WPF: How to make canvas auto-resize?

I would like my Canvas to automatically resize to the size of its items, so that the ScrollViewer scroll bars have the correct range. Can this be done in XAML?

<ScrollViewer HorizontalScrollBarVisibility="Auto" x:Name="_scrollViewer">
    <Grid x:Name ="_canvasGrid" Background="Yellow">
        <Canvas x:Name="_canvas" HorizontalAlignment="Left" VerticalAlignment="Top" Background="Green"></Canvas>
        <Line IsHitTestVisible="False" .../>
    </Grid>
</ScrollViewer>

In the above code the canvas always has size 0, though it doesn't clip its children.

No this is not possible (see snippet from MSDN below). However, if you want to have scrollbars and auto-resizing, consider using a Grid instead, and use the Margin property to position your items on this Grid.. Grid will tell the ScrollViewer how big he wants to be, and you will get the scrollbars.. Canvas will always tells the ScrollViewer he doesn't need any size.. :)

Grid lets you enjoy both worlds - As long as you're putting all elements into a single cell, you get both: Arbitrary positioning and auto-sizing. In general it is good to remember that most panel controls (DockPanel, StackPanel, etc) can be implemented via a Grid control.

From MSDN:

> Canvas is the only panel element that has no inherent layout characteristics. A Canvas has default Height and Width properties of zero, unless it is the child of an element that automatically sizes its child elements. Child elements of a Canvas are never resized, they are just positioned at their designated coordinates. This provides flexibility for situations in which inherent sizing constraints or alignment are not needed or wanted. For cases in which you want child content to be automatically resized and aligned, it is usually best to use a Grid element.

Hope this helps

I'm just copying illef's answer here but in answer to PilotBob, you just define a canvas object like this

public class CanvasAutoSize : Canvas
{
    protected override System.Windows.Size MeasureOverride(System.Windows.Size constraint)
    {
        base.MeasureOverride(constraint);
        double width = base
            .InternalChildren
            .OfType<UIElement>()
            .Max(i => i.DesiredSize.Width + (double)i.GetValue(Canvas.LeftProperty));

        double height = base
            .InternalChildren
            .OfType<UIElement>()
            .Max(i => i.DesiredSize.Height + (double)i.GetValue(Canvas.TopProperty));

        return new Size(width, height);
    }
}

and then use CanvasAutoSize in your XAML.

            <local:CanvasAutoSize VerticalAlignment="Top" HorizontalAlignment="Left"></local:CanvasAutoSize>

I prefer this solution to the one presented above that uses the grid as it works through attached properties and just requires setting less properties on the elements.

I think you can resize Canvas by overriding MeasureOverride or ArrangeOverride methods.

This job is not difficult.

You can see this post. http://illef.tistory.com/entry/Canvas-supports-ScrollViewer

I hope this helps you.

Thank you.

Essentially it requires a complete rewrite of Canvas. Previous proposed solutions that override MeasureOverride fail because the default Canvas.Left/.Top &c properties invalidate Arrangment, but ALSO need to invalidate measure. (You get the right size the first time, but the size doesn't change if you move elements after the initial layout).

The Grid solution is more-or-less reasonable but binding to Margins in order to get x-y displacement can wreak havoc on other code (particalary in MVVM). I struggled with the Grid view solution for a while, but complications with View/ViewModel interactions and scrolling behaviour finally drove me to this. Which is simple and to the point, and Just Works.

It's not THAT complicated to re-implement ArrangeOverride and MeasureOverride. And you're bound to write at least as much code elsewhere dealing with Grid/Margin stupidity. So there you are.

Here's a more complete solution. non-zero Margin behaviour is untested. If you need anything other than Left and Top, then this provides a starting point, at least.

WARNING: You must use AutoResizeCanvas.Left and AutoResizeCanvas.Top attached properties instead of Canvas.Left and Canvas.Top. Remaining Canvas properties have not been implemented.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace Mu.Controls
{
    public class AutoResizeCanvas : Panel
    {



        public static double GetLeft(DependencyObject obj)
        {
            return (double)obj.GetValue(LeftProperty);
        }

        public static void SetLeft(DependencyObject obj, double value)
        {
            obj.SetValue(LeftProperty, value);
        }

        public static readonly DependencyProperty LeftProperty =
            DependencyProperty.RegisterAttached("Left", typeof(double),
            typeof(AutoResizeCanvas), 
            new FrameworkPropertyMetadata(0.0, OnLayoutParameterChanged));

        private static void OnLayoutParameterChanged(
                DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            // invalidate the measure of the enclosing AutoResizeCanvas.
            while (d != null)
            {
                AutoResizeCanvas canvas = d as AutoResizeCanvas;
                if (canvas != null)
                {
                    canvas.InvalidateMeasure();
                    return;
                }
                d = VisualTreeHelper.GetParent(d);
            }
        }




        public static double GetTop(DependencyObject obj)
        {
            return (double)obj.GetValue(TopProperty);
        }

        public static void SetTop(DependencyObject obj, double value)
        {
            obj.SetValue(TopProperty, value);
        }

        public static readonly DependencyProperty TopProperty =
            DependencyProperty.RegisterAttached("Top", 
                typeof(double), typeof(AutoResizeCanvas),
                new FrameworkPropertyMetadata(0.0, OnLayoutParameterChanged));

