bool to image c# the reference object is not set to an instance of an object

Question

I have a boolean on a DataGridTextColumn and I want to print an image if it's True and another image if it's False.

This is the DataGrid:

<DataGrid x:Name="DonneesBrutes" IsReadOnly="True" ItemsSource="{Binding Path=ResultatCollectionGrande}" Margin="10,65,0,0" AutoGenerateColumns="False" EnableRowVirtualization="True" RowDetailsVisibilityMode="VisibleWhenSelected">

            <DataGrid.Columns>
                <DataGridTextColumn x:Name="PrisEnCompte" Width="50" Binding="{Binding Path=Flag,Converter={StaticResource BoolImageConverter}}" Header="PEC"></DataGridTextColumn>

This is the Windows.Resources, where I define the converter and which images are used:

<Window.Resources>
    <Image x:Key="TrueImage" Source="booleanTrue.png"/>
    <Image x:Key="FalseImage" Source="booleanFalse.png"/>
    <local:BoolToImage TrueImage="{StaticResource TrueImage}" FalseImage="{StaticResource FalseImage}" x:Key="BoolImageConverter"/>
</Window.Resources>

And there is the converter:

public class BoolToImage : IValueConverter
{
    public Image TrueImage { get; set; }
    public Image FalseImage { get; set; }

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    var viewModel = (ViewModel)(value as System.Windows.Controls.ListBoxItem).DataContext;
    if (!(value is bool))
    {
        return null;
    }

        bool b = (bool) viewModel.ActiviteCollection.FirstOrDefault().Flag;
    if (b)
    {
        return this.TrueImage;
    }
    else
    {
        return this.FalseImage;
    }
}

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

When I declare my var viewModel I have the error the reference object is not set to an instance of an object I think it's the declaration (value as System.Windows.Controls.ListBoxItem) which is wrong, But I don't find how to correct this.

A precision, viewModel.ActiviteCollection.FirstOrDefault().Flag; is the boolean I send to the DataGridTextColumn I want to convert as Image.

I hope I'm precise enough, I can edit my post if you need more informations.

Thanks for your help.

Greetings,

Flo.

`I have the error the reference object is not set to an instance of an object I think it's the declaration (value as System.Windows.Controls.ListBoxItem)`. Any chance you could simply put a breakpoint in your converter and check for yourself? See what type value is at runtime, it should help you debug your issue. — decPL, Sep 30 '14 at 09:09
Ad 1. If value is true, how do you expect to be able to cast it to System.Windows.Controls.ListBoxItem? Ad 2. http://msdn.microsoft.com/en-us/library/ktf38f66%28v=vs.71%29.aspx — decPL, Sep 30 '14 at 09:16

score 2 · Accepted Answer · answered Sep 30 '14 at 09:12

The value in your Convert method is the actual value to convert (the value of property Flag).

Hence, value as System.Windows.Controls.ListBoxItem, will return null.

Use this instead:

if ((bool)value)
{
}

See the use of value in the documentation of IValueConverter on MSDN.

bool to image c# the reference object is not set to an instance of an object

1 Answers1