Why are ActualWidth and ActualHeight 0.0 in this case?

Question

I have a Grid inside a Canvas defined like this:

<Canvas x:Name="outerCanvas">
    <Grid Grid.Row="1" Name="cGrid" ShowGridLines="True" Width="{Binding Path=ActualWidth, RelativeSource={RelativeSource AncestorType={x:Type Canvas}}}" Height="{Binding Path=ActualHeight, RelativeSource={RelativeSource AncestorType={x:Type Canvas}}}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition  />
            <RowDefinition />
        </Grid.RowDefinitions>
        <Rectangle Name="rectangle1" Stroke="Black" Fill="AntiqueWhite" />
        <Rectangle Grid.Row="0" Grid.Column="1" Grid.ColumnSpan="1" Grid.RowSpan="1" Name="rectangle2" Stroke="Black" Fill="AliceBlue" />
        <Rectangle Grid.Row="0" Grid.Column="2" Grid.ColumnSpan="1" Grid.RowSpan="1" Name="rectangle3" Stroke="Black" Fill="Aqua" />
        <Rectangle Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="3" Grid.RowSpan="1" Name="rectangle4" Stroke="Black" Fill="DarkViolet" />
    </Grid>
</Canvas>

My problem is that, on the Window constructor, after InitializeComponents() either Grid.ColumnDefinitions[0].ActualWidth or "any rectangle".ActualWidth are all set to 0.0 (the same for heights). I'm not figuring out what to do to get this information. any help?

Observations:

1. I'm not definig the outer canvas width and height but if I do, it does't solve my problem.
2. At runtime I can see that this Canvas/Grid occupies the entire window space, so every rectangle inside it has ActualWidths and ActualHeights
3. The grid's width/height is bound to the canvas but I tried removing this binding and my problem persisted

Answer 1

ActualHeight and ActualWidth are not set until the control is measured and arranged. Usually there is nothing in InitializeComponent() that causes a measure, so when it returns these will still be zero.

You can force these to be computed earlier by simply calling the window's Measure() and Arrange() methods manually after the window's InitializeComponent() returns.

If you are sizing to content:

window.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
window.Arrange(new Rect(0, 0, window.DesiredWidth, window.DesiredHeight));

If you are using an explicit window size:

window.Measure(new Size(Width, Height));
window.Arrange(new Rect(0, 0, window.DesiredWidth, window.DesiredHeight));

Answer 2

Ray is correct (+1) that this is due to the fact that the measure and arrange pass has not executed yet. However, rather than force another layout pass (expensive), you can just wait until your control has loaded before accessing the ActualXxx properties:

public MyWindow()
{
    Loaded += delegate
    {
        // access ActualWidth and ActualHeight here
    };

}

HTH,
Kent