        



        protected override Size MeasureOverride(Size constraint)
        {
            Size availableSize = new Size(double.MaxValue, double.MaxValue);
            double requestedWidth = MinimumWidth;
            double requestedHeight = MinimumHeight;
            foreach (var child in base.InternalChildren)
            {
                FrameworkElement el = child as FrameworkElement;

                if (el != null)
                {
                    el.Measure(availableSize);
                    Rect bounds, margin;
                    GetRequestedBounds(el,out bounds, out margin);

                    requestedWidth = Math.Max(requestedWidth, margin.Right);
                    requestedHeight = Math.Max(requestedHeight, margin.Bottom);
                }
            }
            return new Size(requestedWidth, requestedHeight);
        }
        private void GetRequestedBounds(
                            FrameworkElement el, 
                            out Rect bounds, out Rect marginBounds
                            )
        {
            double left = 0, top = 0;
            Thickness margin = new Thickness();
            DependencyObject content = el;
            if (el is ContentPresenter)
            {
                content = VisualTreeHelper.GetChild(el, 0);
            }
            if (content != null)
            {
                left = AutoResizeCanvas.GetLeft(content);
                top = AutoResizeCanvas.GetTop(content);
                if (content is FrameworkElement)
                {
                    margin = ((FrameworkElement)content).Margin;
                }
            }
            if (double.IsNaN(left)) left = 0;
            if (double.IsNaN(top)) top = 0;
            Size size = el.DesiredSize;
            bounds = new Rect(left + margin.Left, top + margin.Top, size.Width, size.Height);
            marginBounds = new Rect(left, top, size.Width + margin.Left + margin.Right, size.Height + margin.Top + margin.Bottom);
        }


        protected override Size ArrangeOverride(Size arrangeSize)
        {
            Size availableSize = new Size(double.MaxValue, double.MaxValue);
            double requestedWidth = MinimumWidth;
            double requestedHeight = MinimumHeight;
            foreach (var child in base.InternalChildren)
            {
                FrameworkElement el = child as FrameworkElement;

                if (el != null)
                {
                    Rect bounds, marginBounds;
                    GetRequestedBounds(el, out bounds, out marginBounds);

                    requestedWidth = Math.Max(marginBounds.Right, requestedWidth);
                    requestedHeight = Math.Max(marginBounds.Bottom, requestedHeight);
                    el.Arrange(bounds);
                }
            }
            return new Size(requestedWidth, requestedHeight);
        }

        public double MinimumWidth
        {
            get { return (double)GetValue(MinimumWidthProperty); }
            set { SetValue(MinimumWidthProperty, value); }
        }

        public static readonly DependencyProperty MinimumWidthProperty =
            DependencyProperty.Register("MinimumWidth", typeof(double), typeof(AutoResizeCanvas), 
            new FrameworkPropertyMetadata(300.0,FrameworkPropertyMetadataOptions.AffectsMeasure));



        public double MinimumHeight
        {
            get { return (double)GetValue(MinimumHeightProperty); }
            set { SetValue(MinimumHeightProperty, value); }
        }

        public static readonly DependencyProperty MinimumHeightProperty =
            DependencyProperty.Register("MinimumHeight", typeof(double), typeof(AutoResizeCanvas), 
            new FrameworkPropertyMetadata(200.0,FrameworkPropertyMetadataOptions.AffectsMeasure));

        
        
    }


}

I see you've got a workable solution, but I thought I'd share.

