VisualTreeHelper.GetDescendantBounds returns 'Empty' (Infinity) - wpf

Question

In my WPF app I dynamically load a XAML drawing from XML at runtime. This drawing is a complex series of nested canvas and geometry 'Path's (for example):
<?xml version="1.0" encoding="utf-8"?>
<Canvas Width="1593" Height="1515">
<Canvas.Resources />
<Canvas>
<Path Fill="…" Data="…"/>
<Path Fill="…" Data="…"/>
<Path Fill="…" Data="…"/>
<Canvas>
<Canvas>
<Path Stroke="…" StrokeThickness="…" StrokeMiterLimit="…" StrokeLineJoin="…" StrokeEndLineCap="…" Data="…"/>
<Path Stroke="…" StrokeThickness="…" StrokeMiterLimit="…" StrokeLineJoin="…" StrokeEndLineCap="…" Data="…"/>
</Canvas>
</Canvas>
<Path Fill="…" Data="…"/>
<Path Fill="…" Data="…"/>
<Path Fill="…" Data="…"/>
</Canvas>
</Canvas>
The outer canvas' Height/Width are incorrectly set, as many of the Path expressions exceeds these dimensions. I don't have any control over this source XML, so I'm required to fix it up at runtime after the drawing is loaded. To load the drawing I use code similar to the following:
public static Canvas LoadDrawing(string xaml)
{
Canvas drawing = null;
using (var stringreader = new StringReader(xaml))
{
using (var xmlReader = new XmlTextReader(stringreader))
{
drawing = (Canvas)XamlReader.Load(xmlReader);
}
}
return drawing;
}
Then, I attempt to reset the canvas size, using the following code:
var canvas = LoadDrawing(…);
someContentControOnTheExistingPage.Content = canvas;
var bounds = VisualTreeHelper.GetDescendantBounds(canvas); // << 'bounds' is empty here.
canvas.Width = bounds.Width;
canvas.Height = bounds.Height;
Except, at the point where I create the canvas element, the bounds is empty. However, if I just wire a simple button and invoke GetDescendantBounds() interactively on the same canvas, then I receive expected height/width.
My takeaway is that GetDescendantBounds() does not work unless the layout with the new control has completed. So my questions are:
Is there a way I can force a layout computation prior to running GetDescendantBounds()? Or…
Is there another way I can get the bounds/extents of a visual tree, prior adding it to its parent?
Thanks
-John

Answer 1

Is there a way I can force a layout computation prior to running GetDescendantBounds?
Yes, call the Arrange and Measure methods of the Canvas:
var canvas = LoadDrawing("...");
someContentControOnTheExistingPage.Content = canvas;
canvas.Arrange(new Rect(someContentControOnTheExistingPage.RenderSize));
canvas.Measure(someContentControOnTheExistingPage.RenderSize);
var bounds = VisualTreeHelper.GetDescendantBounds(canvas);
canvas.Width = bounds.Width;
canvas.Height = bounds.Height;

Answer 2

First you need to add this line in your xaml string.
xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'
This is C# code example to find control and properties.
public void LoadXaml()
{
string canvasString = #"<Canvas Name='canvas1' xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' Width = '1593' Height = '1515'> </Canvas>";
var canvas = LoadDrawing(canvasString);
//Use this line you will find height and width.
Canvas canvasCtrl = (Canvas)LogicalTreeHelper.FindLogicalNode(canvas, "canvas1");
// var bounds = VisualTreeHelper.GetDescendantBounds(canvas); // << 'bounds' is empty here.
canvas.Width = canvasCtrl.Width; //bounds.Width;
canvas.Height = canvasCtrl.Height; //bounds.Height;
}