Why ActualSize is not updating its value on WPF?

Question

I have the following code:

<Window x:Class="UnderstandSizing.Window2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window2" Height="300" Width="300">
<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" x:Name="Column1" />
        <ColumnDefinition Width="Auto" />
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <TextBox Grid.Column="0" Text="{Binding ActualWidth,ElementName=Column1,Mode=OneWay,UpdateSourceTrigger=PropertyChanged}" />
</Grid>
</Window>

I expected to see at the textbox the value of the width of the column Column1 but all I can see is 0.

I've seen this and this but everywhere says that is related only to Silverlight, not WPF.

Edit Fixed a typo. Also to note that the Output window do not show any binding issue . What is strange to me is that it is working in the designer. It stops working only on runtime.

Answer 1

Interesting...

As DanM found, ColumnDefinition.ActualWidth is not a dependency property so you wont get binding updates when it changes.

A workaround is to put a hidden control in the column and bind to it's ActualWidth like this:

    <ContentControl Visibility="Hidden" Grid.Column="0" HorizontalAlignment="Stretch" x:Name="hidden"/>
    <TextBox Grid.Column="0" Text="{Binding ActualWidth,ElementName=hidden,Mode=OneWay}" />

Answer 2

Wouldn't this be a circular reference? The width of "Column1" is dependent on the Text of your TextBox, and the Text of your TextBox is dependent on the width of "Column1". I don't see how WPF could ever possibly resolve a value for this unless you explicitly set the width of either "Column1" or your TextBox.

Edit

Oh, I see the problem. ActualWidth is a double not a dependency property, so you will never receive an update when the value gets calculated.

You need to use @Robert Levy's suggestion of putting a dummy control in the space occupied by your TextBox and bind to the ActualWidth of that instead.