<Canvas x:Name="topCanvas">
	<Grid x:Name="topGrid" Width="{Binding ElementName=topCanvas, Path=ActualWidth}" Height="{Binding ElementName=topCanvas, Path=ActualHeight}">
		...Content...
	</Grid>
</Canvas>

The above technique will allow you to nest a grid inside a canvas and have dynamic resizing. Further use of dimension binding makes it possible to mix dynamic material with static material, perform layering, etc. There are too many possibilities to mention, some harder than others. For example I use the approach to simulate animatating content moving from one grid location to another - doing the actual placement at the animation's completion event. Good luck.

void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
    autoSizeCanvas(canvas1);
}
    
void autoSizeCanvas(Canvas canv)
{
    int height = canv.Height;
    int width = canv.Width;
    foreach (UIElement ctrl in canv.Children)
    {
        bool nullTop = ctrl.GetValue(Canvas.TopProperty) == null || Double.IsNaN(Convert.ToDouble(ctrl.GetValue(Canvas.TopProperty))),
                nullLeft = ctrl.GetValue(Canvas.LeftProperty) == null || Double.IsNaN(Convert.ToDouble(ctrl.GetValue(Canvas.LeftProperty)));
        int curControlMaxY = (nullTop ? 0 : Convert.ToInt32(ctrl.GetValue(Canvas.TopProperty))) +
            Convert.ToInt32(ctrl.GetValue(Canvas.ActualHeightProperty)
            ),
            curControlMaxX = (nullLeft ? 0 : Convert.ToInt32(ctrl.GetValue(Canvas.LeftProperty))) +
            Convert.ToInt32(ctrl.GetValue(Canvas.ActualWidthProperty)
            );
        height = height < curControlMaxY ? curControlMaxY : height;
        width = width < curControlMaxX ? curControlMaxX : width;
    }
    canv.Height = height;
    canv.Width = width;
}

In the function, i'm trying to find the maximum X position and Y position, where controls in the canvas can reside.

Use the function only in Loaded event or later and not in constructor. The window has to be measured before loading..

Binding the Height/Width to the actual size of the control within the canvas worked for me:

        <ScrollViewer VerticalScrollBarVisibility="Visible" HorizontalScrollBarVisibility="Visible">
            <Canvas Height="{Binding ElementName=myListBox, Path=ActualHeight}"
                    Width="{Binding ElementName=myListBox, Path=ActualWidth}">
                <ListBox x:Name="myListBox" />
            </Canvas>
        </ScrollViewer>

As an improvement to @MikeKulls's answer, here's a version which does not throw an exception when there are no UI elements in the canvas or when there are UI elements without Canvas.Top or Canvas.Left properties:

public class AutoResizedCanvas : Canvas
{
    protected override System.Windows.Size MeasureOverride(System.Windows.Size constraint)
    {
        base.MeasureOverride(constraint);
        double width = base
            .InternalChildren
            .OfType<UIElement>()
            .Where(i => i.GetValue(Canvas.LeftProperty) != null)
            .Max(i => i.DesiredSize.Width + (double)i.GetValue(Canvas.LeftProperty));
        
        if (Double.IsNaN(width))
        {
            width = 0;
        }

        double height = base
            .InternalChildren
            .OfType<UIElement>()
            .Where(i => i.GetValue(Canvas.TopProperty) != null)
            .Max(i => i.DesiredSize.Height + (double)i.GetValue(Canvas.TopProperty));

        if (Double.IsNaN(height))
        {
            height = 0;
        }

        return new Size(width, height);
    }
}

I have also encountered this problem, my issue was that the grid wasn't auto-resizing when the Canvas did resize thanks to the overrided MeasureOverride function.

my problem: https://stackoverflow.com/questions/30726257/wpf-measureoverride-loop

I was able to achieve the result you are looking for by simply adding a new size changed event to the control which contained the data that was causing the canvas to grow. After the canvas reaches the extent of the scroll viewer it will cause the scroll bars to appear. I just assigned the following lambda expression to the size changed event of the control:

text2.SizeChanged += (s, e) => { DrawingCanvas.Height = e.NewSize.Height; 
                                 DrawingCanvas.Width = e.NewSize.Width; };

What worked for me is the following: Like the original poster's example in their question, I nested a canvas in a grid. The grid is within a scrollviewer. Instead of attempting to change the canvas size, I changed the grid size, both height and width in my case, and the canvas followed the size of the grid minus any margins. I set the grid size programmatically, although I would think binding would work as well. I got the desired size of the grid programmatically as well.

Using Grid will auto size to content without setting extra parameters, but just the VerticalAlignment and HorizontalAlignment. For example:

<Grid VerticalAlignment="Top" HorizontalAlignment="Center">
    <Polygon Points="0, 0, 0, 100, 80, 100" Panel.ZIndex="6" Fill="Black" Stroke="Black" StrokeStartLineCap="Round" StrokeDashCap="Round" StrokeLineJoin="Round" StrokeThickness="10"/>
</Grid>

As I understood the canvas does not inform the scrollviewer when controls are added to the canvas that increase the size of the canvas. The scrollviewer does not know that the canvas size increased. What worked for me was just to change the width of the canvas programmatically after adding the control.

mycanvas.Children.Add(myTextBlock);
mycanvas.Width += 10.0

;

<viewbox>
    <canvas>
        <uielements /> 
    </canvas>
</viewbox>