WPF Converter strange behavior

Question

I am binding the Width of a TextBox to the ActualWidth of its containing Grid, as follows:

<TextBlock
    Grid.Column="0"
    Text="WorkPortalView UserControl"
    TextTrimming="CharacterEllipsis"
    Foreground="White"
    Margin="5" 
    Width="{Binding ActualWidth, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Grid}}, Converter={StaticResource textWidthConverter}, diag:PresentationTraceSources.TraceLevel=High}" />

As shown, I am using a Converter, as follows:

class TextWidthConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            double? d = value as double?;
            if (d == null)
                return null;

            return (d / 2);
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotSupportedException("Cannot convert back");
        }
    }

The strange thing is, that the code shown works fine. However, if I replace the return (d / 2) statement with return (d - 75D), it seems to set off an infinite loop of property changes.

Here is the output from the diagnostics trace tag for d / 2:

System.Windows.Data Warning: 71 :     Lookup ancestor of type Grid:  queried Grid (hash=19487651)
System.Windows.Data Warning: 70 :   RelativeSource.FindAncestor found Grid (hash=19487651)
System.Windows.Data Warning: 76 : BindingExpression (hash=35097890): Activate with root item Grid (hash=19487651)
System.Windows.Data Warning: 106 : BindingExpression (hash=35097890):   At level 0 - for Grid.ActualWidth found accessor DependencyProperty(ActualWidth)
System.Windows.Data Warning: 102 : BindingExpression (hash=35097890): Replace item at level 0 with Grid (hash=19487651), using accessor DependencyProperty(ActualWidth)
System.Windows.Data Warning: 99 : BindingExpression (hash=35097890): GetValue at level 0 from Grid (hash=19487651) using DependencyProperty(ActualWidth): '417.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=35097890): TransferValue - got raw value '417.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=35097890): TransferValue - user's converter produced '208.8'
System.Windows.Data Warning: 87 : BindingExpression (hash=35097890): TransferValue - using final value '208.8'

Here is the output from the diagnostics trace tag for d - 75D:

System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '429.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '429.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '354.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '354.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '441.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '441.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '366.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '366.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '453.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '453.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '378.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '378.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '465.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '465.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '390.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '390.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '477.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '477.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '402.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '402.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '489.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '489.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '414.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '414.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '501.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '501.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '426.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '426.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '513.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '513.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '438.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '438.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '525.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '525.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '450.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '450.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '537.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '537.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '462.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '462.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '549.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '549.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '474.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '474.6'
System.Windows.Data Warning: 94 : BindingExpression (hash=58235710): Got PropertyChanged event from Grid (hash=43090134) for ActualWidth
System.Windows.Data Warning: 99 : BindingExpression (hash=58235710): GetValue at level 0 from Grid (hash=43090134) using DependencyProperty(ActualWidth): '561.6'
System.Windows.Data Warning: 78 : BindingExpression (hash=58235710): TransferValue - got raw value '561.6'
System.Windows.Data Warning: 80 : BindingExpression (hash=58235710): TransferValue - user's converter produced '486.6'
System.Windows.Data Warning: 87 : BindingExpression (hash=58235710): TransferValue - using final value '486.6'

Any thoughts on what explains this behavior of the Converter?

@Wonko: No, but it does seem to have something to do with the interaction with the other column (Width=Auto on both columns) that gets triggered when I do not leave enough Width for the second column; setting the equation to d - 100D (leaving more room for the other column) seems to correct the problem. I still don't know the triggering condition to be able to know what is "enough" room for the other column, why it doesn't just cutoff the other column, or make some other final determination. — Bill, May 10 '12 at 17:55

score 1 · Answer 1 · answered May 10 '12 at 18:26

1

Apparently, the grid needs more room for the textblock. You might be able to find the cutoff point of this behavior at d-87D since the grid seems to need an additional 12 px on each textblock width change.

By the way, it might be better to use star sizing for the grid column, if all you want is to give a relative size to the textblock.

answered May 10 '12 at 18:26

Eren Ersönmez

38,383
7
71
92

Thanks Eren. +1 for good suggestions. I think the answer to my question is that this is a bug. I think the answer is that I must have been triggering a Grid size change by not allowing the second column enough room. Still, I would think one "auto" adjustment to the Grid size should correct the layout and the layout should resolve to a stable adjusted setting, not result in an infinite loop. As such, this is arguably a bug I think. – Bill May 10 '12 at 18:41
@Bill This is not a bug. Please note that "auto" means "give me a size just enough to fit my contents". It determines the size based on its contents, not based on the space available. If it turns out it needs 100 px for its contents, it would be the same as setting the width to a fixed size of 100 px. In your case, the size of the second columns contents aren't changing; therefore, the "auto" sized column doesn't alter its size. – Eren Ersönmez May 11 '12 at 06:11
I am not clear how your description of the behavior of Auto ties into the behavior of the converter that I described. – Bill May 16 '12 at 02:22
@Bill I meant, you can assume a fixed size for the auto sized column because of that behavior. Therefore, since the grid's size is the sum of the two columns, the grid's size has to increase whenever the size of the first column increases (since the second column has fixed size). That change in the grid size triggers another change in the column size because of the binding, which triggers a change in the grid size, and so on. Hope I'm making some sense :) – Eren Ersönmez May 16 '12 at 06:34

score 0 · Answer 2 · edited Sep 11 '13 at 02:09

0

This appears to be a bug. The behavior is reasonable for one "auto adjust" iteration, but should not result in an infinite loop. Please see discussions above.

I ended up switching to DockPanel to achieve the desired layout.

edited Sep 11 '13 at 02:09

Dave Clemmer

3,741
12
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answered May 10 '12 at 18:44

Bill

1,407
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22

WPF Converter strange behavior

2 Answers2