C#/WPF: ListView Sorting: NullReference Exception when sorting double/decimals, but not for strings?

Question

Does anyone know why I'm getting a NullReference Expception at following line:

var field = (string)((Binding)((GridViewColumnHeader)e.OriginalSource).Column.DisplayMemberBinding).Path.Path;

when using this example: http://www.switchonthecode.com/comment/reply/263/2980 (based on: http://www.switchonthecode.com/tutorials/wpf-tutorial-using-the-listview-part-2-sorting)

Thanks a lot!

Cheers, Joseph

PS: This only happens when I'm sorting double/decimals, not for strings?

EDIT:

I found the problem. This is how my XAML looks like:

            <GridViewColumn Header="Double">
                <GridViewColumn.CellTemplate>
                    <DataTemplate>
                        <TextBlock Text="{Binding TotalValues, Mode=OneWay, StringFormat=\{0:0\'0.00\}, Converter={StaticResource GridValueConverter}}" TextAlignment="Right" Width="auto"/>
                    </DataTemplate>
                </GridViewColumn.CellTemplate>
            </GridViewColumn>
            <GridViewColumn Header="Comments" DisplayMemberBinding="{Binding Path=Comments, Mode=OneWay}" Width="auto"/>

The problem here is that I'm trying to get the DisplayMemberBinding, but for the double's I'm using a DataTemplate.. does anyone know how I can change the line to make it work for every Column-Type?

Answer 1

At a glance, it looks like the line should only be working for strings since youre casting it to a string before assigning. You might do this instead:

object field = ((Binding)((GridViewColumnHeader)e.OriginalSource).Column.DisplayMemberBinding).Path.Path;

Which won't throw any null reference exception (which is being caused by the (string) cast when the value isn't a string). If you would cast using as, you wouldn't get the exception upon cast but that won't work for value types (and would still only work for string), ie:

var field = (((Binding)((GridViewColumnHeader)e.OriginalSource).Column.DisplayMemberBinding).Path.Path as string);

so with field as an object, you can use field.GetType() to compare against typeof(string), typeof(double),... to figure out what type it is and do whatever else you need to with it.