Wednesday, November 17, 2010

WPF quirks: The case of the exploding MaxWidth Line

I had this control that consisted of a textbox and a squigly red line in case of a required value error. In order to do it, I added the textbox and the line to a Grid, without specifying column or row values, so that the line would come above the textbox. It all worked well until I wanted to make the control align to the left, thus growing with the content entered in it. To my surprise, the control would stretch to the content width when in edit mode and go to 202 pixels outside it. The only things in the template were a textbox and a line inside a grid and a border, so I proceeded on inspecting all of their attributes in search for the culprit. It so happens that the Line was it!

Update: The Microsoft guys responded in a day to my bug report, but they couldn't reproduce it. It seems this behavior can be reproduced only in a Grid column with Width="Auto". Frankly, I was a bit surprised to see that, lacking that grid column, a line with Stretch="Fill" and HorizontalAlignment to "Left" would still expand the container to its maximum size. End update.
The code looked like this:

<Style x:Key="StyleRequiredUnderline" TargetType="{x:Type Line}">
<Setter Property="HorizontalAlignment" Value="Left"/>
<Setter Property="Stretch" Value="Fill"/>
<Setter Property="X1" Value="0"/>
<Setter Property="X2" Value="1"/>
<Setter Property="Y1" Value="0"/>
<Setter Property="Y2" Value="0"/>
<Setter Property="MaxWidth" Value="200"/>
</Style>

<Grid>
<TextBlock x:Name="TextBlock" />
<Line Name="RequiredUnderline" Style="{StaticResource StyleRequiredUnderline}"
/>
</Grid>
As you can see, the line is Left aligned, it has no specified Width, the only giveaway is the Stretch property set to Fill. Now, you think that was the problem, but it was not! See that I have a MaxWidth of 200. That was per request.

It appears that if I remove the MaxWidth setting, the line goes DOWN to the normal size of the parent inner width. MaxWidth, not MinWidth, mind you. Ok, so I've tried some other things. Stretch to None makes the line be 1px long. Setting X2 to 200 makes the line take 200px, same as setting Width. HorizontalAlignment to Stretch makes the line go to the center of the space if it is bigger than the 200 MaxWidth.

The solution? I've bound the Width of the line to the ActualWidth of the TextBlock above. Another option would have been to surround the line with a scrollviewer or some other control that would allow the line to be as long as it wanted without showing a scrollbar or stretching to the size of its content. Either solution seems bad.

Is this a bug? I think so. If the Stretch property should have affected the space the line takes, then it should have done so when MaxWidth was set to Infinity, but it did not. Well, hopes it helps someone. Final code piece for the line:

<Grid>
<TextBlock x:Name="TextBlock" />
<Line Name="RequiredUnderline" Style="{StaticResource StyleRequiredUnderline}"
Width="{Binding ActualWidth,ElementName=TextBlock}" />
</Grid>


Update: The fix I've exemplified above doesn't work when the HorizontalAlignment of the grid is Stretch or has a Width and the TextBox doesn't have a Left HorizontalAlignment. I have tried to replace the Line with a ScrollViewer with hidden scrolling on the horizontal and disabled on the vertical and having a MaxWidth of 200, Inside placing the troublesome Line. I've tried all kinds of panels and combinations of HorizontalAlignment, HorizontalContentAlignment, ClipToBounds, etc. All to no avail.

Finally, the solution was to create a simple control that would ignore the dimensions of the child controls, demanding no space. I named it ClipContainer and here is its source:

public class ClipContainer : ContentControl
{
protected override Size MeasureOverride(Size availableSize)
{
base.MeasureOverride(availableSize);
return new Size(0, 0);
}
}

<Grid>
<TextBlock x:Name="TextBlock" />
<local:ClipContainer>
<Line Name="RequiredUnderline" Style="{StaticResource StyleRequiredUnderline}" />
</local:ClipContainer>
</Grid>
I've put the line in this control, MaxWidth and all, and the control stretched to the size of its container and clipped all of its contents.

Also, like any responsible developer , I went to Microsoft Connect to add this issue as a bug. Here is the bug report. Vote it up if it affects you.

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