property is 'Viewbox' (type 'Rect') FormatException:'System.FormatException: input string was not in a correct format.

I've tried to change"_rect" to a String, but the same error is thrown. Why does the error says "converter failed to convert value 'ViewboxRect' (type 'String')" while that property is a Rect and not a String?

Thank you.

score 0 · Answer 1 · answered Feb 02 '15 at 13:38

In my opinion, your binding does not seem to be right. Your are binding to a MainWindow's property so it should be:

<Rectangle Grid.Row="1">
    <Rectangle.Fill>
        <ImageBrush Stretch="UniformToFill"  ImageSource="descarga.png" ViewboxUnits="Absolute"
                    Viewbox="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:MainWindow}}, Path=ViewboxRect}"/>
    </Rectangle.Fill>
</Rectangle>

Moreover you should consider that Rect does not implement INotifyPropertyChanged, so your UI will be not aware about changements you make in your Rectangle_MouseMove method.

WPF. FormatException when binding an ImageBrush Viewbox property to an object

1 Answers1