The oh so awesome AG_E_PARSER_BAD_PROPERTY_VALUE

I'm trying to extend the datagridcolumn a bit so I can make column widths percentage based, rather then absolute in silverlight. This way no matter what size the grid, the columns take up a specified percentage of the grid.

Anyway, this is my first step

public static class DataGridC开发者_运维问答olumnBehaviors
{
    public static readonly DependencyProperty WidthPercentageProperty =
        DependencyProperty.RegisterAttached("WidthPercentage", typeof(double?), typeof(DataGridColumnBehaviors),
            new PropertyMetadata(null, OnWidthPercentagePropertyChanged));

    public static double? GetWidthPercentage(DependencyObject d)
    {
        return (double?)d.GetValue(WidthPercentageProperty);
    }

    public static void SetWidthPercentage(DependencyObject d, double? value)
    {
        d.SetValue(WidthPercentageProperty, value);
    }

    public static void OnWidthPercentagePropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {

    }
}

And in the XAML I'm doing

            <data:DataGridTemplateColumn MinWidth="200" 
                                         dataBehaviors:DataGridColumnBehaviors.WidthPercentage="5.0"
                                         Header="Name">
                <data:DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <TextBlock Text="{Binding Name}" />
                    </DataTemplate>
                </data:DataGridTemplateColumn.CellTemplate>
                <data:DataGridTemplateColumn.CellEditingTemplate>
                    <DataTemplate>
                        <TextBox Text="{Binding Name, Mode=TwoWay}" />
                    </DataTemplate>
                </data:DataGridTemplateColumn.CellEditingTemplate>
            </data:DataGridTemplateColumn>

This is producing the following message at runtime

AG_E_PARSER_BAD_PROPERTY_VALUE [Line: 85 Position: 100]

Line 85 is this:

dataBehaviors:DataGridColumnBehaviors.WidthPercentage="5.0

Any ideas?

You can't convert from a double to a double? at the CLR level like that. And you almost certainly don't want to.

Silverlight uses doubles and then uses double.NaN and double.PositiveInfinity to represent 'special' values.